Confusing the mind by crossing the hands

Cognitive Brain Research 14 (2002) 153–163www.elsevier.com/ locate /bres

Research report

Confusing the mind by crossing the handsa , b b*David I. Shore , Emily Spry , Charles Spence

aDepartment of Psychology, McMaster University, Hamilton, Ontario, Canada L8S 4K1bDepartment of Experimental Psychology, Oxford University, Oxford, UK

Accepted 15 August 2001

Abstract

Observers made temporal order judgements (TOJs) regarding which of two tactile stimuli presented to either hand (at stimulus onsetasynchronies of up to 200 ms) occurred first. When the observers’ hands were placed in an uncrossed posture (i.e., each hand in its ownhemispace), performance was accurate, with a just noticeable difference (JND; the smallest interval which produces 75% correctperformance) of 34 ms. By contrast, when the hands were crossed over the midline, performance declined such that 124 ms was neededfor accurate performance. In a second experiment, we presented visual instead of tactile stimuli to evaluate the relative contribution ofmotor and perceptual confusions to the effect. While performance with crossed hands was significantly worse than with uncrossed hands(JND536 vs. 31 ms, respectively), this difference was negligible compared to that with tactile stimuli. In a third experiment, experiencedobservers showed a robust crossed-hands deficit which was not improved by using different fingers on either hand. We argue that thedecline in tactile discrimination performance when the hands are crossed reflects a failure to represent appropriately near simultaneousbimanual tactile stimuli, and stands in marked contrast to many recent observations of efficient remapping of singly-presented tactilestimuli. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Temporal order judgement; Tactile stimuli; Just noticeable difference; Motor and perceptual confusion; Tactile discrimination performance

11. Confusing the mind by crossing the hands responses, respectively). To our knowledge, no one hasattempted to investigate this apparent crossed hands deficit

Investigations of human temporal perception have oc- in the intervening years; although researchers have pro-cupied psychological research for more than two centuries vided evidence that other perceptual deficits may occur(e.g., Refs. [38,53,61,62]). The present research builds on when the limbs are crossed. We first review this literaturean anecdotal finding to emerge from Drew’s [14] studies of on perceptual deficits, before describing a series of experi-temporal perception and the phenomenon of prior entry ments designed both to replicate and extend Drew’s[67] reported at the turn of 19th century. Drew used a seminal finding.temporal order judgement (TOJ) task in which a pair ofelectrocutaneous tactile stimuli were presented to an 1.1. Perceptual deficits associated with crossing theobserver (one to either hand) at a single fixed temporal handsasynchrony. The observers’ accuracy in determining thetemporal ordering of the two stimuli was found to be James [27] in his Principles of Psychology highlighted areduced when the hands were placed in a crossed as tactile illusion of spatial distortion with crossed limbs firstcompared to an uncrossed posture (56 vs. 75% correct

1Drew [14] collected data from only two observers (one in the crossed-hands posture) across various days with various attentional parametersand thus there were a number of examples for the uncrossed posture butonly one example with the crossed posture. The authors have tried to

*Corresponding author. Tel.: 11-905-525-9140, ext. 23013; fax: 11- examine the bulk of the data objectively to arrive at the figures of 56 and905-529-6225. 74% accuracy—we used Drew’s Table 18 for the crossed hands data and

E-mail address: [email protected] (D.I. Shore). Tables 4, 9, and 14 for the uncrossed data.

0926-6410/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0926-6410( 02 )00070-8

154 D.I. Shore et al. / Cognitive Brain Research 14 (2002) 153 –163

described by Aristotle, in his Metaphysica IV, 6 and De impaired in detecting two near simultaneous stimuli whenSomniis 2 [37]. In this ‘Aristotle illusion’, observers cross their hands were crossed.two adjacent fingers one over the other (such as D3 over One potential problem with interpreting Aglioti et al.’sD2) and then touch the crossed fingertips to an object [1] study is related to the fact that uniform stimulus(such as one’s nose). Under such conditions, people presentation cannot be assured if tactile stimuli are pre-frequently experience the sensation of touching two objects sented manually (i.e., by finger taps). It is well-known that(i.e., that they have two noses [48,55,65]). Moreover, if presenting two near-threshold stimuli at even slightlydifferent objects are presented to each fingertip, then their different intensities frequently leads either to the maskingrelative location will be incorrectly perceived (e.g., Ref. of the weaker stimulus [7,22,52,68], or else to the fusing of[6]). This illusion of double sensation (or ‘tactile diplopia’) the two stimuli into a single percept located in-between theis thought to reflect a failure by observers to consider their two stimulated locations (e.g., Refs. [14,70]). Consequent-fingers as crossed (i.e., they apparently fail to update their ly, if the experimenter inadvertently delivered one of thespatial representation of the tactile stimuli to account for stimuli with less force, or for a shorter duration, in thethe new posture). crossed hands compared to uncrossed hands posture, then

A second example of the performance decrement that the observed results might have been obtained.can occur when the hands are crossed was reported by While there are no recent reports on the effect ofHenri [24]. Observers cross their arms over and then clasp crossing the hands on TOJs, two papers have used atheir hands together with the thumbs pointing downward. simultaneous /successive judgement task to look at theNext, the hands are rotated toward the body until the effects of posture change on tactile temporal perceptionthumbs point upward. Once this posture has been adopted, [5,19]. In both studies, an adaptive staircase procedure wasobservers are required to move a single finger identified by used to determine the minimum interval needed to reliablythe experimenter (either by verbal command or by point- judge the two stimuli as being sequential rather thaning). The majority of people fail to move the correct digit, simultaneous. Geffen et al. [19] reported no significanttypically moving the mirror-symmetrical digit on their effect of crossing the hands on simultaneity thresholds (57other hand instead [11,32,69]. The crucial role of vision in ms for crossed versus 58 ms for uncrossed). Axelrod et al.producing this illusion (known as the Japanese illusion) is [5] provided no statistical comparison of these conditions,shown by the fact that if the finger to be moved is touched, but based on the graphical representation of their results,response errors rarely occur [11]. the difference of 2 ms is unlikely to have been significant

Consider that observers see the indicated finger on one (46 ms for crossed versus 44 ms for uncrossed). Thus,side of the body midline (e.g., left), and that if the hands there appears to be no deficit in judgements of simultaneitywere clasped normally this would correspond to the finger associated with crossing the hands. The contrast betweenof the opposing hand (i.e., right). Moving the visualized these null results and the dramatic performance decrementfinger (i.e., thought to be on the right hand) results in the reported by Drew [14] may suggest that the TOJ andobserved error because the corresponding finger on the simultaneous /successive tasks may be modulated by dif-right hand is actually on the right side of the body (in this ferent factors. However, given that Drew tested only aunusual and contorted body posture). Data consistent with single observer at just one relative interval, a replication ofthis explanation were reported by Van Riper ([69], Table 6) his finding is clearly required before any firm conclusionswho showed that varying the amount of visual information can be drawn about the consequences of crossing the hands(i.e., by using a blindfold, an occluding shield with an on tactile TOJs.image of the fingers or a view of the actual fingers),affected the number of errors.

More recently, Aglioti et al. [1] assessed the effect of 2. Experiment 1adopting a crossed-hands posture on the detection of twonear-simultaneous tactile stimuli. The goal of their study 2.1. Methodwas to assess tactile extinction in three groups of neglectpatients; however, we will focus on the data reported from Nine right-handed observers (five females) were pre-12 control observers. The experimenter in Aglioti et al.’s sented with pairs of suprathreshold vibrotactile stimuli (12

2study lightly tapped the dorsal surface of the observers’ ms duration), one to either the index or second finger ofhand(s) using his /her finger. Observers wore a blindfold each hand, and were required to make unspeeded temporaland adopted either a crossed or uncrossed posture with the order judgements (TOJs) regarding which finger washands placed in either separate hemifields (i.e., across the stimulated first. Observers responded by pressing a re-midline), or else in the same hemifield (left or right).Crossing the hands led to a significant reduction in thenumber of double stimulations perceived (90%) in the 2Although the pulse was delivered to the stimulator for 12 ms, it takescontrol group when compared to performance in the several ms for the stimulator to start moving, so even at the shortest SOAuncrossed posture (98%). Apparently, observers were (10 ms) the vibrations would not have been presented simultaneously.

D.I. Shore et al. / Cognitive Brain Research 14 (2002) 153 –163 155

sponse key with the finger next to the stimulated finger, blocks, and the order of presentation counterbalancedensuring a compatible stimulus–response mapping. The across observers. These short blocks (less than 5 min each)actual finger stimulated (index versus second finger) was limited any possibility that tiredness (associated with anycounterbalanced across observers with the other finger of increased tonic muscle activation in the crossed posture)each hand being used for responding. Observers were could account for performance differences reported be-required to fixate a central visual fixation point in an tween the two postures. While no explicit eye-monitoringotherwise dark testing chamber, and so could not see their was performed, observers were reminded to maintainarms during the experimental blocks. Additionally, small fixation throughout the trials and there was no reason forfoam cubes were placed between the arms in the crossed- them to look at one or the other side since both hands werehands posture to reduce any contact between them. The stimulated and there was no prior knowledge about whichright arm was always crossed over the left. The spatial was more likely on a particular trial (cf. Ref. [57]).separation between the vibrotactile stimulators (placed 25cm to either side of the midline) was kept constant 2.2. Resultsthroughout the experiment. Two arrow lights (pointing tothe right or left) just beneath fixation provided feedback The results are shown in Fig. 1: Fig. 1A represents theconcerning which response had been made (note that this results of a probit analysis that converts the raw proportiondid not indicate the accuracy of the response). White noise of ‘right hand first’ responses at each SOA to its stan-was played continuously over a centrally positioned loud- dardized z-score equivalent, allowing a best-fitting straightspeaker cone to mask any sounds made by the operation of line to be calculated for each observer [17]. Only thethe tactile stimulators. intermediate 8 points were used for this analysis because

There were 10 possible stimulus onset asynchronies the longest intervals showed evidence of a ceiling effect(SOAs) between the stimuli (2200, 290, 255, 230, 210, for the uncrossed posture (cf. Ref. [62]). The slope of this10, 30, 55, 90, or 200 ms; where negative values indicate line was calculated for each individual and submitted to athat the left hand was stimulated first) presented according within-observer analysis of variance (ANOVA) with handto the method of constant stimuli [12,53]. In order to position (crossed versus uncrossed) as the only factor. Thefacilitate response acquisition, observers completed two effect of posture was highly significant, F(1,8)533.7, P,

blocks of 32 practice trials in which the SOA values were 0.0005.doubled. The practice blocks were followed by six blocks Traditionally, the just noticeable difference (JND; theof 80 experimental trials, with the posture (uncrossed smallest interval needed to reliably indicate temporalversus crossed; see Fig. 1B) alternated between successive order) is reported for psychophysical data such as these.

Fig. 1. Proportion of right hand first responses across stimulus onset asynchrony (SOA) from nine observers are presented in (A) for the crossed anduncrossed hand postures shown in (B). These data are the result of a probit analysis which converts raw proportion response into normally distributedz-scores [17]. The y-axis remains proportional but is not linear. The just noticeable difference (JND) shown in (C) was calculated based on the averageslope across individual observers (see text for details). Error bars in (A) indicate the between observer standard error of the mean. There are no error bars in(C) because the JND could not be calculated for those observers evidencing a negative slope (see text for details).


This value is calculated by subtracting the SOA needed to formance in the crossed posture, it seems unlikely that itachieve 75% performance from that needed to achieve could account completely for our data, given that observers25% performance and dividing by 2 [12]. The resulting responded with the hand which was stimulated first, thusvalue is monotonically related to the slope of the line. Fig. ensuring high stimulus-response compatibility [9]. More-1C shows the mean JNDs for the uncrossed and crossed over, response coding conflicts are thought to affectpostures determined from the mean slope data. Note that performance in speeded responding paradigms more thanthis value could not be determined independently for all in unspeeded perceptual tasks, such as the TOJ task usedobservers because several of them obtained a slightly here (e.g., Refs. [4,9] see also Refs. [63,73]). However,negative slope value for the crossed hands posture. Since even if response-coding conflicts contributed to the per-the JNDs were determined on the group data, it was not formance decrement seen in the present data, this stillpossible to calculate a standard error and so no error bars represents a failure to maintain an accurate representationare presented in Fig. 1C. We used the slope values (which within the tactile /proprioceptive /kinaesthetic system—aare not limited to positive values) in our statistics and failure which may represent a perceptual deficit (i.e., notreport the JND value for the group average. A negative knowing which stimulus was presented first), a motoricslope value indicates that even though observers were deficit (i.e., not knowing which hand to respond with), orinstructed to press the button beside the finger that was some unknown combination of the two.stimulated first, they often pressed the button on the The suggestion that the performance deficit in theopposite hand instead. The three observers who evidenced crossed posture of Experiment 1 reflects a genuine per-such a negative slope showed a normal pattern of re- ceptual confusion, caused by the observers’ failure tosponses in the uncrossed condition. Removal of these accurately represent the two tactile stimuli appropriately, isobservers did not eliminate the highly robust slope differ- consistent with observer’s verbal reports after the experi-ence between postures, F(1,5)516.86, P,0.01, with JNDs ment; they frequently reported that, they simply did notof 31 and 109 ms for the uncrossed and crossed postures, know which finger had been stimulated first in the crossedrespectively. posture. However, to evaluate the relative contribution of

Analysis of the observers’ average response latencies motor and perceptual processes more objectively, we(from the onset of the first vibrotactile stimulus) showed conducted a second experiment in which visual rather thanthat even though the observers were informed that the TOJ tactile stimuli were presented, while observers adoptedtask was entirely unspeeded, they took 247 ms longer on either an uncrossed or crossed posture. If the poor per-average to respond when their hands were crossed (M5 formance seen in Experiment 1 reflects a difficulty in the821 ms) than when their hands were uncrossed (M5574 programming of the appropriate motor response then ams; cf. Refs. [23,53]). similar deficit should be evident in the present experiment.

However, if the deficit in the crossed posture of Experi-2.3. Discussion ment 1 was specifically related to problems at the per-

ceptual level associated with representing two near-These results clearly show that people can accurately simultaneous tactile stimuli instead, then no deficit should

discriminate the temporal order of pairs of tactile stimuli be evident when the experiment is repeated with visualwhen their hands are placed in an uncrossed posture, stimuli.confirming previous findings [13,25,62]. However, ourresults also show that performance was dramaticallyimpaired when observers crossed their hands over the 3. Experiment 2midline (see the 3-fold increase in JNDs reported in thecrossed relative to the uncrossed posture shown in Fig. 3.1. Method1C). This result provides both a replication and extensionof the results first reported by Drew [14] more than a Five experienced psychophysical observers (includingcentury ago. Since the initial preparation of this manuscript the first and third author) volunteered for this study. Theit has been brought to our attention that a similar deficit in only change in apparatus and methods from the previouscrossed hands tactile TOJs has also been reported by study was the addition of two red light emitting diodesYamamoto and Kitazawa [74]. (LEDs) placed at the end of each of the responding fingers.

The observers’ task was to indicate which of the two visual2.3.1. Perceptual versus motor confusion stimuli was presented first by pressing the response key

One obvious explanation for the performance decrement under the appropriate LED. For half of the eight blocks,in the crossed hands posture of the present study relates to their hands were placed in a crossed posture.the possibility of a response coding conflict between theresponding hand’s anatomical label (as a ‘left’ or ‘right’ 3.2. Resultshand) and its spatial position (left versus right) relative tothe other hand (cf. Ref. [42]). While any such response The data from this experiment are presented in Fig. 2.coding conflict would only be expected to affect per- The slope data were subjected to a within-observer


Fig. 2. Proportion of right light first responses across stimulus onset asynchrony (SOA) from five observers are presented in (A) for the crossed anduncrossed hand postures indicated in (B). The just noticeable difference (JND) presented in (C) represents the average value calculated independently foreach observer. Error bars in (A) and (C) represent the standard error of the mean.

ANOVA with the single factor of hand posture (crossed with a TOJ task can reduce the observed JND ([62],versus uncrossed). There was a small but significant effect Experiment 4). Thus, the improved precision observed inof posture, F(1,4)59.0, P,0.05, with all five observers the second Experiment might have been caused by thisshowing larger JNDs in the crossed than in the uncrossed factor rather than the sensory modality being stimulated.posture (M536 versus 31 ms, respectively). To examine this explanation, the same experienced ob-

servers from Experiment 2 repeated the task with vibrotac-3.3. Discussion tile rather than visual stimuli. With a few exceptions noted

below, this Experiment is a replication of Experiment 1The goal of Experiment 2 was to evaluate the contribu- with experienced observers.

tion of motor confusion to the 3-fold increase in JNDs seen We argued in Section 2.3, that the response corre-with tactile stimuli in the previous experiment. The critical spondence between the responding and stimulated fingerdifference between the two experiments was the modality was very high; however, it is possible that this stimulus–stimulated. The small deficit for crossed versus uncrossed response arrangement introduced additional confusionpostures observed in the present visual experiment con- since the stimulated and responding finger could betrasted with the large deficit observed previously with thought of as either to the left or right of each other and

3tactile stimuli. Since the response demands were extremely this relation would be different for the two hands. In ordersimilar (stimulated finger in Experiment 2 versus finger to eliminate any such confusion, participants in Experi-beside the stimulated finger in Experiment 1), a motor ment 3 responded with the stimulated finger—the mi-confusion account of the poorer precision with tactile croswitch was mounted under the tactile vibrator so thatstimuli can be discounted. The deficit observed in Experi- the observer simply had to press down on the vibrator thatment 1 appears to be a difficulty in perceiving the relative was stimulated first.locations of the two tactile stimuli. Before drawing this One additional factor was introduced into the presentconclusion, however, two additional differences between experiment in an attempt to elucidate the potential sourcethe experiments need to be ruled out as potential explana- of the perceptual deficit seen in Experiment 1. Ourtions: the relative experience of the observers and the intuition was that some of the confusion between the twoprecise response demands. tactile stimuli may have stemmed from their similarity.

That is, the same finger (on either hand) was stimulatedwith a vibration of similar amplitude and frequency. There

4. Experiment 3 were no features to help observers distinguish between thetwo stimuli. Based on behavioral [51] and neuroimaging

In the first Experiment, where performance was worse,3the observers were inexperienced, whereas in the second, We thank Jason Ivanoff and Raymond Klein for bringing this relation to

all five observers were experienced. Extensive practice our attention.


[18,20] data we reasoned that if different fingers on either ANOVA with the single factor of finger (same versushand were stimulated, then different cortical regions would different finger). This comparison was not significant,respond (cf. Ref. [47]) and that this may aid observers in F(1,4),1.0. Upon further inspection, the slope of thesedistinguishing between the two stimuli. Thus, between five observers was not significantly different from zeroblocks of trials, the observers used either the same fingers t(4)51.02 for same finger comparison and t(4)51.22 for(index or ring finger) or different fingers. If this reduced the different finger comparison. Even when pooled to-the degree of confusion, then performance should be better gether this data was not significantly different from zero,(lower JNDs) for the different finger condition than for the t(9)51.68. By way of comparison, the similar statistic onsame finger condition. the slope data from the crossed posture in Experiment 1

revealed a significant effect, t(8)52.10, P,0.05, one-4.1. Methods tailed, showing that the slope was indeed different from

zero. Of some interest, the JNDs in the present experimentThe same five experienced psychophysical observers as were much larger than those seen in crossed-posture

participated in the previous experiment volunteered for this condition of Experiment 1. Compare the JND of 124 msstudy. The tactile vibrators were mounted on the response reported in Experiment 1 with the 233 ms JND for themicroswitches and observers indicated which stimulus was same finger condition and 181 ms for the different fingerpresented first by pressing down on that vibrator. For half condition of the present experiment.of the eight blocks, the same finger on either hand wasstimulated (two blocks for the index finger and two blocks 4.3. Discussionfor the ring finger). For the remaining four blocks, differentfingers were stimulated on either hand. For two blocks the The goals of Experiment 3 were to assess the role ofring finger of the right hand and index finger of the left expertise in the difference between the previous twohand were used. The use of same-finger or different-finger Experiments, to examine the effect of using the samealternated across blocks with the relation for the first block finger for perceiving and responding, and to comparebeing counterbalanced across observers. Note that the arms performance when stimulation was to the same digit onwere crossed in all conditions for this experiment. either hand versus to different digits. Clearly, being an

experienced psychophysical observer does not defend4.2. Results against the crossed-hands deficit seen in Experiment 1. In

fact, the JNDs were non-significantly larger in this groupThe data from this experiment are presented in Fig. 3. of observers than those seen previously. This may stem

The slope data were subjected to a within-observer from the fact that in the present experiment, the observersmaintained a crossed-hands posture throughout the 1-htesting period, whereas in the previous experiment theyuncrossed their hands after every block. With regard towhether it was the perceiving finger or the one next to itthat responded, again we saw no evidence that this factoraffected performance arguing that motor confusions werenot responsible for the crossed-hands deficit. Finally, thecomparison of same versus different fingers was suggestiveof a benefit for the different finger condition; however, thistrend was not significant. Future research should examineother factors that could differentiate the two stimulidelivered to either hand.

5. General discussion

The most important result to emerge from the presentexperiments is the dramatic decrement in the precision oftactile temporal order judgements seen when stimuli aredelivered to each hand in the crossed posture. A motoric

Fig. 3. Proportion of right hand first responses across stimulus onset account of this result was discounted in Experiment 2 byasynchrony (SOA) from five experienced psychophysical observers. The showing a negligible increase in JNDs when observershands were crossed for both conditions shown here. The two different

made visual rather than tactile TOJs. The role of expertisefilled symbols refer to whether the same (diamond) or different (square)and confusion regarding which finger to respond with wasfingers (index or ring) on either hand were stimulated. Error bars

represented the standard error of the mean. ruled out in the third experiment. Finally, a suggestive


advantage was observed for stimulating different as op- search from our laboratory [58] has shown that people doposed to the same finger on either hand. The crossed-hands not have any problem in determining the spatial location ofdeficit seen in Experiments 1 and 3 clearly replicate and individual tactile stimuli, even when their hands areextend the seminal work of Drew [14] and provides further crossed in complete darkness (see also Ref. [71]).evidence of the perceptual deficits that may ensue when It is still possible that the present perceptual deficit maythe hands are crossed over the midline (cf. Refs. stem from a dominance of a visual frame of reference as in[6,11,24,65]). the case of the Japanese Illusion discussed earlier

The results of the study by Aglioti et al. [1] may offer [24,32,69]. Even though observers could not see theirone possible explanation for the present data: that is, hands, there was some visual information in the form of aobservers may have perceived only one tactile stimulus fixation light and feedback signals which could haveeven though two were clearly presented (cf. Ref. [14], p. provided a sparse visual framework/ reference point on550). According to this explanation, the single percept which observers could base their TOJs (e.g., Ref. [15]).must have been mislocalized away from the location of the Even without this information, one could argue thatfirst stimulus, otherwise, observers would have reported observers may imagine a visual reference frame (cf. Ref.the location of the only stimulus perceived. This is [35]) as did the blind-folded observers in the experimentsconsistent with Drew’s observation that the single percept of Van Riper [69]. Many studies have now shown thatsometimes reported by the observer in his study tended to visual cues can play a dominant role in localizing tactilegravitate toward a location between the two stimuli. To be and proprioceptive sensations in space [8,21,40,46,65,66].clear, we are proposing that the same mechanism which If observers accurately remapped the location of each limbcauses the deficit in the Aglioti et al. study may be at play and thus knew which side of the body the first stimulushere. While such a mechanism may help to account for the had been presented on and which hand was located there,poor performance seen at the shortest SOAs, this binding / then performance should have been equivalent across theventriloquism effect breaks down rapidly once the SOA two postures. Thus, the marked deficit observed mayreaches approximately 30 ms [52], and hence cannot indicate a relative failure of remapping the tactile stimuliexplain the perceptual confusions that were evident in our into a visual reference frame. Note that this failure couldobservers performance at the longer intervals. only be attributed to the presentation of two near-simulta-

One might wonder whether one stimulus might have neous stimuli as people have relatively little difficultymasked the other stimulus. However, once again, Sherrick when only one stimulus is presented (e.g., Refs. [4,9]).[52] reported that masking effects only occur when stimuli One way to assess the relative role of a visual referenceare presented within 20 ms of each other, whereas the frame would be to vary the degree of visual informationdeficit observed here for TOJs with a crossed-hands (cf. Ref. [69]). One could replicate the experiment reportedposture extends out to beyond 100 ms (see Fig. 1A). While here with the lights on, versus with the eyes closed tono one has tested if the temporal window of masking is manipulate the amount of visual information which waslarger with a crossed-hands posture, it seems unlikely that available, and hence perhaps the extent to which visionit would extend out to the 200-ms SOA used in the present may dominate.study. Consider that the thresholds obtained for the simul-taneity task with crossed hands averaged about 50 ms[5,19] and so at this interval observers must have perceived 5.2. One versus two stimulithe two stimuli as distinct. Although the masking effectcannot account for the present deficit, it would neverthe- It is important to note that our participants’ difficulty inless be interesting for future research to investigate the crossed posture condition contrasts with numerouswhether such masking effects are modulated by crossing previous studies showing that people can correctly de-the hands. termine the veridical location from which unimanual tactile

stimuli are presented when the hands are crossed [4,9,59].One potentially important difference is that two vibrotac-

5.1. Role of visual reference frame tile stimuli were presented in close temporal succession inthe present study, whereas only one stimulus was pre-

In considering the importance of visual information, one sented in the previous studies. It may be that while peoplemight propose that since the experiments were conducted can easily remap the location of a single tactile stimulusin the dark, a lack of vision (i.e., not being able to see when the hands are crossed, they are unable to keep trackone’s arms) may have led to poor performance in the of multiple near-simultaneous stimuli, such as the bimanu-crossed-hands posture. Indeed, numerous studies have now al stimulus pairs used here. Only further research willshown that vision of the hand and/or arm frequently plays reveal whether this problem with processing multiplea dominant role in determining both where the arm is felt tactile stimuli presented to the crossed hands reflects anto be (proprioception), and where tactile sensations are attentional limitation [5], or whether instead the confusionlocalized in space [8,21,59,65,66]. However, recent re- may be related to the fact that although somatosensory


stimuli are initially projected contralaterally, there is also are put into conflict. This apparent conflict may provide theextensive ipsilateral processing of somatosensory stimuli locus of confusion that observers report and which resultspresented to the digits (e.g., Refs. [26,45]). Iwamura et al. in their poor performance. Clearly, when the two referencehave shown in the macaque that neurons in the postcentral frames provide consistent information (i.e., with uncrossedsomatosensory cortex can be driven by stimulation of hands), merging the representations should lead to goodeither hand. It is possible that while the bilateral projection overall performance, as we observed. See also Yamamotoof hand regions to somatosensory cortical areas may, under and Kitazawa [74] for further thoughts on the source of thecertain conditions, facilitate the sensory remapping of perceptual deficit seen here.visuotactile space when the hands cross the midline (i.e., insituations in which only a single tactile target stimulus is 5.4. Contrasting visual and tactile stimulationpresented; cf. Refs. [4,9,56,60]), it may also lead toproblems in the resolution of near-simultaneous pairs of One possible reason for the difference in the size of thetactile stimuli which activate both hemispheres (cf. Refs. perceptual deficit observed when stimulation was visual[7,36]). In this regard, it is important to consider that versus tactile is the relative ease with which space is codedstimulation of the digits of one hand produces bilateral in these two modalities. Space is an indispensable attributeactivation in secondary somatosensory cortex but only in vision, coded on the retina and maintained in a roughlycontralateral activation in primary somatosensory cortex retinotopic arrangement throughout the processing stream.[28,49]. It may be that the difficulties evidenced in the In the tactile modality, the allocentric location of apresent experiment arise within cortical areas receiving stimulus can only be determined after information frombilateral activation from both hands. However, at present proprioception and other senses is integrated. This differ-this remains an interesting question for future neuroimag- ence may be critical in understanding the observed deficiting research. in the tactile modality.

The observation of any deficit in the visual modality,5.3. Frames of reference albeit small (6 ms) is somewhat surprising given the

unspeeded nature of the task. It cannot be determined fromThe manipulation of crossing the hands across the this experiment if this is a motor confusion problem or if

midline (as performed in the present experiment) con- there is indeed some perceptual effect on visual temporalfounds several different possible frames of reference (e.g., order judgements from crossing the hands. As reviewed inegocentric (trunk or head-centered), room-centered, or the Introduction, there are a number of results showing thatvisual). Although these distinctions may turn out to be visual judgements can be affected by crossing the handsimportant in future research, for present purposes, we will (e.g., Refs. [4,9]); however, all of these results wererefer to all of these frames of reference as allocentric (since obtained with speeded responses. If the small deficit in thethey are all changed by crossing the hands). This contrasts present experiment is truly a perceptual effect (more timewith a somatotopic representation which refers specifically needed between two visual stimuli for reliable temporalto the location of stimuli on the body surface, and which is order judgements when the hands are crossed versusunaffected by hand crossing. With respect to what frame of uncrossed) then it may be the first observation of such anreference is the left side defined? Is it in somatotopically effect with an unspeeded task. The cause of this difficultydefined coordinates or some more abstract spatially defined will have to await future research.frame-of-reference, or both [39]? Observers might definethe left with respect to the sensory receptor surface 5.5. Relation between judgements of order and of(somatotopic) and/or with respect to a higher-order spatial simultaneityframe-of-reference. In the uncrossed posture, sensory andhigher-order, spatial frames of references are aligned with A comparison between the present findings and previouseach other, whereas in the crossed hands posture they are results using a similar posture manipulation and a simulta-placed in opposition. neous /successive judgement task [5,19] reinforces the

This line of thinking provides an alternative way to claim made by Hirsh and Sherrick [25] that although theaccount for the present data while still maintaining the two tasks appear to be superficially similar, they mayproposal that the tactile stimuli are remapped into a visual actually be qualitatively different. Hirsh and Sherrickframe of reference. Perhaps the two available frames of argued that TOJs require more information than simul-reference, somatotopic and allocentric, are integrated and taneity judgements, for not only do you have to perceiveboth contribute to performance equally. Consider that that two events occurred rather than one, but also which ofnormally these two coordinate frames would provide the two stimuli occurred first. Additionally, the presentrelatively congruent information regarding the location of findings may indicate that different underlying neuraltactile stimuli in space and so a combination of these mechanisms are tapped by the two tasks (cf. Refs. [2,64]).would serve to enhance performance (cf. Ref. [44]), With simultaneity judgements, no deficit in temporalwhereas in the present, somewhat unusual situation, they processing is seen when the hands are crossed (compared


with the critical condition where they were uncrossed but our results in terms of the temporal ordering of the twoaway from the midline; [5,19]), whereas we show a robust stimuli being unaffected by crossing the hands, but thatdeficit when comparing these conditions (cf. [14]). Argu- observers’ awareness of their correct spatial location isments for the similarity of processing in these two tasks confused, we should acknowledge the logical possibilitycome from the claim that language impaired individuals that observers could have spatially remapped the twoshow poorer performance than controls on both tasks (e.g., stimuli correctly, but simply confused their relative tempo-Ref. [16]) and the claim that either task can be used to ral ordering instead (i.e., participants may have remappedassess relative hemispheric specialization for temporal the stimuli perfectly but have confused them in time).processing (e.g., Refs. [19,41]). However, the present Future research needs to disentangle the relative coding ofresearch prompts the suggestion that future research would space and time within the temporal order judgement taskbe well served by delineating how these two tasks differ and different postures.and to what extent they tap different perceptual and/orneural mechanisms.

6. Conclusions5.6. Informing neural models of tactile perception

The present results highlight a performance limitation onRecent neuroimaging studies have demonstrated sub- people’s ability to process multiple tactile stimuli pre-

stantial ipsilateral representation of the digits in both sented in close temporal proximity when they adopted aprimary and secondary somatosensory cortex crossed hands posture. This deficit represents a genuine[26,33,34,43,45,50,54]. For example, Iwamura et al. [26] perceptual limitation rather than a difficulty with motorhave shown in the macaque that neurons in the postcentral programming, as shown by the fact that crossing the handssomatosensory cortex can be driven by stimulation of had very little effect on the accuracy of visual TOJs.either hand. Importantly, activity from stimulation of Critically, our results provide the first empirical demon-ipsilateral digits relies on cross-cortical connections since stration that people find it difficult to reconstruct theablation of the contralateral primary somatosensory cortex appropriate spatiotemporal ordering of rapidly presented(SI) eliminates this effect (see also Ref. [31], though see bimanual pairs of tactile stimuli, when presented in aalso Ref. [43]). Further evidence for the role of transcallos- crossed posture. This result stands in marked contrast toal fibres comes from the finding that the ipsilateral activity the results of numerous previous studies showing thatis delayed [30,33,43] and somewhat smaller in amplitude single tactile events can be localized appropriately in the[43]. In the present context, it is important to note that the crossed posture, presumably due to the maintenance of anipsilateral response is enhanced by simultaneous stimula- up-to-date representation of visuotactile space [21,56], buttion of the contralateral hand [50]. These findings, com- is consistent with another line of research showing thatbined with the recent neuropsychological data showing that perceptual confusions occur in a number of other crossedsplit-brain patients fail to remap single tactile stimuli hands situations.across the midline [56,60], suggest that remapping maytake place very early in the tactile processing stream, ratherthan later downstream in areas such as the pre-frontal Acknowledgementscortex [21]. No one has yet tested the effect of crossing thehands on the relative activity produced by ipsi- and We would like to thank the Rotman Research Institutecontralateral stimulation. This information may in the and the Killam Trust funds for supporting post-doctoralfuture allow for a more neurally based explanation of the fellowships to DIS during the development of these ideas,present data to be posited and would clearly inform the NSERC of Canada for an operating grant to DIS, and4theories of tactile remapping. the Oxford McDonnell-Pew Centre for Cognitive Neuro-

science for a grant to DIS. We would also like to thank Jim5.7. Space and time in somatosensory information- Enns, Erin Austin, Salvador Soto-Faraco, Jason Ivanoff,processing Ray Klein, Steffan Kennett and two anonymous reviewers

for comments on earlier drafts of the manuscript.It should be noted that there is an intimate relationship

between space and time in tactile information processing.In fact, researchers have argued that there may, to some

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Confusing the mind by crossing the hands