Perception amp Psychophysics2003 65 (5) 779-788

In a recent study of temporal order judgments (TOJs)subjects judged which one of two sites on the hand wasstimulated first (Craig amp Busey 2003) The stimuli weremoving patterns generated on two tactile arrays At briefstimulus onset asynchronies (SOAs) the subjects basedtheir TOJs to a large extent on the direction of motionacross the fingers even though the motion was irrelevantto the TOJ For example when the patterns were pre-sented to the index and middle fingerpads on the pronatedleft hand and the patterns moved from right to left thesubjects showed a strong bias to respond that the indexf inger had received the f irst stimulus If the patternsmoved from left to right the subjects showed a strongbias to respond that the middle finger had received thefirst stimulus The condition was referred to as consis-tent when for example the movement at the two siteswas from right to left and the index finger site (the siteto the right) led the site to the left The term consistentwas used because the direction of motion across the twofingers was consistent with the type of stimulation re-ceived in haptic exploration If the left hand is moved tothe right across a surface the surface relative to the fin-gerpad moves from right to left and surface features areencountered by the index fingerpad before they are en-countered by the middle finger Inconsistent movement

is generated by reversing the direction of motion acrossthe fingerpads (making it left to right) while the indexfingerpad still receives the first pattern (Craig amp Busey2003)

At SOAs of less than 50 msec consistent conditionsled to correct responding on as many as 80 of the trialswhereas inconsistent conditions led to correct respondingon as few as 20 of the trials The use of moving as com-pared with static patterns did not lower overall perfor-mance in TOJsmdashthat is overall correct performance wasthe same for the moving patterns as for the static patternsBecause the moving patterns did not alter sensitivity theeffect of movement appears to be one of biasing the sub-jectsrsquo responses (Craig amp Busey 2003)

In the earlier study testing initially involved adjacentfingers on the same hand On the basis of the results fromthese tests it was thought that moving stimuli affectedTOJs because of the close coupling of the movement ofobjects across the hand and the temporal order in whichfeatures are received at the skinrsquos surface This explana-tion became less plausible as additional conditions weretested Moving patterns produced similar biases in TOJswhen the sites of stimulation were fingers on oppositehands (bilateral) Moreover the bias was the same whetherthe two hands were placed close together or far apart(80 cm) When two hands are placed 80 cm apart it is un-likely that there is a very close coupling of the temporalorder of features arriving at the two hands and the direc-tion of movement What all the conditions had in com-mon was that the leading pattern had to be moving to-ward the other location or the trailing pattern had to bemoving away from the other location however no con-

779 Copyright 2003 Psychonomic Society Inc

This research was supported by National Institutes of Health GrantDC00095 National Institute on Deafness and Other CommunicationDisorders I thank Roger Rhodes for his assistance in these experimentsand Carol Rhodes for the illustrations Correspondence concerning thisarticle should be addressed to J C Craig Department of PsychologyIndiana University Bloomington IN 47405 (e-mail craigjindianaedu)

The effect of hand position and pattern motion on temporal order judgments

JAMES C CRAIGIndiana University Bloomington Indiana

Subjects made temporal order judgments (TOJs) of tactile stimuli presented to the fingerpads Thesubjects judged which one of two locations had been stimulated first The tactile stimuli were patternsthat simulated movement across the fingerpads Although irrelevant to the task the direction of move-ment of the patterns biased the TOJs If the pattern at one location moved in the direction of the sec-ond location the subjects tended to judge the first location as leading the second location If the pat-tern moved in the opposite direction that location was judged as trailing In a series of experimentsthe effect of the spatial position of the hands and fingers on TOJs and the perception of the directionof pattern movement were examined Changing the position of the hands so that the patterns no longermoved directly toward each other reduced or eliminated the effect of motion on TOJs In a variation ofAristotlersquos illusion the moving patterns were presented to crossed and uncrossed fingers The resultsindicated that contrary to Aristotlersquos illusion the subjects processed the moving patterns relative toan environmental framework rather than to the local direction of motion on the fingerpads Present-ing the patterns to crossed hands produced results similar to those obtained with crossed fingers Thesubjects processed the patterns according to an environmental framework

780 CRAIG

ditions were examined in which the motion was not di-rectly toward or away from the sites of stimulation In thepresent study the term leading toward is used to describetrials in which the temporally leading pattern is movingin the direction of the site of the temporally trailing pat-tern similar to the consistent condition Leading awayrefers to trials in which the leading pattern is movingaway from the other site of stimulation similar to the in-consistent condition On leading-away trials it is thetrailing stimulus that moves in the direction of the lead-ing stimulus

One of the aims of the present study was to examineadditional conditions that might affect the TOJ bias In allof the conditions previously tested the two tactile displayswere placed parallel to each other and horizontally infront of the subjects The patterns moved in the lateral-medial direction relative to the body In the present studythe effects of altering the spatial positions of the displayson TOJs were investigated To examine further the ex-planation that the coupling of temporal order and move-ment across the skin surface accounts for the TOJ biaswe presented the moving patterns to the f ingers andhands in positions in which such coupling was unlikelyFor example the subjects made TOJs of moving patternspresented to two fingers crossed over one another (Ex-periment 2) and to two hands crossed over one another(Experiment 3) The subjects were very unlikely to haveengaged in extensive haptic exploration with their handsor fingers crossed

The second major aim of the study was to explore thespatial frame used by subjects in processing moving pat-terns The hand is a unique sensory organ It can be usedboth to manipulate objects and to gain perceptual infor-mation In view of the fact that the hand has a number ofdegrees of freedom the local pattern of sensory infor-mation received at a particular site on the hand often hasto be interpreted in light of the position of the hand inspace Some studies have shown that the perception ofthe pattern of local motion on the hand or other skin sur-faces depends on the position of the stimulated part inspace (Oldfield amp Phillips 1983 Parsons amp Shimojo1987 Sekiyama 1991) For example the letter ldquobrdquodrawn on the palm facing the observer will generally beperceived as the letter ldquobrdquo but when the same local pat-tern is drawn on the palm facing away from the observerthe letter ldquodrdquo is perceived In a study of selective atten-tion target and distractor stimuli were presented to fin-gertips on opposite hands The amount of interferencedeclined as the hands were moved farther apart indicat-ing that the task was sensitive to an external spatialframe of reference rather than to a somatotopic frame ofreference (Driver amp Grossenbacher 1996) Under somecircumstances however subjectsrsquo spatial judgments re-main unchanged as the position of the stimulated sites isaltered (Benedetti 1988 Rieser amp Pick 1976) How andunder what circumstances spatial perception is altereddepends on a number of variables One of the difficultiesin studying the effects of spatial position on pattern per-ception is that subjects may attempt to adopt the perspec-

tive of the experimenter and respond with the pattern thatthey think the experimenter intended to present

A previous study (Rinker amp Craig 1994) in whichmoving patterns were used attempted to avoid the prob-lem of the results being biased because of task require-ments In that study subjects attempted to identify thedirection of motion of a target pattern presented to thethumb A second irrelevant moving pattern was pre-sented to the index finger When the two patterns weremoving in opposite directions the irrelevant pattern in-terfered with the identification of the target pattern Thistesting was done with the two displays parallel to eachother in the transverse plane (horizontally in front of thesubject) The two displays were then oriented verticallyThe subjects gripped the two displays between the thumband the index finger The task was the same attend se-lectively to the thumb and report the direction of motionThe change in orientation of the displays altered the pat-tern of interference from the irrelevant stimulus The ir-relevant stimulus that had produced no interferencewhen the two displays were horizontal now resulted ininterference and vice versa In the present study the biasin TOJs seen with moving stimuli was used as a way ofexamining the perception of moving stimuli

As was noted one of the aims of the present study wasto examine how the spatial orientation of the handswould affect the perception of the direction of motionand the TOJ bias If the local pattern of motion is keptthe same at the two sites does the bias disappear as thehands are moved and the direction of motion is no longertoward one of the sites of stimulation Does changingthe position of the hands in space alter TOJs Do sub-jects show evidence of perceptual constancy in process-ing the moving patterns

These questions were examined in three experimentsIn Experiment 1 the displays and the index fingers con-tacting the displays were changed from being parallel toone another (0ordm) to having the displays form a 90ordm angleand a 180ordm angle In Experiment 2 the moving patternswere presented to two fingers on the same hand when thefingers were both uncrossed and crossed The intent wasto replicate conditions in which subjects fail to show per-ceptual constancy and greatly mislocalize stimuli spa-tially (Benedetti 1988) In Experiment 3 the patternswere presented to uncrossed and crossed hands The lat-ter condition has been shown to interfere greatly withTOJs of static stimuli (Shore Spry amp Spence 2002 Ya-mamoto amp Kitazawa 2001a 2001b)

There are several advantages to using TOJs to exam-ine the interaction between tactile motion and limb posi-tion First the effects of TOJs are relatively large and re-liable Second subjects are not asked to judge theposition of the fingers and hands in space or to make anyjudgments of motion In fact in these studies neither theposition of the limbs nor the direction of motion is rele-vant to the TOJs Subjects do not have to attempt to in-terpret instructions about whether the direction of mo-tion should be judged solely on the basis of the localpattern of motion on the fingerpads (somatotopic frame-

TACTILE TEMPORAL ORDER JUDGMENTS 781

work) or relative to the position of the hands in space(environmentally based framework) In haptic explo-ration or in manipulating an object complex patterns ofmotion across the fingerpads are generated but attentionis rarely focused on these patterns per se Rather the ob-server is trying to judge the shape of an object its posi-tion in space or whether some parts can be moved rela-tive to others Observers are generally more concernedwith the environmental rather than the somatotopicframe of reference Third the question has been raised instudies of haptic and tactile spatial judgments of whetherthe judgments are verbally mediated (Rieser amp Pick1976) Inasmuch as subjects are making no judgments ofmotion the effects of pattern motion on TOJs are un-likely to be mediated verbally

EXPERIMENT 1

In Experiment 1 the effect on the TOJ bias of alteringthe position of the hands in space and the directions ofmotion in an environmental frame were examined Fig-ure 1 shows the position of the left and right index fingersat various positions Arrows indicate the direction of mo-tion on the fingers The previous study had examined onlythe position marked ldquo0ordmrdquo At 90ordm the motion was no longermoving directly from one display to the other If the mo-tions at the two displays were extended toward each othertheir trajectories would intersect at a 90ordm angle midwaybetween the two displays and about 5 cm below a line seg-ment connecting the two displays At 180ordm the motions atthe two sites were parallel to each other If the TOJ biasdepends on the direction of motion in an environmentalframework and if perception of the direction of motionchanges with hand position the size of the TOJ bias mightdecrease perhaps to zero as the angle between the twohands changed from 0ordm to 90ordm to 180ordm The question waswhether there would be any change in TOJ performanceas a function of the placement of the hands

MethodSubjects The subjects in all the experiments were undergradu-

ate students at Indiana University Generally the subjects had par-ticipated in several tactile experiments They were selected for partic-ipation in each experiment as scheduling permitted They receivedan hourly wage for their participation Four women and 2 men weretested

Apparatus Two tactile displays generated the stimuli Each dis-play consisted of 144 tactors arranged in a 6-column 3 24-rowarray The tactors were activated with square waves 230 pps Thetwo displays were interfaced with a PC The arrays were similar tothose used in the Optacon a reading aid for the blind (Bliss KatcherRogers amp Shepard 1970) The arrays fit against the distal portionof the fingerpads The patterns covered an area 17 3 11 mm

Procedure The subjectrsquos task was to indicate which of the twosites of stimulation received the stimulus f irst The subjects placedthe index finger of the left hand on one display and the index fin-ger of the right hand on the other display The moving patterns weregenerated by activating pairs of columns sequentially on the dis-play A representation of the patterns is shown in Figure 2 Eachpair of columns was activated for 13 msec The SOA betweencolumns was 13 msec The pair of columns occupied five differentlocations on the display for a total duration of 65 msec and covered

a distance of 11 mm The voltage to each tactor was set so as to pro-duce a moderate stimulus intensity The resulting sensation was oneof a light mechanical stimulus moving across the fingerpad

The subjects initiated each trial with a foot pedal Following thepresentation of the pair of stimuli the subjects responded by meansof two foot pedals to indicate which of the two stimuli they judgedto have been presented first The foot pedals were placed side byside The subjects typically used the left foot to depress the leftpedal and the right foot to depress the right pedal No trial-by-trialfeedback was provided The subjects were instructed that the di-rection of motion was irrelevant to the TOJ

The patterns presented to the two fingers moved in the same di-rection on both fingers The direction of motion and the temporalorder were determined randomly on a trial-by-trial basis The di-rection of motion was uncorrelated with the temporal order Threepositions of the arrays (and the positions of the hands and fingers)were tested in Experiment 1 (Figure 1) The distance between thetwo arrays for these three positions was 10 cm A single positionwas tested each session The directions of movement of the patternsacross the f ingerpads were the same for the three positions Theorder of testing of the positions was random with the constraint thateach position was tested once before being tested again

Six SOAs were tested 400 200 100 52 26 and 13 msec Eachblock of 50 trials tested a different SOA In each experimental ses-sion the six SOAs were tested in order from longest to shortest Webegan with the 400-msec SOA At this SOA there was no bias andthe subjects were generally correct This provided a clear demon-stration of the nature of the task for the subjects and showed themthat the direction of the pattern movement was irrelevant to the TOJThe subjects were generally able to complete two sets of six SOAseach (a total of 600 trials) in a single session To eliminate auditorycues from the two displays the subjects wore earplugs and ear-phones through which white noise was presented For each of thethree conditions (0ordm 90ordm and 180ordm) each of the 6 subjects com-pleted four sets of the six SOAs a total of 7200 trials at each of thethree positions

Figure 1 A representation of the position of the index fingersin Experiment 1 0ordm 90ordm and 180ordm The arrows indicate the di-rection of motion of the patterns across the fingerpads for onetype of trial In a second type of trial the motion at both finger-pads was in the opposite direction

782 CRAIG

Results and DiscussionThe results for the three positions of the arrays are

shown in Figure 3 The percentage of correct responsesis plotted as a function of SOA The top panel shows theresults when the two displays were at 0ordm Three functionsare presented The top function marked ldquoleading to-wardrdquo refers to trials on which the pattern at the tempo-rally leading site was moving toward the site at which thetrailing pattern was presented As was noted in the pre-vious study (Craig amp Busey 2003) this condition wasreferred to as consistent movement The lower functionldquoleading awayrdquo refers to trials on which the temporallyleading stimulus was moving away from the site of thetrailing stimulus previously referred to as inconsistentmovement The middle function is the mean of the upperand lower functions approximately equal to the overalllevel of correct performance These results were similarto the results obtained in the previous study There wasa bias to respond that the pattern that was moving towardthe second location was the temporally leading patternWhen the pattern moving toward the second locationwas in fact the leading pattern the subjects tended to becorrect When the pattern moving away from the secondlocation was the leading pattern the subjects tended tobe incorrect (lower function) An analysis of variance(ANOVA) showed a significant effect of trial type [lead-ing toward vs leading away F(15) = 2437 p lt 01]

The middle panel presents the results from the 90ordmcondition Although the direction of motion at the twosites was no longer directly toward each other there wasstill an effect of the direction of pattern motion and a biasfavoring the leading-toward trials [F(15) = 1378 p lt05] In the lower panel the 180ordm condition the patternswere now moving parallel to each other in an environ-mental frame rather than toward one another howeverthe same labels for the functions were retainedmdashthat isldquoleading towardrdquo referred to the same frame of reference

as in the other panels (local pattern of motion or soma-totopic) The same labels for the local directions of mo-tion were retained to show more clearly the effect of thepositions of the hand and specifically that pattern mo-tion no longer affected TOJs [F(15) = 088 p = 39]These results indicate that the TOJ bias depended as wasexpected on the spatial position of the sites of stimula-tion As the position changed so did the TOJ bias

To see more clearly the effect that hand position andthe direction of motion (environmental frame) has onTOJs we calculated the difference in performance ateach SOA between the leading-toward and the leading-away trial types These differences are plotted in Figure 4and show that the magnitude of the TOJ bias declinedwith the angle between the two displays An ANOVAshowed a significant effect of position of the arrays[F(210) = 1017 p lt 01] An ANOVA performed onjust the 0ordm and 90ordm positions showed no significant dif-ference in the size of the TOJ bias between these two po-sitions [F(15) = 398 p = 10] however a separateanalysis of the three briefest SOAs where the TOJ biasis most evident did show a significant effect of position[F(15) = 757 p lt 05] This result suggests that the ef-fect of altering the angle between the arrays did not re-sult in an all-or-none effect but rather that the TOJ biaswas a continuous function of the relative direction ofmotion across the two hands

EXPERIMENT 2

In Experiment 2 subjects made TOJs of moving pat-terns presented to two f ingers on the same hand Theissue was again whether the subjects would correctlyperceive the direction of motion (environmental frame)as reflected in TOJs when the positions of the sensorysurfaces were altered spatially In Experiment 2 the spa-tial positions were altered by crossing the fingers The

Figure 2 A representation of the tactile display and moving patterns Each panel represents one frame of thetactile pattern moving from left to right

TACTILE TEMPORAL ORDER JUDGMENTS 783

reason for being particularly interested in the effect ofcrossing fingers comes from a series of experiments byBenedetti (1985 1986a 1986b 1988 1991) and is re-lated to Aristotlersquos illusion In Aristotlersquos illusion twofingers on the same hand are crossed such as the middleand the index fingers and a rod is placed between the twofingertips The observer typically perceives two rodsBenedetti examined and quantified some additional per-ceptual effects of crossing the fingers In Benedettirsquos(1988) finding most relevant for the present experimentsubjects crossed finger D3 over D4 The subjects weretouched simultaneously on the tips of the two fingers andwere asked to localize the two points of stimulation rel-ative to one another When the third finger (right hand)was placed above the fourth finger the subjects accu-rately perceived the stimulus presented to the third fin-ger as being directly above the stimulus presented to thefourth finger The subjects then moved their fingers fromthis position and crossed D3 over D4 so that D3 was nowto the right of D4 The unexpected finding was that thesubjects were almost completely unable to take thecrossed position of the fingers into account in localizingthe points The subjects continued to localize D4 directlyabove D3 not in its new position to the right of D3 Inother words the subjects did not take into account thefact that the fingers were now crossed (Benedetti 1988)Benedetti pointed this out as an example of a failure ofperceptual constancy

The question in Experiment 2 was whether subjects withcrossed fingers would perceive the direction of motionas though their fingers remained uncrossed Figure 5shows representations of the patterns with fingers un-crossed and crossed Suppose that the two patterns weremoving from left to right and the middle finger (un-crossed condition) received the stimulus first (panel A)Previous results indicate that in this leading-toward con-dition subjects are very likely to correctly select themiddle finger as leading the index finger Suppose thatthe identical patterns same directions of motion aregenerated on the displays but now the fingers are crossed(Figure 5 panel B) The index finger would now be re-ceiving the pattern f irst If subjects correctly took ac-count of the fact that the patterns were being processedby crossed fingers they should tend to respond that theindex finger was the leading site This result would beevidence for perceptual constancy If however asBenedettirsquos (1988) results predict subjects fail to per-ceive that the middle finger is now to the right of theindex finger the TOJ bias could disappear or subjectscould respond that the middle finger trails the index fin-ger (somatotopic frame) Such results would show a lackof perceptual constancy

MethodSubjects Six women and 2 men were testedProcedure The procedure was similar to that used in Experi-

ment 1 The subjectsrsquo task was to respond with the location that re-

Figure 3 Percent correct in judging temporal order as a func-tion of the stimulus onset asynchrony (SOA) in milliseconds Thethree panels present the data from the three positions of the handsin Experiment 1 0ordm 90ordm and 180ordm In the results from the 0ordm con-dition the top function represents those trials on which the lead-ing pattern was moving toward the other site of stimulation Thebottom function represents those trials on which the leading pat-tern was moving away from the other site of stimulation Theldquomeanrdquo function represents overall percent correct the mean ofthe upper and lower functions The 90ordm results are labeled in thesame way as the 0ordm results as are the 180ordm results For the 180ordm re-sults the same local patterns of motion (somatotopic frame) onthe fingerpads are labeled as they are in the other results al-though the patterns are now moving parallel to one another (en-vironmental frame) Error bars represent plusmn1 standard error ofthe mean

784 CRAIG

ceived the stimulus first The index and middle fingers of the lefthand were the sites of stimulation A single display was used Inorder for the two fingers to contact the array the array was rotated90ordm In this configuration what had been rows of the display be-came columns and vice versa The stimuli consisted of a patternthree tactors in width and six tactors in height The stimuli movedlaterally across the fingerpads with each element of the pattern oc-cupying a position in the proximaldistal orientation for 13 msecThere were 13 msec between onsets of successive positions acrossthe f ingerpad The patterns occupied eight positions along thearray a distance of 88 mm The total duration for each pattern was104 msec In the testing sessions the blocks of trials alternated be-tween crossed and uncrossed conditions In the first block in eachsession the fingers were uncrossed Six SOAsmdash400 200 100 5226 and 13 msecmdashwere tested in each session Blocks consisted of25 trials Eight subjects were tested for four blocks for each of thesix SOAs a total of 4800 trials for the uncrossed condition and4800 trials for the crossed condition

Results and DiscussionThe results with the uncrossed fingers are shown in

Figure 6 The functions are labeled as in Figure 3 Thedata from the two directions of motion left and right arepresented in separate panels to make it easier to comparethe results from the uncrossed condition with the resultsfrom the crossed condition The top panel shows the re-sults when the two patterns were moving to the right Aswas expected when the fingers were uncrossed and whenthe pattern at the leading site was moving in the directionof the second site the subjects were very likely to re-spond correctly (leading-toward trials) When the patternat the leading site was moving in a direction away fromthe second site the subjects were at brief SOAs veryunlikely to respond correctly (leading-away trials) Theseresults are similar to those seen in previous measure-

ments For both the moving-right and the moving-leftconditions there was a significant effect of trial type[F(17) = 10382 p lt 001 and F(17) = 9238 p lt 001respectively]

The results with the crossed fingers are shown in Fig-ure 7 To help visualize the effects of motion the resultsfrom the two directions of motion are presented sepa-rately as they were in Figure 6 The top panel shows theresults when the two patterns were moving to the rightIn discussing these results the terms left and right referto positions of the patterns on the array (environmentalframe) not to the fingers (either crossed or uncrossed)The top function marked ldquoleading towardrdquo refers tothose trials on which the pattern on the left side of thearray was presented first These results show that whenboth patterns were moving from left to right there was astrong bias to respond that the site on the left (in thiscase the index finger) was leading When the site on theleft received the stimulus first the subjects were correct(top function) In this crossed-finger condition the leftside of the array was contacted by the left index finger(Figure 5) Thus the leading pattern was moving towardthe site of the trailing pattern (leading toward)mdashthat isindex toward middle finger Moreover the results werethe same whether the middle finger (uncrossed condi-tion) or the index finger (crossed condition) was con-tacting the left side of the array When the site on the leftwas trailing the subjects were incorrect (bottom func-tion)

The pattern of results in Figure 7 (top panel) is pre-dicted by perceptual constancy If the fact that the fin-gers are crossed had not been taken into account and thesubjects had responded as though their fingers were un-

Figure 4 The difference in percent correct between the leading-toward andleading-away functions in Figure 3mdashthat is the size of the TOJ bias plotted asa function of the angle between the positions of the fingers The parameter is thestimulus onset asynchrony (SOA) in milliseconds between the two patterns

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

780 CRAIG

ditions were examined in which the motion was not di-rectly toward or away from the sites of stimulation In thepresent study the term leading toward is used to describetrials in which the temporally leading pattern is movingin the direction of the site of the temporally trailing pat-tern similar to the consistent condition Leading awayrefers to trials in which the leading pattern is movingaway from the other site of stimulation similar to the in-consistent condition On leading-away trials it is thetrailing stimulus that moves in the direction of the lead-ing stimulus

One of the aims of the present study was to examineadditional conditions that might affect the TOJ bias In allof the conditions previously tested the two tactile displayswere placed parallel to each other and horizontally infront of the subjects The patterns moved in the lateral-medial direction relative to the body In the present studythe effects of altering the spatial positions of the displayson TOJs were investigated To examine further the ex-planation that the coupling of temporal order and move-ment across the skin surface accounts for the TOJ biaswe presented the moving patterns to the f ingers andhands in positions in which such coupling was unlikelyFor example the subjects made TOJs of moving patternspresented to two fingers crossed over one another (Ex-periment 2) and to two hands crossed over one another(Experiment 3) The subjects were very unlikely to haveengaged in extensive haptic exploration with their handsor fingers crossed

The second major aim of the study was to explore thespatial frame used by subjects in processing moving pat-terns The hand is a unique sensory organ It can be usedboth to manipulate objects and to gain perceptual infor-mation In view of the fact that the hand has a number ofdegrees of freedom the local pattern of sensory infor-mation received at a particular site on the hand often hasto be interpreted in light of the position of the hand inspace Some studies have shown that the perception ofthe pattern of local motion on the hand or other skin sur-faces depends on the position of the stimulated part inspace (Oldfield amp Phillips 1983 Parsons amp Shimojo1987 Sekiyama 1991) For example the letter ldquobrdquodrawn on the palm facing the observer will generally beperceived as the letter ldquobrdquo but when the same local pat-tern is drawn on the palm facing away from the observerthe letter ldquodrdquo is perceived In a study of selective atten-tion target and distractor stimuli were presented to fin-gertips on opposite hands The amount of interferencedeclined as the hands were moved farther apart indicat-ing that the task was sensitive to an external spatialframe of reference rather than to a somatotopic frame ofreference (Driver amp Grossenbacher 1996) Under somecircumstances however subjectsrsquo spatial judgments re-main unchanged as the position of the stimulated sites isaltered (Benedetti 1988 Rieser amp Pick 1976) How andunder what circumstances spatial perception is altereddepends on a number of variables One of the difficultiesin studying the effects of spatial position on pattern per-ception is that subjects may attempt to adopt the perspec-

tive of the experimenter and respond with the pattern thatthey think the experimenter intended to present

A previous study (Rinker amp Craig 1994) in whichmoving patterns were used attempted to avoid the prob-lem of the results being biased because of task require-ments In that study subjects attempted to identify thedirection of motion of a target pattern presented to thethumb A second irrelevant moving pattern was pre-sented to the index finger When the two patterns weremoving in opposite directions the irrelevant pattern in-terfered with the identification of the target pattern Thistesting was done with the two displays parallel to eachother in the transverse plane (horizontally in front of thesubject) The two displays were then oriented verticallyThe subjects gripped the two displays between the thumband the index finger The task was the same attend se-lectively to the thumb and report the direction of motionThe change in orientation of the displays altered the pat-tern of interference from the irrelevant stimulus The ir-relevant stimulus that had produced no interferencewhen the two displays were horizontal now resulted ininterference and vice versa In the present study the biasin TOJs seen with moving stimuli was used as a way ofexamining the perception of moving stimuli

As was noted one of the aims of the present study wasto examine how the spatial orientation of the handswould affect the perception of the direction of motionand the TOJ bias If the local pattern of motion is keptthe same at the two sites does the bias disappear as thehands are moved and the direction of motion is no longertoward one of the sites of stimulation Does changingthe position of the hands in space alter TOJs Do sub-jects show evidence of perceptual constancy in process-ing the moving patterns

These questions were examined in three experimentsIn Experiment 1 the displays and the index fingers con-tacting the displays were changed from being parallel toone another (0ordm) to having the displays form a 90ordm angleand a 180ordm angle In Experiment 2 the moving patternswere presented to two fingers on the same hand when thefingers were both uncrossed and crossed The intent wasto replicate conditions in which subjects fail to show per-ceptual constancy and greatly mislocalize stimuli spa-tially (Benedetti 1988) In Experiment 3 the patternswere presented to uncrossed and crossed hands The lat-ter condition has been shown to interfere greatly withTOJs of static stimuli (Shore Spry amp Spence 2002 Ya-mamoto amp Kitazawa 2001a 2001b)

There are several advantages to using TOJs to exam-ine the interaction between tactile motion and limb posi-tion First the effects of TOJs are relatively large and re-liable Second subjects are not asked to judge theposition of the fingers and hands in space or to make anyjudgments of motion In fact in these studies neither theposition of the limbs nor the direction of motion is rele-vant to the TOJs Subjects do not have to attempt to in-terpret instructions about whether the direction of mo-tion should be judged solely on the basis of the localpattern of motion on the fingerpads (somatotopic frame-

TACTILE TEMPORAL ORDER JUDGMENTS 781

work) or relative to the position of the hands in space(environmentally based framework) In haptic explo-ration or in manipulating an object complex patterns ofmotion across the fingerpads are generated but attentionis rarely focused on these patterns per se Rather the ob-server is trying to judge the shape of an object its posi-tion in space or whether some parts can be moved rela-tive to others Observers are generally more concernedwith the environmental rather than the somatotopicframe of reference Third the question has been raised instudies of haptic and tactile spatial judgments of whetherthe judgments are verbally mediated (Rieser amp Pick1976) Inasmuch as subjects are making no judgments ofmotion the effects of pattern motion on TOJs are un-likely to be mediated verbally

EXPERIMENT 1

In Experiment 1 the effect on the TOJ bias of alteringthe position of the hands in space and the directions ofmotion in an environmental frame were examined Fig-ure 1 shows the position of the left and right index fingersat various positions Arrows indicate the direction of mo-tion on the fingers The previous study had examined onlythe position marked ldquo0ordmrdquo At 90ordm the motion was no longermoving directly from one display to the other If the mo-tions at the two displays were extended toward each othertheir trajectories would intersect at a 90ordm angle midwaybetween the two displays and about 5 cm below a line seg-ment connecting the two displays At 180ordm the motions atthe two sites were parallel to each other If the TOJ biasdepends on the direction of motion in an environmentalframework and if perception of the direction of motionchanges with hand position the size of the TOJ bias mightdecrease perhaps to zero as the angle between the twohands changed from 0ordm to 90ordm to 180ordm The question waswhether there would be any change in TOJ performanceas a function of the placement of the hands

MethodSubjects The subjects in all the experiments were undergradu-

ate students at Indiana University Generally the subjects had par-ticipated in several tactile experiments They were selected for partic-ipation in each experiment as scheduling permitted They receivedan hourly wage for their participation Four women and 2 men weretested

Apparatus Two tactile displays generated the stimuli Each dis-play consisted of 144 tactors arranged in a 6-column 3 24-rowarray The tactors were activated with square waves 230 pps Thetwo displays were interfaced with a PC The arrays were similar tothose used in the Optacon a reading aid for the blind (Bliss KatcherRogers amp Shepard 1970) The arrays fit against the distal portionof the fingerpads The patterns covered an area 17 3 11 mm

Procedure The subjectrsquos task was to indicate which of the twosites of stimulation received the stimulus f irst The subjects placedthe index finger of the left hand on one display and the index fin-ger of the right hand on the other display The moving patterns weregenerated by activating pairs of columns sequentially on the dis-play A representation of the patterns is shown in Figure 2 Eachpair of columns was activated for 13 msec The SOA betweencolumns was 13 msec The pair of columns occupied five differentlocations on the display for a total duration of 65 msec and covered

a distance of 11 mm The voltage to each tactor was set so as to pro-duce a moderate stimulus intensity The resulting sensation was oneof a light mechanical stimulus moving across the fingerpad

The subjects initiated each trial with a foot pedal Following thepresentation of the pair of stimuli the subjects responded by meansof two foot pedals to indicate which of the two stimuli they judgedto have been presented first The foot pedals were placed side byside The subjects typically used the left foot to depress the leftpedal and the right foot to depress the right pedal No trial-by-trialfeedback was provided The subjects were instructed that the di-rection of motion was irrelevant to the TOJ

The patterns presented to the two fingers moved in the same di-rection on both fingers The direction of motion and the temporalorder were determined randomly on a trial-by-trial basis The di-rection of motion was uncorrelated with the temporal order Threepositions of the arrays (and the positions of the hands and fingers)were tested in Experiment 1 (Figure 1) The distance between thetwo arrays for these three positions was 10 cm A single positionwas tested each session The directions of movement of the patternsacross the f ingerpads were the same for the three positions Theorder of testing of the positions was random with the constraint thateach position was tested once before being tested again

Six SOAs were tested 400 200 100 52 26 and 13 msec Eachblock of 50 trials tested a different SOA In each experimental ses-sion the six SOAs were tested in order from longest to shortest Webegan with the 400-msec SOA At this SOA there was no bias andthe subjects were generally correct This provided a clear demon-stration of the nature of the task for the subjects and showed themthat the direction of the pattern movement was irrelevant to the TOJThe subjects were generally able to complete two sets of six SOAseach (a total of 600 trials) in a single session To eliminate auditorycues from the two displays the subjects wore earplugs and ear-phones through which white noise was presented For each of thethree conditions (0ordm 90ordm and 180ordm) each of the 6 subjects com-pleted four sets of the six SOAs a total of 7200 trials at each of thethree positions

Figure 1 A representation of the position of the index fingersin Experiment 1 0ordm 90ordm and 180ordm The arrows indicate the di-rection of motion of the patterns across the fingerpads for onetype of trial In a second type of trial the motion at both finger-pads was in the opposite direction

782 CRAIG

Results and DiscussionThe results for the three positions of the arrays are

shown in Figure 3 The percentage of correct responsesis plotted as a function of SOA The top panel shows theresults when the two displays were at 0ordm Three functionsare presented The top function marked ldquoleading to-wardrdquo refers to trials on which the pattern at the tempo-rally leading site was moving toward the site at which thetrailing pattern was presented As was noted in the pre-vious study (Craig amp Busey 2003) this condition wasreferred to as consistent movement The lower functionldquoleading awayrdquo refers to trials on which the temporallyleading stimulus was moving away from the site of thetrailing stimulus previously referred to as inconsistentmovement The middle function is the mean of the upperand lower functions approximately equal to the overalllevel of correct performance These results were similarto the results obtained in the previous study There wasa bias to respond that the pattern that was moving towardthe second location was the temporally leading patternWhen the pattern moving toward the second locationwas in fact the leading pattern the subjects tended to becorrect When the pattern moving away from the secondlocation was the leading pattern the subjects tended tobe incorrect (lower function) An analysis of variance(ANOVA) showed a significant effect of trial type [lead-ing toward vs leading away F(15) = 2437 p lt 01]

The middle panel presents the results from the 90ordmcondition Although the direction of motion at the twosites was no longer directly toward each other there wasstill an effect of the direction of pattern motion and a biasfavoring the leading-toward trials [F(15) = 1378 p lt05] In the lower panel the 180ordm condition the patternswere now moving parallel to each other in an environ-mental frame rather than toward one another howeverthe same labels for the functions were retainedmdashthat isldquoleading towardrdquo referred to the same frame of reference

as in the other panels (local pattern of motion or soma-totopic) The same labels for the local directions of mo-tion were retained to show more clearly the effect of thepositions of the hand and specifically that pattern mo-tion no longer affected TOJs [F(15) = 088 p = 39]These results indicate that the TOJ bias depended as wasexpected on the spatial position of the sites of stimula-tion As the position changed so did the TOJ bias

To see more clearly the effect that hand position andthe direction of motion (environmental frame) has onTOJs we calculated the difference in performance ateach SOA between the leading-toward and the leading-away trial types These differences are plotted in Figure 4and show that the magnitude of the TOJ bias declinedwith the angle between the two displays An ANOVAshowed a significant effect of position of the arrays[F(210) = 1017 p lt 01] An ANOVA performed onjust the 0ordm and 90ordm positions showed no significant dif-ference in the size of the TOJ bias between these two po-sitions [F(15) = 398 p = 10] however a separateanalysis of the three briefest SOAs where the TOJ biasis most evident did show a significant effect of position[F(15) = 757 p lt 05] This result suggests that the ef-fect of altering the angle between the arrays did not re-sult in an all-or-none effect but rather that the TOJ biaswas a continuous function of the relative direction ofmotion across the two hands

EXPERIMENT 2

In Experiment 2 subjects made TOJs of moving pat-terns presented to two f ingers on the same hand Theissue was again whether the subjects would correctlyperceive the direction of motion (environmental frame)as reflected in TOJs when the positions of the sensorysurfaces were altered spatially In Experiment 2 the spa-tial positions were altered by crossing the fingers The

Figure 2 A representation of the tactile display and moving patterns Each panel represents one frame of thetactile pattern moving from left to right

TACTILE TEMPORAL ORDER JUDGMENTS 783

reason for being particularly interested in the effect ofcrossing fingers comes from a series of experiments byBenedetti (1985 1986a 1986b 1988 1991) and is re-lated to Aristotlersquos illusion In Aristotlersquos illusion twofingers on the same hand are crossed such as the middleand the index fingers and a rod is placed between the twofingertips The observer typically perceives two rodsBenedetti examined and quantified some additional per-ceptual effects of crossing the fingers In Benedettirsquos(1988) finding most relevant for the present experimentsubjects crossed finger D3 over D4 The subjects weretouched simultaneously on the tips of the two fingers andwere asked to localize the two points of stimulation rel-ative to one another When the third finger (right hand)was placed above the fourth finger the subjects accu-rately perceived the stimulus presented to the third fin-ger as being directly above the stimulus presented to thefourth finger The subjects then moved their fingers fromthis position and crossed D3 over D4 so that D3 was nowto the right of D4 The unexpected finding was that thesubjects were almost completely unable to take thecrossed position of the fingers into account in localizingthe points The subjects continued to localize D4 directlyabove D3 not in its new position to the right of D3 Inother words the subjects did not take into account thefact that the fingers were now crossed (Benedetti 1988)Benedetti pointed this out as an example of a failure ofperceptual constancy

The question in Experiment 2 was whether subjects withcrossed fingers would perceive the direction of motionas though their fingers remained uncrossed Figure 5shows representations of the patterns with fingers un-crossed and crossed Suppose that the two patterns weremoving from left to right and the middle finger (un-crossed condition) received the stimulus first (panel A)Previous results indicate that in this leading-toward con-dition subjects are very likely to correctly select themiddle finger as leading the index finger Suppose thatthe identical patterns same directions of motion aregenerated on the displays but now the fingers are crossed(Figure 5 panel B) The index finger would now be re-ceiving the pattern f irst If subjects correctly took ac-count of the fact that the patterns were being processedby crossed fingers they should tend to respond that theindex finger was the leading site This result would beevidence for perceptual constancy If however asBenedettirsquos (1988) results predict subjects fail to per-ceive that the middle finger is now to the right of theindex finger the TOJ bias could disappear or subjectscould respond that the middle finger trails the index fin-ger (somatotopic frame) Such results would show a lackof perceptual constancy

MethodSubjects Six women and 2 men were testedProcedure The procedure was similar to that used in Experi-

ment 1 The subjectsrsquo task was to respond with the location that re-

Figure 3 Percent correct in judging temporal order as a func-tion of the stimulus onset asynchrony (SOA) in milliseconds Thethree panels present the data from the three positions of the handsin Experiment 1 0ordm 90ordm and 180ordm In the results from the 0ordm con-dition the top function represents those trials on which the lead-ing pattern was moving toward the other site of stimulation Thebottom function represents those trials on which the leading pat-tern was moving away from the other site of stimulation Theldquomeanrdquo function represents overall percent correct the mean ofthe upper and lower functions The 90ordm results are labeled in thesame way as the 0ordm results as are the 180ordm results For the 180ordm re-sults the same local patterns of motion (somatotopic frame) onthe fingerpads are labeled as they are in the other results al-though the patterns are now moving parallel to one another (en-vironmental frame) Error bars represent plusmn1 standard error ofthe mean

784 CRAIG

ceived the stimulus first The index and middle fingers of the lefthand were the sites of stimulation A single display was used Inorder for the two fingers to contact the array the array was rotated90ordm In this configuration what had been rows of the display be-came columns and vice versa The stimuli consisted of a patternthree tactors in width and six tactors in height The stimuli movedlaterally across the fingerpads with each element of the pattern oc-cupying a position in the proximaldistal orientation for 13 msecThere were 13 msec between onsets of successive positions acrossthe f ingerpad The patterns occupied eight positions along thearray a distance of 88 mm The total duration for each pattern was104 msec In the testing sessions the blocks of trials alternated be-tween crossed and uncrossed conditions In the first block in eachsession the fingers were uncrossed Six SOAsmdash400 200 100 5226 and 13 msecmdashwere tested in each session Blocks consisted of25 trials Eight subjects were tested for four blocks for each of thesix SOAs a total of 4800 trials for the uncrossed condition and4800 trials for the crossed condition

Results and DiscussionThe results with the uncrossed fingers are shown in

Figure 6 The functions are labeled as in Figure 3 Thedata from the two directions of motion left and right arepresented in separate panels to make it easier to comparethe results from the uncrossed condition with the resultsfrom the crossed condition The top panel shows the re-sults when the two patterns were moving to the right Aswas expected when the fingers were uncrossed and whenthe pattern at the leading site was moving in the directionof the second site the subjects were very likely to re-spond correctly (leading-toward trials) When the patternat the leading site was moving in a direction away fromthe second site the subjects were at brief SOAs veryunlikely to respond correctly (leading-away trials) Theseresults are similar to those seen in previous measure-

ments For both the moving-right and the moving-leftconditions there was a significant effect of trial type[F(17) = 10382 p lt 001 and F(17) = 9238 p lt 001respectively]

The results with the crossed fingers are shown in Fig-ure 7 To help visualize the effects of motion the resultsfrom the two directions of motion are presented sepa-rately as they were in Figure 6 The top panel shows theresults when the two patterns were moving to the rightIn discussing these results the terms left and right referto positions of the patterns on the array (environmentalframe) not to the fingers (either crossed or uncrossed)The top function marked ldquoleading towardrdquo refers tothose trials on which the pattern on the left side of thearray was presented first These results show that whenboth patterns were moving from left to right there was astrong bias to respond that the site on the left (in thiscase the index finger) was leading When the site on theleft received the stimulus first the subjects were correct(top function) In this crossed-finger condition the leftside of the array was contacted by the left index finger(Figure 5) Thus the leading pattern was moving towardthe site of the trailing pattern (leading toward)mdashthat isindex toward middle finger Moreover the results werethe same whether the middle finger (uncrossed condi-tion) or the index finger (crossed condition) was con-tacting the left side of the array When the site on the leftwas trailing the subjects were incorrect (bottom func-tion)

The pattern of results in Figure 7 (top panel) is pre-dicted by perceptual constancy If the fact that the fin-gers are crossed had not been taken into account and thesubjects had responded as though their fingers were un-

Figure 4 The difference in percent correct between the leading-toward andleading-away functions in Figure 3mdashthat is the size of the TOJ bias plotted asa function of the angle between the positions of the fingers The parameter is thestimulus onset asynchrony (SOA) in milliseconds between the two patterns

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

TACTILE TEMPORAL ORDER JUDGMENTS 781

work) or relative to the position of the hands in space(environmentally based framework) In haptic explo-ration or in manipulating an object complex patterns ofmotion across the fingerpads are generated but attentionis rarely focused on these patterns per se Rather the ob-server is trying to judge the shape of an object its posi-tion in space or whether some parts can be moved rela-tive to others Observers are generally more concernedwith the environmental rather than the somatotopicframe of reference Third the question has been raised instudies of haptic and tactile spatial judgments of whetherthe judgments are verbally mediated (Rieser amp Pick1976) Inasmuch as subjects are making no judgments ofmotion the effects of pattern motion on TOJs are un-likely to be mediated verbally

EXPERIMENT 1

In Experiment 1 the effect on the TOJ bias of alteringthe position of the hands in space and the directions ofmotion in an environmental frame were examined Fig-ure 1 shows the position of the left and right index fingersat various positions Arrows indicate the direction of mo-tion on the fingers The previous study had examined onlythe position marked ldquo0ordmrdquo At 90ordm the motion was no longermoving directly from one display to the other If the mo-tions at the two displays were extended toward each othertheir trajectories would intersect at a 90ordm angle midwaybetween the two displays and about 5 cm below a line seg-ment connecting the two displays At 180ordm the motions atthe two sites were parallel to each other If the TOJ biasdepends on the direction of motion in an environmentalframework and if perception of the direction of motionchanges with hand position the size of the TOJ bias mightdecrease perhaps to zero as the angle between the twohands changed from 0ordm to 90ordm to 180ordm The question waswhether there would be any change in TOJ performanceas a function of the placement of the hands

MethodSubjects The subjects in all the experiments were undergradu-

ate students at Indiana University Generally the subjects had par-ticipated in several tactile experiments They were selected for partic-ipation in each experiment as scheduling permitted They receivedan hourly wage for their participation Four women and 2 men weretested

Apparatus Two tactile displays generated the stimuli Each dis-play consisted of 144 tactors arranged in a 6-column 3 24-rowarray The tactors were activated with square waves 230 pps Thetwo displays were interfaced with a PC The arrays were similar tothose used in the Optacon a reading aid for the blind (Bliss KatcherRogers amp Shepard 1970) The arrays fit against the distal portionof the fingerpads The patterns covered an area 17 3 11 mm

Procedure The subjectrsquos task was to indicate which of the twosites of stimulation received the stimulus f irst The subjects placedthe index finger of the left hand on one display and the index fin-ger of the right hand on the other display The moving patterns weregenerated by activating pairs of columns sequentially on the dis-play A representation of the patterns is shown in Figure 2 Eachpair of columns was activated for 13 msec The SOA betweencolumns was 13 msec The pair of columns occupied five differentlocations on the display for a total duration of 65 msec and covered

a distance of 11 mm The voltage to each tactor was set so as to pro-duce a moderate stimulus intensity The resulting sensation was oneof a light mechanical stimulus moving across the fingerpad

The subjects initiated each trial with a foot pedal Following thepresentation of the pair of stimuli the subjects responded by meansof two foot pedals to indicate which of the two stimuli they judgedto have been presented first The foot pedals were placed side byside The subjects typically used the left foot to depress the leftpedal and the right foot to depress the right pedal No trial-by-trialfeedback was provided The subjects were instructed that the di-rection of motion was irrelevant to the TOJ

The patterns presented to the two fingers moved in the same di-rection on both fingers The direction of motion and the temporalorder were determined randomly on a trial-by-trial basis The di-rection of motion was uncorrelated with the temporal order Threepositions of the arrays (and the positions of the hands and fingers)were tested in Experiment 1 (Figure 1) The distance between thetwo arrays for these three positions was 10 cm A single positionwas tested each session The directions of movement of the patternsacross the f ingerpads were the same for the three positions Theorder of testing of the positions was random with the constraint thateach position was tested once before being tested again

Six SOAs were tested 400 200 100 52 26 and 13 msec Eachblock of 50 trials tested a different SOA In each experimental ses-sion the six SOAs were tested in order from longest to shortest Webegan with the 400-msec SOA At this SOA there was no bias andthe subjects were generally correct This provided a clear demon-stration of the nature of the task for the subjects and showed themthat the direction of the pattern movement was irrelevant to the TOJThe subjects were generally able to complete two sets of six SOAseach (a total of 600 trials) in a single session To eliminate auditorycues from the two displays the subjects wore earplugs and ear-phones through which white noise was presented For each of thethree conditions (0ordm 90ordm and 180ordm) each of the 6 subjects com-pleted four sets of the six SOAs a total of 7200 trials at each of thethree positions

Figure 1 A representation of the position of the index fingersin Experiment 1 0ordm 90ordm and 180ordm The arrows indicate the di-rection of motion of the patterns across the fingerpads for onetype of trial In a second type of trial the motion at both finger-pads was in the opposite direction

782 CRAIG

Results and DiscussionThe results for the three positions of the arrays are

shown in Figure 3 The percentage of correct responsesis plotted as a function of SOA The top panel shows theresults when the two displays were at 0ordm Three functionsare presented The top function marked ldquoleading to-wardrdquo refers to trials on which the pattern at the tempo-rally leading site was moving toward the site at which thetrailing pattern was presented As was noted in the pre-vious study (Craig amp Busey 2003) this condition wasreferred to as consistent movement The lower functionldquoleading awayrdquo refers to trials on which the temporallyleading stimulus was moving away from the site of thetrailing stimulus previously referred to as inconsistentmovement The middle function is the mean of the upperand lower functions approximately equal to the overalllevel of correct performance These results were similarto the results obtained in the previous study There wasa bias to respond that the pattern that was moving towardthe second location was the temporally leading patternWhen the pattern moving toward the second locationwas in fact the leading pattern the subjects tended to becorrect When the pattern moving away from the secondlocation was the leading pattern the subjects tended tobe incorrect (lower function) An analysis of variance(ANOVA) showed a significant effect of trial type [lead-ing toward vs leading away F(15) = 2437 p lt 01]

The middle panel presents the results from the 90ordmcondition Although the direction of motion at the twosites was no longer directly toward each other there wasstill an effect of the direction of pattern motion and a biasfavoring the leading-toward trials [F(15) = 1378 p lt05] In the lower panel the 180ordm condition the patternswere now moving parallel to each other in an environ-mental frame rather than toward one another howeverthe same labels for the functions were retainedmdashthat isldquoleading towardrdquo referred to the same frame of reference

as in the other panels (local pattern of motion or soma-totopic) The same labels for the local directions of mo-tion were retained to show more clearly the effect of thepositions of the hand and specifically that pattern mo-tion no longer affected TOJs [F(15) = 088 p = 39]These results indicate that the TOJ bias depended as wasexpected on the spatial position of the sites of stimula-tion As the position changed so did the TOJ bias

To see more clearly the effect that hand position andthe direction of motion (environmental frame) has onTOJs we calculated the difference in performance ateach SOA between the leading-toward and the leading-away trial types These differences are plotted in Figure 4and show that the magnitude of the TOJ bias declinedwith the angle between the two displays An ANOVAshowed a significant effect of position of the arrays[F(210) = 1017 p lt 01] An ANOVA performed onjust the 0ordm and 90ordm positions showed no significant dif-ference in the size of the TOJ bias between these two po-sitions [F(15) = 398 p = 10] however a separateanalysis of the three briefest SOAs where the TOJ biasis most evident did show a significant effect of position[F(15) = 757 p lt 05] This result suggests that the ef-fect of altering the angle between the arrays did not re-sult in an all-or-none effect but rather that the TOJ biaswas a continuous function of the relative direction ofmotion across the two hands

EXPERIMENT 2

In Experiment 2 subjects made TOJs of moving pat-terns presented to two f ingers on the same hand Theissue was again whether the subjects would correctlyperceive the direction of motion (environmental frame)as reflected in TOJs when the positions of the sensorysurfaces were altered spatially In Experiment 2 the spa-tial positions were altered by crossing the fingers The

Figure 2 A representation of the tactile display and moving patterns Each panel represents one frame of thetactile pattern moving from left to right

TACTILE TEMPORAL ORDER JUDGMENTS 783

reason for being particularly interested in the effect ofcrossing fingers comes from a series of experiments byBenedetti (1985 1986a 1986b 1988 1991) and is re-lated to Aristotlersquos illusion In Aristotlersquos illusion twofingers on the same hand are crossed such as the middleand the index fingers and a rod is placed between the twofingertips The observer typically perceives two rodsBenedetti examined and quantified some additional per-ceptual effects of crossing the fingers In Benedettirsquos(1988) finding most relevant for the present experimentsubjects crossed finger D3 over D4 The subjects weretouched simultaneously on the tips of the two fingers andwere asked to localize the two points of stimulation rel-ative to one another When the third finger (right hand)was placed above the fourth finger the subjects accu-rately perceived the stimulus presented to the third fin-ger as being directly above the stimulus presented to thefourth finger The subjects then moved their fingers fromthis position and crossed D3 over D4 so that D3 was nowto the right of D4 The unexpected finding was that thesubjects were almost completely unable to take thecrossed position of the fingers into account in localizingthe points The subjects continued to localize D4 directlyabove D3 not in its new position to the right of D3 Inother words the subjects did not take into account thefact that the fingers were now crossed (Benedetti 1988)Benedetti pointed this out as an example of a failure ofperceptual constancy

The question in Experiment 2 was whether subjects withcrossed fingers would perceive the direction of motionas though their fingers remained uncrossed Figure 5shows representations of the patterns with fingers un-crossed and crossed Suppose that the two patterns weremoving from left to right and the middle finger (un-crossed condition) received the stimulus first (panel A)Previous results indicate that in this leading-toward con-dition subjects are very likely to correctly select themiddle finger as leading the index finger Suppose thatthe identical patterns same directions of motion aregenerated on the displays but now the fingers are crossed(Figure 5 panel B) The index finger would now be re-ceiving the pattern f irst If subjects correctly took ac-count of the fact that the patterns were being processedby crossed fingers they should tend to respond that theindex finger was the leading site This result would beevidence for perceptual constancy If however asBenedettirsquos (1988) results predict subjects fail to per-ceive that the middle finger is now to the right of theindex finger the TOJ bias could disappear or subjectscould respond that the middle finger trails the index fin-ger (somatotopic frame) Such results would show a lackof perceptual constancy

MethodSubjects Six women and 2 men were testedProcedure The procedure was similar to that used in Experi-

ment 1 The subjectsrsquo task was to respond with the location that re-

Figure 3 Percent correct in judging temporal order as a func-tion of the stimulus onset asynchrony (SOA) in milliseconds Thethree panels present the data from the three positions of the handsin Experiment 1 0ordm 90ordm and 180ordm In the results from the 0ordm con-dition the top function represents those trials on which the lead-ing pattern was moving toward the other site of stimulation Thebottom function represents those trials on which the leading pat-tern was moving away from the other site of stimulation Theldquomeanrdquo function represents overall percent correct the mean ofthe upper and lower functions The 90ordm results are labeled in thesame way as the 0ordm results as are the 180ordm results For the 180ordm re-sults the same local patterns of motion (somatotopic frame) onthe fingerpads are labeled as they are in the other results al-though the patterns are now moving parallel to one another (en-vironmental frame) Error bars represent plusmn1 standard error ofthe mean

784 CRAIG

ceived the stimulus first The index and middle fingers of the lefthand were the sites of stimulation A single display was used Inorder for the two fingers to contact the array the array was rotated90ordm In this configuration what had been rows of the display be-came columns and vice versa The stimuli consisted of a patternthree tactors in width and six tactors in height The stimuli movedlaterally across the fingerpads with each element of the pattern oc-cupying a position in the proximaldistal orientation for 13 msecThere were 13 msec between onsets of successive positions acrossthe f ingerpad The patterns occupied eight positions along thearray a distance of 88 mm The total duration for each pattern was104 msec In the testing sessions the blocks of trials alternated be-tween crossed and uncrossed conditions In the first block in eachsession the fingers were uncrossed Six SOAsmdash400 200 100 5226 and 13 msecmdashwere tested in each session Blocks consisted of25 trials Eight subjects were tested for four blocks for each of thesix SOAs a total of 4800 trials for the uncrossed condition and4800 trials for the crossed condition

Results and DiscussionThe results with the uncrossed fingers are shown in

Figure 6 The functions are labeled as in Figure 3 Thedata from the two directions of motion left and right arepresented in separate panels to make it easier to comparethe results from the uncrossed condition with the resultsfrom the crossed condition The top panel shows the re-sults when the two patterns were moving to the right Aswas expected when the fingers were uncrossed and whenthe pattern at the leading site was moving in the directionof the second site the subjects were very likely to re-spond correctly (leading-toward trials) When the patternat the leading site was moving in a direction away fromthe second site the subjects were at brief SOAs veryunlikely to respond correctly (leading-away trials) Theseresults are similar to those seen in previous measure-

ments For both the moving-right and the moving-leftconditions there was a significant effect of trial type[F(17) = 10382 p lt 001 and F(17) = 9238 p lt 001respectively]

The results with the crossed fingers are shown in Fig-ure 7 To help visualize the effects of motion the resultsfrom the two directions of motion are presented sepa-rately as they were in Figure 6 The top panel shows theresults when the two patterns were moving to the rightIn discussing these results the terms left and right referto positions of the patterns on the array (environmentalframe) not to the fingers (either crossed or uncrossed)The top function marked ldquoleading towardrdquo refers tothose trials on which the pattern on the left side of thearray was presented first These results show that whenboth patterns were moving from left to right there was astrong bias to respond that the site on the left (in thiscase the index finger) was leading When the site on theleft received the stimulus first the subjects were correct(top function) In this crossed-finger condition the leftside of the array was contacted by the left index finger(Figure 5) Thus the leading pattern was moving towardthe site of the trailing pattern (leading toward)mdashthat isindex toward middle finger Moreover the results werethe same whether the middle finger (uncrossed condi-tion) or the index finger (crossed condition) was con-tacting the left side of the array When the site on the leftwas trailing the subjects were incorrect (bottom func-tion)

The pattern of results in Figure 7 (top panel) is pre-dicted by perceptual constancy If the fact that the fin-gers are crossed had not been taken into account and thesubjects had responded as though their fingers were un-

Figure 4 The difference in percent correct between the leading-toward andleading-away functions in Figure 3mdashthat is the size of the TOJ bias plotted asa function of the angle between the positions of the fingers The parameter is thestimulus onset asynchrony (SOA) in milliseconds between the two patterns

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

782 CRAIG

Results and DiscussionThe results for the three positions of the arrays are

shown in Figure 3 The percentage of correct responsesis plotted as a function of SOA The top panel shows theresults when the two displays were at 0ordm Three functionsare presented The top function marked ldquoleading to-wardrdquo refers to trials on which the pattern at the tempo-rally leading site was moving toward the site at which thetrailing pattern was presented As was noted in the pre-vious study (Craig amp Busey 2003) this condition wasreferred to as consistent movement The lower functionldquoleading awayrdquo refers to trials on which the temporallyleading stimulus was moving away from the site of thetrailing stimulus previously referred to as inconsistentmovement The middle function is the mean of the upperand lower functions approximately equal to the overalllevel of correct performance These results were similarto the results obtained in the previous study There wasa bias to respond that the pattern that was moving towardthe second location was the temporally leading patternWhen the pattern moving toward the second locationwas in fact the leading pattern the subjects tended to becorrect When the pattern moving away from the secondlocation was the leading pattern the subjects tended tobe incorrect (lower function) An analysis of variance(ANOVA) showed a significant effect of trial type [lead-ing toward vs leading away F(15) = 2437 p lt 01]

The middle panel presents the results from the 90ordmcondition Although the direction of motion at the twosites was no longer directly toward each other there wasstill an effect of the direction of pattern motion and a biasfavoring the leading-toward trials [F(15) = 1378 p lt05] In the lower panel the 180ordm condition the patternswere now moving parallel to each other in an environ-mental frame rather than toward one another howeverthe same labels for the functions were retainedmdashthat isldquoleading towardrdquo referred to the same frame of reference

as in the other panels (local pattern of motion or soma-totopic) The same labels for the local directions of mo-tion were retained to show more clearly the effect of thepositions of the hand and specifically that pattern mo-tion no longer affected TOJs [F(15) = 088 p = 39]These results indicate that the TOJ bias depended as wasexpected on the spatial position of the sites of stimula-tion As the position changed so did the TOJ bias

To see more clearly the effect that hand position andthe direction of motion (environmental frame) has onTOJs we calculated the difference in performance ateach SOA between the leading-toward and the leading-away trial types These differences are plotted in Figure 4and show that the magnitude of the TOJ bias declinedwith the angle between the two displays An ANOVAshowed a significant effect of position of the arrays[F(210) = 1017 p lt 01] An ANOVA performed onjust the 0ordm and 90ordm positions showed no significant dif-ference in the size of the TOJ bias between these two po-sitions [F(15) = 398 p = 10] however a separateanalysis of the three briefest SOAs where the TOJ biasis most evident did show a significant effect of position[F(15) = 757 p lt 05] This result suggests that the ef-fect of altering the angle between the arrays did not re-sult in an all-or-none effect but rather that the TOJ biaswas a continuous function of the relative direction ofmotion across the two hands

EXPERIMENT 2

In Experiment 2 subjects made TOJs of moving pat-terns presented to two f ingers on the same hand Theissue was again whether the subjects would correctlyperceive the direction of motion (environmental frame)as reflected in TOJs when the positions of the sensorysurfaces were altered spatially In Experiment 2 the spa-tial positions were altered by crossing the fingers The

Figure 2 A representation of the tactile display and moving patterns Each panel represents one frame of thetactile pattern moving from left to right

TACTILE TEMPORAL ORDER JUDGMENTS 783

reason for being particularly interested in the effect ofcrossing fingers comes from a series of experiments byBenedetti (1985 1986a 1986b 1988 1991) and is re-lated to Aristotlersquos illusion In Aristotlersquos illusion twofingers on the same hand are crossed such as the middleand the index fingers and a rod is placed between the twofingertips The observer typically perceives two rodsBenedetti examined and quantified some additional per-ceptual effects of crossing the fingers In Benedettirsquos(1988) finding most relevant for the present experimentsubjects crossed finger D3 over D4 The subjects weretouched simultaneously on the tips of the two fingers andwere asked to localize the two points of stimulation rel-ative to one another When the third finger (right hand)was placed above the fourth finger the subjects accu-rately perceived the stimulus presented to the third fin-ger as being directly above the stimulus presented to thefourth finger The subjects then moved their fingers fromthis position and crossed D3 over D4 so that D3 was nowto the right of D4 The unexpected finding was that thesubjects were almost completely unable to take thecrossed position of the fingers into account in localizingthe points The subjects continued to localize D4 directlyabove D3 not in its new position to the right of D3 Inother words the subjects did not take into account thefact that the fingers were now crossed (Benedetti 1988)Benedetti pointed this out as an example of a failure ofperceptual constancy

The question in Experiment 2 was whether subjects withcrossed fingers would perceive the direction of motionas though their fingers remained uncrossed Figure 5shows representations of the patterns with fingers un-crossed and crossed Suppose that the two patterns weremoving from left to right and the middle finger (un-crossed condition) received the stimulus first (panel A)Previous results indicate that in this leading-toward con-dition subjects are very likely to correctly select themiddle finger as leading the index finger Suppose thatthe identical patterns same directions of motion aregenerated on the displays but now the fingers are crossed(Figure 5 panel B) The index finger would now be re-ceiving the pattern f irst If subjects correctly took ac-count of the fact that the patterns were being processedby crossed fingers they should tend to respond that theindex finger was the leading site This result would beevidence for perceptual constancy If however asBenedettirsquos (1988) results predict subjects fail to per-ceive that the middle finger is now to the right of theindex finger the TOJ bias could disappear or subjectscould respond that the middle finger trails the index fin-ger (somatotopic frame) Such results would show a lackof perceptual constancy

MethodSubjects Six women and 2 men were testedProcedure The procedure was similar to that used in Experi-

ment 1 The subjectsrsquo task was to respond with the location that re-

Figure 3 Percent correct in judging temporal order as a func-tion of the stimulus onset asynchrony (SOA) in milliseconds Thethree panels present the data from the three positions of the handsin Experiment 1 0ordm 90ordm and 180ordm In the results from the 0ordm con-dition the top function represents those trials on which the lead-ing pattern was moving toward the other site of stimulation Thebottom function represents those trials on which the leading pat-tern was moving away from the other site of stimulation Theldquomeanrdquo function represents overall percent correct the mean ofthe upper and lower functions The 90ordm results are labeled in thesame way as the 0ordm results as are the 180ordm results For the 180ordm re-sults the same local patterns of motion (somatotopic frame) onthe fingerpads are labeled as they are in the other results al-though the patterns are now moving parallel to one another (en-vironmental frame) Error bars represent plusmn1 standard error ofthe mean

784 CRAIG

ceived the stimulus first The index and middle fingers of the lefthand were the sites of stimulation A single display was used Inorder for the two fingers to contact the array the array was rotated90ordm In this configuration what had been rows of the display be-came columns and vice versa The stimuli consisted of a patternthree tactors in width and six tactors in height The stimuli movedlaterally across the fingerpads with each element of the pattern oc-cupying a position in the proximaldistal orientation for 13 msecThere were 13 msec between onsets of successive positions acrossthe f ingerpad The patterns occupied eight positions along thearray a distance of 88 mm The total duration for each pattern was104 msec In the testing sessions the blocks of trials alternated be-tween crossed and uncrossed conditions In the first block in eachsession the fingers were uncrossed Six SOAsmdash400 200 100 5226 and 13 msecmdashwere tested in each session Blocks consisted of25 trials Eight subjects were tested for four blocks for each of thesix SOAs a total of 4800 trials for the uncrossed condition and4800 trials for the crossed condition

Results and DiscussionThe results with the uncrossed fingers are shown in

Figure 6 The functions are labeled as in Figure 3 Thedata from the two directions of motion left and right arepresented in separate panels to make it easier to comparethe results from the uncrossed condition with the resultsfrom the crossed condition The top panel shows the re-sults when the two patterns were moving to the right Aswas expected when the fingers were uncrossed and whenthe pattern at the leading site was moving in the directionof the second site the subjects were very likely to re-spond correctly (leading-toward trials) When the patternat the leading site was moving in a direction away fromthe second site the subjects were at brief SOAs veryunlikely to respond correctly (leading-away trials) Theseresults are similar to those seen in previous measure-

ments For both the moving-right and the moving-leftconditions there was a significant effect of trial type[F(17) = 10382 p lt 001 and F(17) = 9238 p lt 001respectively]

The results with the crossed fingers are shown in Fig-ure 7 To help visualize the effects of motion the resultsfrom the two directions of motion are presented sepa-rately as they were in Figure 6 The top panel shows theresults when the two patterns were moving to the rightIn discussing these results the terms left and right referto positions of the patterns on the array (environmentalframe) not to the fingers (either crossed or uncrossed)The top function marked ldquoleading towardrdquo refers tothose trials on which the pattern on the left side of thearray was presented first These results show that whenboth patterns were moving from left to right there was astrong bias to respond that the site on the left (in thiscase the index finger) was leading When the site on theleft received the stimulus first the subjects were correct(top function) In this crossed-finger condition the leftside of the array was contacted by the left index finger(Figure 5) Thus the leading pattern was moving towardthe site of the trailing pattern (leading toward)mdashthat isindex toward middle finger Moreover the results werethe same whether the middle finger (uncrossed condi-tion) or the index finger (crossed condition) was con-tacting the left side of the array When the site on the leftwas trailing the subjects were incorrect (bottom func-tion)

The pattern of results in Figure 7 (top panel) is pre-dicted by perceptual constancy If the fact that the fin-gers are crossed had not been taken into account and thesubjects had responded as though their fingers were un-

Figure 4 The difference in percent correct between the leading-toward andleading-away functions in Figure 3mdashthat is the size of the TOJ bias plotted asa function of the angle between the positions of the fingers The parameter is thestimulus onset asynchrony (SOA) in milliseconds between the two patterns

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

TACTILE TEMPORAL ORDER JUDGMENTS 783

reason for being particularly interested in the effect ofcrossing fingers comes from a series of experiments byBenedetti (1985 1986a 1986b 1988 1991) and is re-lated to Aristotlersquos illusion In Aristotlersquos illusion twofingers on the same hand are crossed such as the middleand the index fingers and a rod is placed between the twofingertips The observer typically perceives two rodsBenedetti examined and quantified some additional per-ceptual effects of crossing the fingers In Benedettirsquos(1988) finding most relevant for the present experimentsubjects crossed finger D3 over D4 The subjects weretouched simultaneously on the tips of the two fingers andwere asked to localize the two points of stimulation rel-ative to one another When the third finger (right hand)was placed above the fourth finger the subjects accu-rately perceived the stimulus presented to the third fin-ger as being directly above the stimulus presented to thefourth finger The subjects then moved their fingers fromthis position and crossed D3 over D4 so that D3 was nowto the right of D4 The unexpected finding was that thesubjects were almost completely unable to take thecrossed position of the fingers into account in localizingthe points The subjects continued to localize D4 directlyabove D3 not in its new position to the right of D3 Inother words the subjects did not take into account thefact that the fingers were now crossed (Benedetti 1988)Benedetti pointed this out as an example of a failure ofperceptual constancy

The question in Experiment 2 was whether subjects withcrossed fingers would perceive the direction of motionas though their fingers remained uncrossed Figure 5shows representations of the patterns with fingers un-crossed and crossed Suppose that the two patterns weremoving from left to right and the middle finger (un-crossed condition) received the stimulus first (panel A)Previous results indicate that in this leading-toward con-dition subjects are very likely to correctly select themiddle finger as leading the index finger Suppose thatthe identical patterns same directions of motion aregenerated on the displays but now the fingers are crossed(Figure 5 panel B) The index finger would now be re-ceiving the pattern f irst If subjects correctly took ac-count of the fact that the patterns were being processedby crossed fingers they should tend to respond that theindex finger was the leading site This result would beevidence for perceptual constancy If however asBenedettirsquos (1988) results predict subjects fail to per-ceive that the middle finger is now to the right of theindex finger the TOJ bias could disappear or subjectscould respond that the middle finger trails the index fin-ger (somatotopic frame) Such results would show a lackof perceptual constancy

MethodSubjects Six women and 2 men were testedProcedure The procedure was similar to that used in Experi-

ment 1 The subjectsrsquo task was to respond with the location that re-

Figure 3 Percent correct in judging temporal order as a func-tion of the stimulus onset asynchrony (SOA) in milliseconds Thethree panels present the data from the three positions of the handsin Experiment 1 0ordm 90ordm and 180ordm In the results from the 0ordm con-dition the top function represents those trials on which the lead-ing pattern was moving toward the other site of stimulation Thebottom function represents those trials on which the leading pat-tern was moving away from the other site of stimulation Theldquomeanrdquo function represents overall percent correct the mean ofthe upper and lower functions The 90ordm results are labeled in thesame way as the 0ordm results as are the 180ordm results For the 180ordm re-sults the same local patterns of motion (somatotopic frame) onthe fingerpads are labeled as they are in the other results al-though the patterns are now moving parallel to one another (en-vironmental frame) Error bars represent plusmn1 standard error ofthe mean

784 CRAIG

ceived the stimulus first The index and middle fingers of the lefthand were the sites of stimulation A single display was used Inorder for the two fingers to contact the array the array was rotated90ordm In this configuration what had been rows of the display be-came columns and vice versa The stimuli consisted of a patternthree tactors in width and six tactors in height The stimuli movedlaterally across the fingerpads with each element of the pattern oc-cupying a position in the proximaldistal orientation for 13 msecThere were 13 msec between onsets of successive positions acrossthe f ingerpad The patterns occupied eight positions along thearray a distance of 88 mm The total duration for each pattern was104 msec In the testing sessions the blocks of trials alternated be-tween crossed and uncrossed conditions In the first block in eachsession the fingers were uncrossed Six SOAsmdash400 200 100 5226 and 13 msecmdashwere tested in each session Blocks consisted of25 trials Eight subjects were tested for four blocks for each of thesix SOAs a total of 4800 trials for the uncrossed condition and4800 trials for the crossed condition

Results and DiscussionThe results with the uncrossed fingers are shown in

Figure 6 The functions are labeled as in Figure 3 Thedata from the two directions of motion left and right arepresented in separate panels to make it easier to comparethe results from the uncrossed condition with the resultsfrom the crossed condition The top panel shows the re-sults when the two patterns were moving to the right Aswas expected when the fingers were uncrossed and whenthe pattern at the leading site was moving in the directionof the second site the subjects were very likely to re-spond correctly (leading-toward trials) When the patternat the leading site was moving in a direction away fromthe second site the subjects were at brief SOAs veryunlikely to respond correctly (leading-away trials) Theseresults are similar to those seen in previous measure-

ments For both the moving-right and the moving-leftconditions there was a significant effect of trial type[F(17) = 10382 p lt 001 and F(17) = 9238 p lt 001respectively]

The results with the crossed fingers are shown in Fig-ure 7 To help visualize the effects of motion the resultsfrom the two directions of motion are presented sepa-rately as they were in Figure 6 The top panel shows theresults when the two patterns were moving to the rightIn discussing these results the terms left and right referto positions of the patterns on the array (environmentalframe) not to the fingers (either crossed or uncrossed)The top function marked ldquoleading towardrdquo refers tothose trials on which the pattern on the left side of thearray was presented first These results show that whenboth patterns were moving from left to right there was astrong bias to respond that the site on the left (in thiscase the index finger) was leading When the site on theleft received the stimulus first the subjects were correct(top function) In this crossed-finger condition the leftside of the array was contacted by the left index finger(Figure 5) Thus the leading pattern was moving towardthe site of the trailing pattern (leading toward)mdashthat isindex toward middle finger Moreover the results werethe same whether the middle finger (uncrossed condi-tion) or the index finger (crossed condition) was con-tacting the left side of the array When the site on the leftwas trailing the subjects were incorrect (bottom func-tion)

The pattern of results in Figure 7 (top panel) is pre-dicted by perceptual constancy If the fact that the fin-gers are crossed had not been taken into account and thesubjects had responded as though their fingers were un-

Figure 4 The difference in percent correct between the leading-toward andleading-away functions in Figure 3mdashthat is the size of the TOJ bias plotted asa function of the angle between the positions of the fingers The parameter is thestimulus onset asynchrony (SOA) in milliseconds between the two patterns

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

784 CRAIG

ceived the stimulus first The index and middle fingers of the lefthand were the sites of stimulation A single display was used Inorder for the two fingers to contact the array the array was rotated90ordm In this configuration what had been rows of the display be-came columns and vice versa The stimuli consisted of a patternthree tactors in width and six tactors in height The stimuli movedlaterally across the fingerpads with each element of the pattern oc-cupying a position in the proximaldistal orientation for 13 msecThere were 13 msec between onsets of successive positions acrossthe f ingerpad The patterns occupied eight positions along thearray a distance of 88 mm The total duration for each pattern was104 msec In the testing sessions the blocks of trials alternated be-tween crossed and uncrossed conditions In the first block in eachsession the fingers were uncrossed Six SOAsmdash400 200 100 5226 and 13 msecmdashwere tested in each session Blocks consisted of25 trials Eight subjects were tested for four blocks for each of thesix SOAs a total of 4800 trials for the uncrossed condition and4800 trials for the crossed condition

Results and DiscussionThe results with the uncrossed fingers are shown in

Figure 6 The functions are labeled as in Figure 3 Thedata from the two directions of motion left and right arepresented in separate panels to make it easier to comparethe results from the uncrossed condition with the resultsfrom the crossed condition The top panel shows the re-sults when the two patterns were moving to the right Aswas expected when the fingers were uncrossed and whenthe pattern at the leading site was moving in the directionof the second site the subjects were very likely to re-spond correctly (leading-toward trials) When the patternat the leading site was moving in a direction away fromthe second site the subjects were at brief SOAs veryunlikely to respond correctly (leading-away trials) Theseresults are similar to those seen in previous measure-

ments For both the moving-right and the moving-leftconditions there was a significant effect of trial type[F(17) = 10382 p lt 001 and F(17) = 9238 p lt 001respectively]

The results with the crossed fingers are shown in Fig-ure 7 To help visualize the effects of motion the resultsfrom the two directions of motion are presented sepa-rately as they were in Figure 6 The top panel shows theresults when the two patterns were moving to the rightIn discussing these results the terms left and right referto positions of the patterns on the array (environmentalframe) not to the fingers (either crossed or uncrossed)The top function marked ldquoleading towardrdquo refers tothose trials on which the pattern on the left side of thearray was presented first These results show that whenboth patterns were moving from left to right there was astrong bias to respond that the site on the left (in thiscase the index finger) was leading When the site on theleft received the stimulus first the subjects were correct(top function) In this crossed-finger condition the leftside of the array was contacted by the left index finger(Figure 5) Thus the leading pattern was moving towardthe site of the trailing pattern (leading toward)mdashthat isindex toward middle finger Moreover the results werethe same whether the middle finger (uncrossed condi-tion) or the index finger (crossed condition) was con-tacting the left side of the array When the site on the leftwas trailing the subjects were incorrect (bottom func-tion)

The pattern of results in Figure 7 (top panel) is pre-dicted by perceptual constancy If the fact that the fin-gers are crossed had not been taken into account and thesubjects had responded as though their fingers were un-

Figure 4 The difference in percent correct between the leading-toward andleading-away functions in Figure 3mdashthat is the size of the TOJ bias plotted asa function of the angle between the positions of the fingers The parameter is thestimulus onset asynchrony (SOA) in milliseconds between the two patterns

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

TACTILE TEMPORAL ORDER JUDGMENTS 785

crossed the bias would have been in the opposite direc-tion The subjects would have responded that the indexfinger trailed the middle finger because with the samepatterns presented to uncrossed fingers the direction ofmovement on the index finger would be moving awayfrom the middle finger An ANOVA showed a signifi-cant effect of trial type [F(17) = 2021 p lt 01]

The bottom panel of Figure 7 shows the results withcrossed fingers when the patterns were moving to theleft These results mirror those in the top panel and showthe same bias Patterns moving toward the other locationwere selected as the leading pattern In this conditionwith both patterns moving from right to left there was astrong bias to respond as though the pattern on the rightwas the leading pattern An ANOVA showed a signifi-cant effect of trial type [F(17) = 5938 p lt 01] As withthe patterns moving to the right (Figure 7 top panel) thesubjectsrsquo performance indicates that when moving pat-terns are processed crossing the fingers does not lead to

a misperception of the direction of motion as referencedexternally

Overall sensitivity in the crossed condition appears tobe somewhat poorer than that in the uncrossed conditionAn ANOVA showed that there was a significant differ-ence in overall performance between the crossed and theuncrossed conditions [F(17) = 1321 p lt 01] This dif-ference is also reflected in the thresholds 75 correctwhich were calculated by linear interpolation In the un-crossed condition the threshold was 128 msec and inthe crossed condition it was 175 msec Sensitivity in theuncrossed condition also appears to be rather poor how-ever the threshold 128 msec is close to the thresholdobtained in the previous study under similar conditionsThat threshold was 101 msec (Craig amp Busey 2003)

EXPERIMENT 3

Several recent studies compared TOJs for stimuli pre-sented to crossed and uncrossed hands (Shore et al2002 Yamamoto amp Kitazawa 2001a 2001b) Subjectsjudged whether a tactile stimulus had been delivered to

Figure 5 A representation of the direction of motion (to theright) and temporal order (1st and 2nd) for patterns presented tofingers uncrossed and crossed Panels A and B are leading-toward trial types as defined by an environmental frame PanelsC and D are leading-away trial types as defined by an environ-mental frame As defined by the local pattern of motion (somato-topic frame) panel B is a leading-away trial type and panel D isa leading-toward trial type

Figure 6 Performance in judging temporal order Patternswere presented to two fingers uncrossed The functions are la-beled as in Figure 3 The top panel presents the results when thetwo patterns were moving to the right The bottom panel presentsthe results when the two patterns were moving to the left Errorbars represent 6 1 standard error of the mean

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

786 CRAIG

the right hand before the left hand or vice versa Cross-ing the hands resulted in large increases in the thresholdfor temporal order as compared with uncrossed condi-tions In one study with uncrossed hands subjects couldcorrectly report the temporal order on more than 80 ofthe trials at temporal intervals of approximately 70 msec(Yamamoto amp Kitazawa 2001a) With the hands crossedsimilar levels of performance were not achieved until theintervals were greater than 300 msec In Shore et alrsquosstudy the thresholds in the crossed condition were insome conditions more than 3 and one half times largerthan the thresholds in the uncrossed condition 34 msecas compared with 124 msec In addition Shore et alshowed that the crossed-hand deficit was evident evenwith experienced subjects In these studies the tactilesignals were brief mechanical stimulimdashthat is staticrather than moving stimuli

In Experiment 3 TOJs were obtained with movingstimuli with hands crossed and uncrossed The first aimof Experiment 3 was to see whether crossed hands wouldperturb TOJs when the stimuli were moving patterns As

compared with static patterns might moving patternsovercome the effects of crossing the hands and producesensitivity similar to that seen in Experiment 1 The sec-ond aim was to see whether subjects use a somatotopicframe or an environmental frame when the hands arecrossed One of the conclusions from Yamamoto and Ki-tazawarsquos (2001a) study was that the relatively poor sen-sitivity they observed was the result of subjectsrsquo havingto adjust for the crossed-hand position The authors sug-gest that subjects assume that their hands are uncrossedand that to make the TOJ remapping is necessary andtime consuming If subjects are assuming that theirhands are uncrossed when making TOJs this should beevident in the effect of the direction of movement ontheir judgments Such an effect would be similar toBenedettirsquos (1988) results with crossed fingers and con-trary to the results of Experiment 2 In short the bias ob-served with crossed fingers should be reversed

MethodSubjects Five women and 3 men were testedProcedure Preliminary testing indicated that crossing the hands

did indeed result in poorer performance as predicted by earlierstudies Thus it was necessary to test longer SOAs than in the pre-vious experiments Specif ically SOAs of 1000 500 200 100 52and 26 msec were tested in 50-trial blocks The same patterns asthose used in Experiment 1 were used in Experiment 3 As in Ex-periments 1 and 2 the subjects were tested in order from the longestto the shortest SOAs within a testing session The subjects weretested with hands crossed right hand crossed over the left hand andwith arms uncrossed Sessions alternated between crossed and uncrossed hands The subjects were tested for eight sessions fourcrossed and four uncrossed The patterns were presented to theindex fingers of the two hands

Results and DiscussionThe results are presented in Figure 8 and are labeled

as in Figure 3 The top panel shows the results with handsuncrossed and the bottom panel shows the results withhands crossed In both conditions the direction of mo-tion affected performance There was a significant effectof trial type in both the uncrossed [F(17) = 1336 p lt01] and the crossed conditions [F(17) = 787 p lt 05]The uncrossed-hands condition was in effect a replica-tion of the 0ordm condition in Experiment 1 and the resultswere similar

The focus of Experiment 3 was on the crossed condi-tion Consistent with the terminology used in Experi-ment 2 the descriptions of the movement of the patternsmdashmoving toward and moving awaymdashrefer to the motion inan environmental framework When the patterns weremoving from left to right the subjects were likely to re-spond that the site of stimulation on the left received thestimulus first If the leading site was on the left the sub-jects tended to be correct If the leading site was on theright the subjects tended to be incorrect The fact thatthe hands were crossed did not affect the direction of thebias These results do not support the conclusion fromYamamoto and Kitazawarsquos (2001a) study that the so-matosensory system has a default option that assumes

Figure 7 Performance in judging temporal order Patterns arepresented to two fingers crossed The top panel presents the re-sults when the two patterns are moving to the right The bottompanel presents the results when the two patterns are moving tothe left The functions are labeled using an environmental framerather than a somatotopic frame (Figure 5) Error bars representplusmn1 standard error of the mean

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

TACTILE TEMPORAL ORDER JUDGMENTS 787

that the hands are uncrossed As with the crossed fin-gers the subjectsrsquo responses indicate that they wereaware of the effect of crossing their hands and were usingan environmental frame

Crossing the hands had a clear effect on sensitivityThere was a significant difference between the meanfunction with uncrossed hands and the mean functionwith crossed hands [F(17) = 2013 p lt 005] From themean performance data in Figure 8 thresholds (75correct points) were calculated With uncrossed handsthe threshold was 65 msec whereas with crossed handsit was 183 msec These results are consistent with the re-sults from both Yamamoto and Kitazawa (2001a) andShore et al (2002) with crossed hands As Shore et alreported even with experienced subjects crossing thehands results in substantial interference in TOJs

GENERAL DISCUSSION

The size of the TOJ bias as measured by the differ-ence between leading-toward and leading-away trialsappears to vary across Experiments 1 2 and 3 Thelargest bias was seen in Experiment 2 In Experiment 2

the subjects made TOJs of ipsilateral stimuli whereas inExperiments 1 and 3 the judgments were made of bilat-eral stimuli In the previous study bilateral conditionsdid not result in substantially smaller biases than didipsilateral conditions Training and experience in judg-ing temporal order did appear to reduce the bias (Craigamp Busey 2003) In the present study the subjects testedin the bilateral conditions had substantially more expe-rience in making TOJs than did the subjects in the ipsi-lateral condition which might account for the smallerbias seen in the bilateral conditions A direct comparisonbetween ipsilateral and bilateral conditions controllingfor the subjectsrsquo experience would be necessary to de-termine whether the size of the TOJ bias is affected

In all three experiments the subjects showed substan-tial TOJ biases even under conditions in which they wereunlikely to have engaged in extensive haptic explorationThe fact that experience in haptic exploration has littleeffect on the TOJ bias is most clearly seen in Experi-ment 2 Subjects have considerable experience in ma-nipulating objects or exploring surfaces in such a waythat tactile features stimulate adjacent fingers on thesame hand as they move across the skinrsquos surface In thistype of exploration the temporal order and the directionof motion are perfectly correlated Subjects have virtu-ally no experience in exploring surfaces with their indexfingers crossed over their middle fingers yet the size ofthe TOJ bias is almost the same in both conditions (Fig-ures 6 and 7) These results and the results with crossedhands offer no support for the explanation that haptic ex-ploration and the close coupling of movement and tem-poral order is necessary for the TOJ bias

The conditions under which temporal order is studiedare similar to the conditions that produce apparent mo-tion stimuli presented to two sites with a brief SOATactile apparent motion can be generated not only be-tween two sites on the same side of the body (ipsilater-ally) but also between bilateral sites (Sherrick 1968a1968b 1970) It is possible that the TOJ bias effect mightbe the result of the interaction between the local patternof motion across the f ingerpads and apparent motiongenerated between the two sites When the local patternof motion at one site points directly at the second site theapparent motion between the two sites is aligned with thelocal motion Under those conditions one might expectmaximum interaction between the two types of motionAltering the alignment between the two types of motionshould result in the local patternrsquos having a smaller effecton apparent motion When the two directions are or-thogonal to each other the local patterns should not af-fect apparent motion

Arguing against a major role for apparent motion isthe range of SOAs over which the bias effect is seenThere is a substantial bias at SOAs ranging from 13 to100 msec Apparent motion depends on SOA Apparentmotion would likely be poor with the pattern durationsused in the present experiments at 13-msec SOAs andnonexistent at 200-msec SOAs yet the TOJ bias is stillevident

Figure 8 Performance in judging temporal order Patternswere presented to two hands The top panel presents the resultswhen the hands were uncrossed The bottom panel presents theresults when the hands were crossed The functions are labeledusing an environmental frame rather than a somatotopic frameError bars represent plusmn1 standard error of the mean

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)

788 CRAIG

The present results suggest that neither haptic explo-ration experience nor apparent motion are major factorsin the TOJ bias but that the direction of motion exter-nally defined is an important factor The results are con-sistent with the view that moving patterns produce a shiftin attention With visual stimuli it has been shown thatsubjects perceive a moving stimulus ahead of its actuallocation in space (Balda amp Klein 1995 PurushothamanPatel Bedell amp Ogmen 1998) This anticipation of thetrajectory of moving stimuli may reflect a spatial shift inattention If a similar effect were generated by movingtactile patterns then for a leading-toward stimulus at-tention would be moving toward the trailing stimulus Injudging temporal order subjects report that their atten-tion is drawn from one location to another (Craig ampBusey 2003) The present results suggest that the extentto which attention shifts from one location to anotherwithin an environmental frame may affect the size of theTOJ bias

The results of these experiments are consistent withthe view that subjects use an environmental frameworkin processing direction of motion This environmentalframework likely results from combining tactile informa-tion with information about the position of the hands andfingers in space a conclusion similar to that reached byDriver and Grossenbacher (1996) Driver and Grossen-bacher reached their conclusion on the basis of the factthat the amount of interference between the two index fin-gers declined as the hands were moved farther apart Inthe case of the TOJ bias changing the distance betweenhands had no effect on the bias (Craig amp Busey 2003)Thus although the TOJ bias is sensitive to changes inspatial position the effects appear to be limited to ori-entation in space and are not a function of the distancebetween the two sites of stimulation

The information that subjects integrate with the tactilestimulation in developing an environmental frameworkmight be proprioceptive or visual No attempt was madein these experiments to differentiate these two sources ofinformation by for example excluding vision Such amanipulation might be carried out in future studies

REFERENCES

Balda M V amp Klein S A (1995) Extrapolation or attention shiftNature 378 565-566

Benedetti F (1985) Processing of tactile spatial information withcrossed fingers Journal of Experimental Psychology Human Per-ception amp Performance 11 517-525

Benedetti F (1986a) Spatial organization of the diplesthetic andnondiplesthetic areas of the fingers Perception 15 285-301

Benedetti F (1986b) Tactile diplopia (diplesthesia) on the humanfingers Perception 15 83-91

Benedetti F (1988) Localization of tactile stimuli and body parts inspace Two dissociated perceptual experiences revealed by a lack ofconstancy in the presence of position sense and motor activity Jour-nal of Experimental Psychology Human Perception amp Performance14 69-76

Benedetti F (1991) Reorganization of tactile perception followingthe simulated amputation of one finger Perception 20 687-692

Bliss J C Katcher M H Rogers C H amp Shepard R P (1970)Optical-to-tactile image conversion for the blind IEEE Transactionson ManndashMachine Systems MMS-11 58-64

Craig J C amp Busey T A (2003) The effect of motion on tactile andvisual temporal order judgments Perception amp Psychophysics 6581-94

Driver J amp Grossenbacher P G (1996) Multimodal spatial con-straints on tactile selective attention In T Innui amp J L McClelland(Eds) Attention and performance XVI Information integration inperception and communication (pp 209-235) Cambridge MA MITPress

Oldfield S R amp Phillips J R (1983) The spatial characteristics oftactile form perception Perception 12 615-626

Parsons L M amp Shimojo S (1987) Perceived spatial organizationof cutaneous patterns on surfaces of the human body in various po-sitions Journal of Experimental Psychology Human Perception ampPerformance 13 488-504

Purushothaman G Patel S S Bedell H E amp Ogmen H(1998) Moving ahead through differential visual latency Nature396 424

Rieser J J amp Pick H L Jr (1976) Reference systems and the per-ception of tactual and haptic orientation Perception amp Psycho-physics 19 117-121

Rinker M A amp Craig J C (1994) The effect of spatial orientationon the perception of moving tactile stimuli Perception amp Psycho-physics 56 356-362

Sekiyama K (1991) Importance of head axes in perception of cuta-neous patterns drawn on vertical body surfaces Perception ampPsychophysics 49 481-492

Sherrick C E (1968a) Bilateral apparent haptic movement Percep-tion amp Psychophysics 4 159-160

Sherrick C E (1968b) Studies of apparent tactual movement InD R Kenshalo (Ed) The skin senses (pp 331-344) Springfield ILThomas

Sherrick C E (1970) Temporal ordering of events in haptic spaceIEEE Transactions on ManndashMachine Systems MMS-11 25-28

Shore D I Spry E amp Spence C (2002) Confusing the mind bycrossing the hands Cognitive Brain Research 14 153-163

Yamamoto S amp Kitazawa S (2001a) Reversal of subjective tem-poral order due to arm crossing Nature Neuroscience 4 759-765

Yamamoto S amp Kitazawa S (2001b) Sensations at the tips of in-visible tools Nature Neuroscience 4 979-980

(Manuscript received August 9 2002revision accepted for publication December 31 2002)