S-R COMPATIBILITY: SPATIAL CHARACTERISTICSSTIMULUS AND RESPONSE CODES 1

PAUL M. FITTS

The Ohio State University

AND CHARLES M. SEEGER

Aero Medical Laboratory

OF

The present paper reports the resultsof two experiments designed to dem-onstrate the utility of the concept ofstimulus-response compatibility2 inthe development of a theory of percep-tual-motor behavior.

A task involves compatible S-Rrelations to the extent that the en-semble of stimulus and response com-binations comprising the task resultsin a high rate of information transfer.Admittedly, degree of compatibilitycan be defined in terms of operationsother than those used to secure ameasure of information, for example,it could be specified in terms of meas-ures of speed or accuracy. However,the present writers prefer the preced-ing definition because of the theoreti-can interpretation that they wish togive to compatibility effects. Thisinterpretation makes use of the idea ofa hypothetical process of informationtransformation or recoding in thecourse of a perceptual-motor activity,and assumes that the degree of com-patibility is at a maximum when re-coding processes are at a minimum.The concept of compatibility can be

1 This research was supported in part by theUnited States Air Force under Contract No. AF33(O38>1OS28 with the Ohio State University-Research Foundation, monitored by the HumanResources Research Center, and was reported atthe 1952 meeting of the Midwestern Psychologi-cal Association. Permission is granted for repro-duction, publication, use and disposal in whole orin part by or for the United States Government.

2 The authors wish to credit Dr. A. M. Smallfor suggesting the use of the term compatibilityin an unpublished paper presented before theErgonomics Research Society in 1951.

extended to cover relations betweenconcurrent stimulus activities, such astake place during simultaneous listen-ing and looking, as well as to relationsbetween concurrent motor responses.However, the present paper will belimited to a consideration of stimulus-response compatibility effects in whichthe relevant information in the stim-ulus source is that generated bychanges in its spatial characteristics,and the relevant aspect of a responseis its direction of movement.

One of the earliest studies of thebehavioral effects of changes in thespatial correspondence of S-R rela-tions is the well-known experiment ofStratton (12) on vision without inver-sion of the retinal image. Recentlythe majority of studies of such effectshave dealt with the spatial relationsbetween machine controls and remotevisual displays that are connected tothem by mechanical or electrical means(1, 3, 5, 9, 13). These relations areimportant for human engineering.They have also become a matter ofconsiderable interest for learning the-ory, in part because of Gagne, Foster,and Baker's (2) proposal that a rever-sal of S-R relations is the only condi-tion leading to negative transfer effectsin perceptual-motor learning. How-ever, studies in both of these areashave consistently dealt with only oneaspect of the compatibility problem.Investigations in the human-engineer-ing area usually have compared -dif-ferent ways of displaying informationwhen the S responds with a single type

199

200 PAUL M. FITTS AND CHARLES M. SEEGER

of control, or else have examined theeffectiveness of several kinds of con-trols or control motions when usedwith a single display. Investigationsof transfer effects have typically con-sidered only reversals of the stimulus-response relations within one S—Rensemble, such as changes in the direc-tion of motion of the control on a par-ticular psychomotor test (8), withoutspecifying the location of the originaltask along a dimension of S-R com-patibility. Only two studies (7, 11)have systematically investigated theeffects of varying both stimulus setsand response sets. Such an approachis indicated by the concept ofcompatibility.

Information theory provides a con-venient formulation of compatibilityin terms of information coding. Shan-non (10) and others have pointed out,in the case of physical communicationchannels, that information can betransmitted at a rate approachingchannel capacity only if messages areoptimally encoded for the particularchannel being used. In such systemsoptimum encoding requires that mes-sages be expressed in a suitable formand that their probability constraintsbe matched to the physical constraintsof the channel. By analogy, it seemsreasonable to hypothesize that man'sperformance of a perceptual-motortask should be most efficient when thetask necessitates a minimum amountof information transformation (encod-ing and/or decoding), in other words,when the information generated bysuccessive stimulus events is appro-priate to the set of responses that mustbe made in the task, or conversely,when the set of responses is appropri-ately matched to the stimulus source.8

A few of the writers who have dis-3 If an incompatible S-R relation exists, then

a considerable amount of recoding of informa-tion (such as the looking for meaningful associ-ations, etc.) may be highly desirable.

cussed the application of informationtheory to psychology have consideredcoding problems, such as the possi-bility of recoding as an aid in remem-bering. However, interest has beenlimited to stimulus coding (6).

Since the coding problem is centralto the topic of compatibility, it will beworth while to consider briefly theavailable ways of choosing the setsof stimuli and the sets of responsesemployed in a visual-motor task. Weshall consider only the case of a finiteseries of discrete code symbols. Astimulus code must utilize one or morestimulus dimensions such as the inten-sity, the wave length, the duration, orthe extent of visible light. A uni-dimensional code is one in which therequired number of unique code sym-bols is provided by the selection ofpoints along a single stimulus dimen-sion. A multidimensional code is onein which points are selected along twoor more stimulus dimensions and eachcode symbol is defined by specifying aunique combination of several stim-ulus characteristics. In either case,several code symbols can be combinedto form a larger multisymbol series.

A response code can be devised bysimilar procedures. A unidimensionalresponse code utilizes a single effectormember and a specified number ofpoints along one of the physical dimen-sions of a particular response con-tinuum, such as the direction, theforce, or the duration of a movement.A multidimensional response code util-izes the complex response of a singleeffector member, such as the motion ofthe arm in positioning of a three-dimensional control, or the responsesof several different effectors actingconcurrently, such as the simultane-ous responses of hands and feet inmoving a set of conventional aircraftcontrols. Both the set of stimuli andthe set of responses required in a par-ticular task can thus be specified as

S-R COMPATIBILITY 201

points lying in the multidimensionalspace comprising the set of all pos-sible discriminable stimuli and motorresponses.

Garner and Hake (4) have pointedout that in order to attain maximumaverage information transmission persymbol, stimulus points should beselected with reference to their posi-tion along a scale of equal discrimina-bility. A similar principle undoubt-edly holds for the selection of a set ofresponses. Discriminability, definedwithout reference to response rate,establishes an upper limit to the maxi-mum average information that can begenerated by the selection of partic-ular stimulus or response points froma set of points lying along a specifieddimension. The problem of S-R com-patibility remains, however, even whensets of stimuli and responses are op-timally chosen with respect to dis-criminability. I t concerns the ques-tion of how the correspondence of thesets of points in the stimulus and theresponse symbol spaces affects infor-mation transfer.

EXPERIMENT I

The first experiment was plannedto test the hypothesis that, in tasksrequiring directional motor responsesto spatial stimulus patterns, effectiveperformance depends to a large extentupon the unique characteristics ofS-R ensembles rather than on specificaspects of particular stimulus or re-sponse sets. Three sets of stimuli(So, S(,, Sc), each comprising eighteasily discriminable light patterns, andthree sets of eight responses (Ra, Rb,Rc) were studied in the nine combina-tions formed by combining each stim-ulus set with each response set.Within each of these nine S-R en-sembles, the pairings of stimuli andresponses were those that the Es

judged would be expected by most Ss,i.e., would agree with population stere-otypes. These sets of stimuli andresponses were chosen as abstractionsof commonly used spatial patterns ofstimuli and responses, i.e., as familiarways of representing points on a two-dimensional surface.

A further consideration in selectingthe sets of stimuli and responses wasthat three of the nine S-R ensembles(Sa-Ra, Sb-Rt,, and Sc-Rc) should rep-resent "corresponding" permutationsand combinations. Correspondence,in this case, was judged by the Es onthe basis of the direct physical simi-larity of the two patterns. The experi-ment therefore provides an incidentaltest of how accurately S-R compatibil-ity can be predicted from a considera-tion of the correspondence, withrespect to the number of codingdimensions employed, between thetwo sets of points employed in stim-ulus and response coding.

Method

Apparatus.—The response required of Ss wasto move a stylus quickly in the direction indi-cated by a stimulus light or lights. The threestimulus and the three response panels used inthe study are shown in Fig. 1 and a drawing ofthe S's position, with the Sa-Rc combination inplace, is shown in Fig. 2.

Stimulus Set A and Response Set A each con-sisted of eight permutations of direction from acentral reference point. The stimulus panelcontained eight lights forming the outline of acircle. The response panel contained eight path-ways radiating from a central point like thespokes of a wheel. Each light and each pathwaywere separated from their neighbors by an angleof 45°. These stimulus and response patternsare characteristic of those provided by a pictorialazimuth or bearing display and an aircraft-typecontrol stick.

Stimulus Panel B consisted of four lights sep-arated by 90°. It provided four single-light posi-tions, plus the four two-light combinations formedby adjacent pairs of lights. Response Panel Bconsisted of four pathways originating at a cen-tral point and separated by 90° intervals. Eachpath, as seen in Fig. 1, branched in a T and per-

202 PAUL M. FITTS AND CHARLES M. SEEGER

oo o

S A

mM

- \ 5

RB ReScale , 6 in.

FIG. 1. The three stimulus panels (So, Ss, and Sc) and the threeresponse panels {£a, Kb, and £c) used in Exp. I. The Ss held a metalstylus on the circular button in the center of the response panel.

mitted Ss to move from the center point to oneof eight terminal positions. The four cornerpoints of the response panel could be reached bytwo alternative pathways; for example, theupper-right corner could be reached either by aright-up sequence or an up-right sequence. TheSs were told that these were equivalent responses.Response Panel B, therefore, involved the choiceof a single directional movement, or of a sequenceof two successive movements, and permitted Ssto terminate their responses in one of eight endstates.

Stimulus Panel C contained a pair of hori-zontally separated lights and a pair of verticallyseparated lights. The set of eight stimulus con-

FIG. 2. The S's station showing the correcttwo-handed response on Panel C to the lowerright-hand light of Stimulus Set A

ditions that it provided included^the four possi-bilities that a single light would come on, plusthe four two-light combinations that could beproduced by the simultaneous presentation ofone of each of the two pairs. Response Panel Cpermitted a left or right response of the left hand,an up or down response of the right hand, plusthe four possible combined movements of the twohands. Stimulus Panel C corresponds in generalto two separate single-scale instruments andResponse Panel C provides a set of responsessimilar to those present whenever two separatehand controls are used.

At the center of each response panel was a§-in. diameter metal disc, surrounded by a thinnonconductive ring. Reaction time was meas-ured as the time taken by S to move the stylusoff this metal button.

The stimulus panels were mounted at a 60°angle to the horizontal. Response panels were30° to the horizontal. The Ss worked from aseated position. The stimulus panels were 15°downward and 28 in. from their eyes, and theresponse panels were at a convenient location infront of them. The Ss watched the stimuluspanels and seldom looked at their hands or theresponse panels.

The JE'S station contained separate selectorswitches for the eight stimulus combinations anda single noiseless activation switch, which turnedon the selected light(s) and simultaneouslystarted a 1/100-sec. timer. The apparatus wasdesigned so that different stimulus and responsepanels could be substituted at S's station withoutany change at E's station. The E sat where hecould observe S's movements and could recorderrors as well as reaction time.

S-R COMPATIBILITY 203

StimulusSets

Response Sets

o o

I u s cms (wean)

FIG. 3. Mean scores for the eight Ss in each of the experimentalgroups. The datum for information lost is the theoretical informa-tion in the stimulus (3 bits) minus the computed value for informationtransmitted, uncorrected for sample size.

Subjects.-—The Ss consisted of 72 airmen atLockbourne Air Force Base, selected on the basisof two-choice reaction-time measures.4 Theirages ranged from 18 to 29 years. All were righthanded. They were excused from drill duringthe time spent as Ss and appeared to be wellmotivated.

The 72 Ss were selected from a larger groupof 153 men so as to form eight equal-sized stratawhich had homogeneous within-group averagetwo-choice reaction-time scores. Each of thenine experimental groups was formed by drawingone S at random from each stratum. Eachgroup of eight Ss was then tested under one of thenine S-R combinations.

Procedure.—The instructions for each of thegroups were similar. Those for the Sa-Ra groupwere as follows:

"Here is a stimulus panel of eight lights and aresponse panel in which you can move this stylusto one of eight places. Hold the stylus in yourright hand. When I say 'center' place it on thiscenter disc. I shall then say 'ready' and a fewseconds later one of the lights will come on. Ifthis light (point) should come on, move thestylus straight up. If this light should come on,

1 All Ss were tested by the second author.

move the stylus quickly to this position (indicateupper-right corner). If you start in the wrongdirection, correct your movement as soon aspossible. Do not try to guess which light willcome on as they will be presented in a randomorder. Work for both speed and accuracysince both reaction time and errors will berecorded."

Each S was given 20 practice trials on hisparticular S-R combination, followed by 40 testtrials. The order of stimuli was randomized,with the restriction of equal frequency for allstimuli at the end of each series, and a furtherrestriction against runs longer than two.

Results

The experiment provides three meas-ures of the effectiveness of each of theS-R ensembles: (a) reaction time, (b)percentage of responses that wereerrors, and (c) average informationlost per stimulus. The means for allthree criterion measures are indicatedin Fig. 3.

204 PAUL M. FITTS AND CHARLES M. SEEGER

Reaction time.—In scoring the two-handed responses that were made onPanel C, reaction time was taken asthe average of the times for the twohands. This procedure is justified bythe finding that in two-handed re-sponses the times for the right andleft hands agree very closely. Theaverage correlation of the two meas-ures was .96 for the three groups thatused Response Panel C, and the dif-ference in the mean reaction time forthe two hands was only .004 sec.However, for all one-handed responseson Panel C, the mean difference inreaction time between the two handswas .11 sec. in favor of the right hand,and 23 out of 24 Ss using Panel Cwere faster when using the right hand.In summary, when Ss had to moveone hand alone, the right hand, whichmoved away from or toward the body,was significantly faster (by about .108sec.) than the left hand, which movedto the right or the left, but when bothhands had to be moved together, theyhad similar reaction times. The timefor two-handed responses was approxi-mately the same as the mean for one-hand responses by the left hand.

The means shown in Fig. 3 are forall stimuli in a set combined. Thetimes for movements that were madein the wrong direction (errors) arecombined with the data for correctresponses, since the mean reactiontimes for erroneous and for correctresponses did not differ significantly.

The mean reaction-time data for thedifferent Ss were tested for homoge-neity of variance by Bartlett's testand no significant departure fromhomogeneity was found, even thoughvariance tended to increase somewhatas the mean reaction time increased(the range of SD's was from .036 sec.for the Sa—Ra ensemble to .097 sec. forthe Sc~Ra combination). A double-classification analysis of variance for

matched groups was then carried out(see Table 1). The most importantfinding is the highly significant inter-action effect. The variance that canbe attributed to interaction is verymuch larger than the variance attrib-utable to the primary effects of eitherstimulus or response sets alone.

The differences in the means for theprimary effects are interpreted as sig-nificantly different from chance, asindicated by the F ratios shown inTable 1 for which the residual term isused as the estimate of error. How-ever, no reliable generalizations aboutthese arbitrarily selected stimulus orresponse codes can be made to situa-tions in which comparisons are madeamong different sets of stimuli or dif-ferent sets of responses.

For every stimulus set there was adifferent best-response set, and forevery response set there was a differ-ent best-stimulus set. For example,Response Set A led to the shortestmean reaction time in combinationwith Stimulus Set A, but to the long-est reaction time in combination withStimulus Set C. The difference ofalmost .4 sec. is 21 times as large asthe estimate of the standard error ofthe difference. The three "best" com-binations are those that were pre-dicted by Es on the basis of the corre-spondence of the spatial codes. A

TABLE 1ANALYSIS OF VARIANCE FOR THE

REACTION-TIME DATA

Source

Matched SsStimulus setsResponse setsS-R interactionResidual

Total

df

1224

5671

MeanSquare

428968677236352

1704921268

F

68.43*28.67*

134.46*

* Significant beyond the 1 % level of confidence whentested against the residual as an estimate of error.

S-R COMPATIBILITY 205

direct test of the significance of thedifference between performance withcorresponding and noncorrespondingS-R ensembles is provided by a single-classification analysis of variance,which is equivalent to a conventionalt test between the means for matchedSs under the two conditions. Such ananalysis was carried out and the dif-ference was found to be highly signifi-cant (p<.001). The writers feel jus-tified in predicting that if other setsof stimuli and sets of responses shouldbe selected with due considerationto the correspondence of the stimulusand response codes, then it is highlyprobable that interaction effects wouldagain account for a large portion of thevariance and spatially correspondingcodes would again give superiorperformance.

The reaction times for the Sa-Ra

and the Sb~Rb combinations did notdiffer significantly, but both weresuperior to that for the Sc—Rc combi-nation. I t might be hypothesizedthat a bidimensional stimulus andresponse coding scheme (two pairs oflights and a two-handed response) is in-efficient. I t seems appropriate, how-ever, to suggest an alternative hypoth-esis, that the inferiority may havebeen due to failure on the part of JEs toobserve some principle of response-response compatibility in selecting theeight alternative movements consti-tuting Response Set C. I t should bementioned that forward and backresponses made with the right handonly averaged .41 sec. for the Sc-Rc

combination, which is comparable tothe mean reaction times for the Sa-Ra

and Sb-Rb combinations, where theright hand was also used.

Errors.—Approximately 10% of allresponses were in error, an error beingdefined as an initial movement in thewrong direction. These error dataagree in general with the time data in

respect to the rankings assigned to thedifferent S—R ensembles. The three"best" combinations within each rowand column of Fig. 3 are the same forthe two criteria. The difference be-tween the best and the worst combi-nations appears to be relatively largerthan was the case with the time scores.It is interesting to note that ResponseSet B, which in combination with itscorresponding stimulus set (Sb) ledto the fewest errors, resulted in themost errors when used in combinationwith another stimulus set (Sc).

The data for responses to separatestimuli revealed two important rela-tions. The first is that those S-Rensembles having lowest mean errorscores also tend to have the most uni-form time scores from one stimulus toanother. The second is that whereverthere is marked variability within theresponses to a set of stimuli, time anderror scores tend to vary together, i.e.,are positively correlated. For thefour S-R ensembles with the greatestnumber of errors the rank correlationsbetween time and error scores for theeight stimuli vary from .65 to .93.

Information lost.—The average in-formation transmitted per stimuluswas computed by Method 1 of Garnerand Hake (4). The data for infor-mation lost, shown in Fig. 3, are thedifferences between the theoreticalinformation in each stimulus event (3bits) and the average informationtransmitted per stimulation.

The results of the information anal-ysis agree closely with the total errorfrequencies and, in fact, add little tothe grosser error analysis. The ranksof the nine groups are identical on thetwo criteria except for a reversal ofthe two last (worst) groups. Thisagreement is not surprising since thesame type of error distribution, a pil-ing up of errors in certain cells of the

206 PAUL M. FITTS AND CHARLES M. SEEGER

matrix of transition probabilities, wasfound for all nine S-R combinations.

It is not possible to compute ameaningful estimate of average rateof information transfer in this experi-ment because of the relatively longtime delay between successive re-sponses. However, short responsetimes were associated with small lossof information both within the eightconditions of each S—R ensemble andbetween the means for the nine differ-ent S-R ensembles. Thus the resultsof the information or error analysis,considered in relation to the reaction-time analysis, provide empirical sup-port for defining compatibility interms of the average rate of informa-tion transmission.

EXPERIMENT II

An important question concerningthe results of the preceding experi-ment is whether the differences be-tween various S—R combinations aretransitory or permanent. An ex-tended learning study was carried outin an effort to answer this question.

Method

The most desirable way to have conductedthis study would have been to practice all of the72 Ss used in Exp. I over an extended period.However, this was not practical. Instead, asingle new group was studied for 32 training ses-sions covering approximately 2J months. TheSs practiced making a single set of eight responses(Response Set A) to each of the three differentsets of eight stimuli employed in the previousstudy. As an alternative Ss could have beenasked to make three sets of responses to asingle set of stimuli, but this was not donebecause less initial habit interference is to beexpected if the same responses are learned tothree sets of stimuli, than if three sets of responsesare learned to a single set of stimuli. ResponseSet A was chosen because there is no ambiguityin the scoring of these responses.

Six male students at Ohio State Universitywere started on the training series but one drop-ped out after 20 sessions and his data are notreported (although they are comparable to the

data that are reported). In order to maintainmotivation Ss were scheduled in pairs (except forthe last 12 sessions of the odd S). One S wasgiven a series of 16 trials on one of the three setsof S-R combinations in an unbroken sequence,while the other S observed. The Ss then ex-changed places. This procedure was continueduntil each S had been tested on each of the threestimulus sets. The sequence of work and restand the order of trials on the three conditionswere balanced. Each stimulus appeared twicein a random order within each run of 16 trials.Each S made a total of 48 responses per session.

Initial instructions and other procedures werethe same as in the preceding experiment. Atperiodic intervals Ss were cautioned to try not tomake errors. In addition to reaction time anderrors, E recorded movement time, the time totraverse the selected pathway of Panel A. Aftereach response Ss were told their reaction timeand if they had made an error, this was pointedout.

On Sessions 27 to 30 inclusive, a secondarytask, mental arithmetic, was carried on by Ssconcurrently with the perceptual-motor task.The E read aloud a series of numbers at a pre-determined rate, and S gave the successive differ-ences between the last two numbers read by E.This secondary task was introduced to test thehypothesis that the least compatible S-R combi-nation would show the most deterioration underconditions of additional load or stress. Standardconditions were resumed on Session 31. Theexperiment was terminated at the end of Session32.

Results

The mean reaction-time and move-ment-time data for the five Ss areshown graphically in Fig. 4. Through-out all of the standard sessions per-formance was consistently best whenSs responded to Stimulus Set A.Stimulus Set B gave almost equallygood results. Stimulus Set C wasmuch the worst of the three. At notime during the 32 days did any of theSs consistently respond as quickly toStimulus Set C as they did to theother two sets of eight stimuli. Themean times for the three stimulus setson Days 17 through 26 inclusive wereas follows: Sa, -272 sec.; Si,, .286 sec.;Sc, .355 sec.

S-R COMPATIBILITY 207

0.90

O80

070

SECO

NDS

— 0*5

TIM

E

020

OIO

000

\

1

\

A

—'—S&~ RAS B - R A

\

\ / \ REACTION T I M E S ^

^ ^ ^ ^ ^

HOVEHBIT TIMES» ^

DESTRACTlON '(-TRIAUS

, - 1

25 30

FIG. 4. Learning curve for five Ss during the 32 training sessionsof Eip . II. All Ss had 16 practice trials on each of the three S—Rcombinations each session.

Movement time in responding toStimulus Set C was slightly but con-sistently slower than to Sets A and B.Averaged over Trials 17 through 26these times were as follows: Sa, -059sec.; Si>, .061 sec.; Sc, .067 sec.

The error data, averaged over all

TABLE 2PERCENTAGES OF MOTOR RESPONSES THAT

W E R E IN ERROR DURING DIFFERENTBLOCKS OF TRAINING SESSIONS*

Sessions

i-67-16

17-2627-30**31-32

S-R Combination

2.55.66.94.48.1

Sb-Ra

3.78.7

10.06.99.4

11.912.511.915.612.5

*SessionB consisted of 16 responses per S-R combi-nation for each of the five Ss.** On Sessions 27, 28, 29, and 30 the Ss responded to

stimulus lights and solved mental arithmetic problemsconcurrently. The number of arithmetic problemsattempted during each session was 50 per S and thepercentage of arithmetic errors for the three sets ofstimulus lights was 4.1, 2.9, and 4.2, respectively.

five Ss for successive blocks of time,are given in Table 2. It can be seenthat the Ss never succeeded in elimi-nating all errors. In fact, errorsoccurred more frequently as trainingprogressed, indicating perhaps that Semphasized speed at the expense ofaccuracy. Errors were made mostfrequently to Stimulus Set C through-out the training period. Thus bothtime and error data indicate consis-tently poorer performance on theSc-Ra condition.

On Days 27-30, when the load onSs was increased by the addition of asecondary task, reaction times weresubstantially slower for all three S-Rcombinations. All five Ss continuedto give the slowest reaction to StimulusC; however, the relative differencesbetween groups were much smallerthan they had been previously. Thefrequency of arithmetic errors was ap-proximately the same for all threegroups, but there was a large differ-

208 PAUL M. FITTS AND CHARLES M. SEEGER

ence between groups in the frequencyof movement errors. The introduc-tion of an additional task late in learn-ing in this case apparently served toreduce the degree of readiness for themotor task, minimizing reaction-timedifferences, but increasing differenceswith respect to errors in the motortask. However, the data are notconclusive on these points and thetest of the hypothesis regarding theeffects of increased load is consideredinconclusive.

DISCUSSION

The results of the two experimentsdemonstrate clearly the importance ofstimulus and response coding for themaximum rate of information transferin a perceptual-motor task. It alsois clear that some S-R compatibilityeffects are relatively unaffected byextended practice.

An interpretation of the basis forthe relative permanence of these ef-fects, which seems appropriate to theconcept of compatibility, is one statedin terms of the capacity to learn todeal with sets of probabilities (proba-bility learning). The response that aperson makes to a particular stimulusevent can be considered to be a func-tion of two sets of probabilities; (a)the probabilities (uncertainties) appro-priate to the situational constraintsestablished by E's instructions, by thereinforcements experienced in the ex-perimental situation, and by otheraspects of the immediate situation;and (b) the more general and morestable expectancies or habits based onS's experiences in many other situa-tions. It is suggested that extendedtraining will nearly always lead tochanges in the former but often willhave relatively little effect on thelatter.

This view holds that S's behavior,notwithstanding long experience in aparticular situation, is never entirelyrelevant to the specific constraints ofthat particular situation. Instead, Sresponds as if additional possibilitieswere present.

This interpretation supports theview that stimulus and response setsare optimally matched when the result-ing ensemble agrees closely with thebasic habits or expectancies of indi-viduals, i.e., with individual and withpopulation stereotypes. It must beremembered, however, that the expec-tancies referred to are those whichhold for the particular situation understudy, and that population stereotypesshould be determined with due regardto the total situation. For example,in the present experiment the JBS deter-mined, by preliminary trial, that Ss"preferred" to move toward ratherthan away from a stimulus light.However, it is known that in manystimulus tasks where the S controlsthe stimulus, the opposite motion rela-tion is the expected one. When Sshave to learn to deal with the proba-bilities inherent in a particular situa-tion, the correspondence of thesespecific expectancies to the more gen-eral expectancies of the individual withrespect to that kind of situation is animportant aspect of the learning task.

Further support for considering S-Rcompatibility to be a function of stim-ulus and response matching comesfrom the positive correlation betweenthe two criteria, time and errors, usedin evaluating performance. This posi-tive correlation would be expected ifadditional information transforma-tions, or re-encoding steps, were addedto a communication system, since eachtransformation would be likely to addan additional time delay and to in-

S-R COMPATIBILITY 209

crease the total probability of errors.In the human it is hypothesized thatthe fastest responses may be the mostaccurate because they involve S-Rensembles in which the transfer ofinformation from stimulus to responseis most direct, i.e., involves the mini-mum number of recoding steps. Theconcept of intervening informationtransformations does not attempt toexplain how such recoding occurswithin the nervous system, but it is inagreement with the subjective reportsof Ss who maintained that it was diffi-cult, in the case of the less compatibleS-R ensembles used in the presentstudy, to be prepared to respond to allof the eight stimulus possibilities.

SUMMARY

Experiment I was planned totest the hypothesis that informationtransfer in a perceptual-motor task isin large measure a function of thematching of sets of stimuli and sets ofresponses. Nine S-R ensembles, in-volving variations of the spatial pat-terns of stimuli and responses, werestudied in an eight-choice situation,using groups of matched Ss.

The results, analyzed in terms ofreaction time, errors, and informationlost, support the hypothesis. Theyindicate that it is not permissible toconclude that any particular set ofstimuli, or set of responses, will pro-vide a high rate of information trans-fer; it is the ensemble of S-R combi-nations that must be considered.

Experiment II was planned to testthe permanence of three selected S-Rcompatibility effects. Five Ss weretrained for 32 days to make a par-ticular set of responses to each of threesets of stimuli. Differences in reac-tion time, movement time, and fre-quency of errors in responding to the

three sets of stimuli persisted over the32 days.

The results are interpreted in termsof probability learning and the neces-sity for (hypothetical) informationtransformation or re-encoding steps.It appears that it is very difficult forSs to learn to deal effectively with theinformation (uncertainties) charac-teristic of a specific situation, if theseuncertainties are different from themore general set of probabilities whichhave been learned in similar life

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(Received for early publicationMay 22, 1953)