MADELEINE M. GROSSCalifornia State University, San Jose, San Jose, California 95114

Hemispheric specialization forprocessing of visually presentedverbal and spatial stimuli*,**

Two "same-different" reaction time experiments, analogous in task demandsmade on the S, were designed to test laterality differences in. perception. Tennormal right-handed Ss performed a verbal task in which they decided whetheror not two three-letter words belonged to the same conceptual class. Tendifferent Ss performed a spatial task in which they decided whether two 16-cellmatrices with 3 blackened cells were identical. Reaction times were found to besensitive to laterality differences in perception. Verbal stimuli were processedfaster when presented in the right visual field, and thus projected directly to theleft cerebral hemisphere; spatial stimuli were processed faster when presented inthe left visual field, and thus projected directly to the right cerebral hemisphere.These results were analyzed in terms of implications regarding hemisphericasymmetries for processing of verbal and spatial material and the nature ofinterhemispheric transfer of information.

A large body of literature, derivedfrom a number of diverse researchmethodologies, lends support to theidea of differential specialization ofthe two cerebral hemispheres in man.One such apparent division offunction, suggested by studies ofpatients with asymmetric cerebraldamage (e.g., Milner, 1968) or withsurgical disconnection of the cerebralhemispheres (Gazzaniga, Bogen, &Sperry, 1965; Gazzaniga & Sperry,1967), is that of separation ofprocessing of linguistic andnonlinguistic information. Perceptuallaterality studies in both the auditoryand visual modalities on normal Sshave also supported the conclusionthat linguistic processing and/orou tput appears to be primarilylocalized in the left cerebralhemisphere, while the righthemisphere is specialized fornonlinguistic (in the visual modality,spatial) processing. This cerebraldominance conformation is especiallypredominant in right-handed Ss, theoverwhelming majority of whom have

*This research was supported by NIMHPredoct01'al Fellowship MH 49556-01 to theauthor and NIMH Grant NB-06501. Thispaper is based upon a dissertation submittedby the author to the PsychologyDepartment of Stanford University inpartial fulfillment of the requirements forthe degree of Doctor of Philosphy,"Hemispheric Specialization for theProcessing of Visually Presented Verbal andSpatial Stimuli: A Reaction Time Analysis."1971.

**1 would like to thank my advisor. Dr.Charles R. Hamilton. for his helpfulsuaestions during all phases of thisresearch. Thanks are also due to Sally P.Springer for being instrumental in firstsuggesting the possibilities of a reaction timeanalysis of hemispheric dominance (personalcommunication. 1969).

speech output centers in the leftcerebral hemisphere.

A number of previous lateralitystudies in normal Ss have used ameasure of percent correct report. Inthe auditory modality, for example,Kimura (1961) found that digitspresented to the right ear wereidentified more accurately than thosepresented to the left ear in a dichoticpresen tation. Conversely, Kimura(1964) found that recognition withthe left ear was superior when the taskwas nonverbal.

In the visual modality, lateralitystudies have utilized stimuli presentedto either the left or the right visualhemifield in order to producecortically lateralized presentations.The structure of the human visualsystem is such that the initialprojection of stimuli presented to theright of fixation (RVF) is solely to theleft cerebral hemisphere, while that ofstimuli presented to the left offixation (LVF) is to the right cortex.Bryden (1965) and Kimura (1966), forexample, have reported a right-fieldadvantage for the perception of verbalmaterial. Conversely, Kimura (1969)found that location of a dot in spacewas superior in the left visual field,while Schell and Satz (1970) found aleft-field superiority in recognition ofa previously shown block design.

If the perceptual asymmetries foundin visual presentations are interpretedas being indicative of a hemisphericlocalization of some part of theperception of or response to thesestimuli, two differentprocessing/output alternatives aresuggested. It may be that all or somenecessary part of verbal processingmust take place in the left hemisphere(at least in right-handed Ss), such that

those verbal stimuli presented to theRVF have the advantage of moredirect access to the appropriateprocessing center. Alternatively, verbalstimuli may be processed in the righthemisphere, but (1) the processingmay be slower and not as accurate,and/or (2) the verbal response mustalways be directed by the lefthemisphere, permitting possibleinformation loss during the callosaltransmission preceding output. Forspatial stimuli, direct presentation tothe right hemisphere should yield aprocessing advantage. Such anadvantage would be expected to bedecreased (if the left hemisphere werealso capable of doing some spatialprocessing) if verbal response wererequired.

Recently, several studies utilizing areaction time (RT) measure ofhemispheric asymmetries wereundertaken. In the auditory modality,Springer (1971a) found that motor RTto right ear verbal targets in a dichoticsituation was shorter than that to leftear targets. In the visual modality, anumber of studies (Filbey &Gazzaniga, 1969; Gibson, Filbey, &Gazzaniga, 1970; Klatzky, 1970;Moscovitch & Catlin, 1970) supportedthe notion that hemisphericspecialization of function for thelinguistic VB spatial processing modes isreflected in RT differences betweenthe visual hemifields.

It was the object of the presentexperiments to utilize a RT measure asa potentially more sensitive index ofdominance within an individual than ispercent correct report, as well as toprovide more extensive data about thenature of the information flowbetween hemispheres than haspreviously been possible. Just as thetransfer of information across thecallosum for processing and/or outputmay yield a performance decrement inaccuracy, it could also yield a slowerRT, attributable either to callosalcrossing time or to information loss.By manipulating stimulus parameters(linguistic VB spatial input) andresponse parameters (verbal VB motorresponse, inter- vs intrahemisphericsensory-motor connections by meansof visual field and responding handcombinations), it should be possible tostudy the laterality effect in terms ofboth processing and output functions.

In the present experiments, bothstimulus and response parameters weremanipulated. Two experiments, one a"verbal" task and one a "spatial" task,were performed, which madeanalogous task (decision and response)demands upon the S. In each, Sperformed a "same"-"different" RTtask. In the verbal task, S decidedwhether two words presentedsimultaneously were in the same

Perception & Psychophysics, 1972, Vol. 12 (4) Copyright 1972, Psychonomic Society, Austin, Texas 357

category (both animals or both partsof the body) or in different categories.In the spatial task, S decided whethertwo 16-cell matrices with 3 cellsblackened were the same (all 3 blackcells in register) or different. In eachexperiment, each S gave verbalresponses, as well as manual responseswith both right and left hands.

EXPERIMENT 1: VERBAL TASKIn Experiment I, Ss decided

whether two three-letter word stimuliwere in the same category or indifferent categories.

MethodSubjects. The Ss were four female

and six male members of the Stanfordcommunity between the ages of 18and 26, who were paid $1.75 persession. All Ss met the thrt!e selectioncritera of (1) having normal orcorrected vision of at least 20/22 ineach eye, as measured with a KeystoneOphthalmic Telebinocular, with anacuity difference no greater thanone-half step between eyes (e.g., 20/20and 20/20-), and having at least 100%stereopsis on the Navy eye-screeningtest scale, (2) being right-handed asdefined by self-label and by the handwith which they wrote when fillingout the experimental questionnaire,and (3) having no history ofneurological disorder or speech defect.The 10 Ss scored an average of 22.7 onthe Crovitz and Zener (1962)handedness scale.

Stimuli. Two sets of eightthree-letter words were prepared, oneset of animals and one of parts of thebody. Three-letter words were chosenin order to preclude any potentialscanning effects [see White (1969) fora discussion of the laterality literatureoriented toward this explanation offield differences] . Studies byColtheart and Merikle (1970), Krueger(1970), and Smith and Haviland(1972) indicate that such words areperceived as units rather than scannedfrom left to right. As an additionalsafeguard, the response was dependentupon a "completed" perception ofboth words, since it was necessary tocompare their "categories." The eightanimal words were: ape, cat, cow, dog,elk, hen, pig, rat. The eight parts ofthe body words were: arm, ear, eye,hip, jaw, leg, rib, toe.

Using these words, 64 stimulusconfigurations were designed. Eachwas composed of two different words,one above the other, drawn eitherfrom the same category or fromdifferent categories. Thirty-two"same" units were designed; 16"same" units were drawn from the"animal" group and 16 from the"body" group. Each word in eachgroup appeared in four "same" pairs,

twice on top and twice on the bottom.. Thirty-two "different" units weremade up, half with an animal word ontop, half with a "body" word on top.Each word in each group appeared infour "different" pairs, twice on topand twice on the bottom. Each of the64 stimulus configurations appearedonce to the left and once to the rightof fixation, making a total of 128stimulus configurations. These 128stimuli were divided into four blocksof 32 stimuli; both the right- andleft-field presentation of a stimulusappeared within a block, randomizedwith the constraint that no more thanfour consecutive correct responseswere identical. Ss received these fourblocks in different assigned orders ineach session, with blocks presented inreverse order one-half of the time.

The stimuli were made with black48-point Helvetica Medium Letrasetletters (lAi in. high, subtending about1 deg 14 min of visual angle at aviewing distance of 23 in.) mountedon white construction paper. The twowords appeared, respectively, at 1 mmabove and below the fixation dot,which appeared in the preexposurefield, centered 37 mm to the left orright of the fixation dot (3 deg 34 minof visual angle). The words were from30 to 34 mm in length, subtendingfrom 2 deg 58 min to 3 deg 16 min ofvisual angle. Therefore, the closestapproach to the fixation point wasabout 2 deg.

Procedure. Experimental stimuliwere exposed in a Gerbrands two-fieldmirror tachistoscope. Centered in onefield was a black fixation dot; thisfield was always on except when teststimuli were being presented, and Swas instructed to maintain fixation onthe center dot. The luminance of thefixation field was 5.9 fL; that of thetest field was 6.2 fL. The E signaledthe onset of a test by saying "now."Stimulus duration was 150 msec, topreclude shifts in fixation.

Reaction time was measured to thenearest millisecond by an electronicdecade counter (Hunter KlocKounter),which was activated simultaneouslywith stimulus onset and was stoppedby S's response. In verbal responsesessions, S spoke into a microphonethat activated a voice relay (Trans-VoxModel Vox-I) that stopped the clock;half the Ss responded "it" to thesame stimuli and "It" to different stim-uli, while half did the reverse. In motorresponse sessions, S pushed a levergrasped between the thumb andforefinger up or down, which stoppedthe clock by means of a mechanicalclosing. Those Ss who verbalized "it"for "same" pushed the lever down for"same" and up for "dif!erent" stimuli;those who verbalized "It" for "same"did the reverse.

Each S served in a practice session,during which he became familiar withthe stimuli and methods ofresponding. Each then served in eightexperimental sessions, consisting of sixpractice trials followed by 128 teststimuli. The Ss were instructed torespond as quickly as possible,consistent with accuracy. Stimuli onwhich errors were made werereinserted into the stimulus sequence2-5 presentations later. Order ofnature of response (verbal or manual)for sessions was counterbalancedwithin a S and across Ss. In the motorsessions, right and left hands were usedin alternate blocks; the alternation wascounterbalanced within a S and acrossSs.

ResultsMean RTs for each visual field (right

or left), response mode (verbal, righthand, or left hand), and response("same" or "different") werecomputed for each session. Thesemeans excluded error or repeat trials;however, the error rate for each S waslow and did not seem to differ withregard to visual field of thepresentation. The mean error rate was4.5%; the range over Ss was 1.2% to11.5%. In addition, extreme RTs weredropped. (The drop rule used entailedexamining the distribution of RTs forall "same" stimuli and all "different"stimuli within a session separately. If a200-msec gap was found in thedistribution, numbers that wereseparated from the main body of thedata by the gap were dropped.Number of dropped stimuli did notdiffer with respect to side ofpresentation.) The means so obtainedfor each session were averaged acrosssessions, yielding 12 means for each S.The pattern of means obtainedaveraged across Ss is shown in Table 1.

An analysis of variance of thesemean RTs (Field by Response Modeby Response, within Ss)was performed. The analysis ofvariance showed that RT for thisverbal processing task was faster forthe RVF than for the LVF, 1,063.2and 1,093.9 msec, respectively[F(l,9) = 11.56; P < .01]. All of the10 Ss showed faster RT to RVFstimuli, a "right-field effect."

Verbal response was slower thanthat of either the right or left hand,1,145.0 msec vs 1,048.5 and1,042.2 msec, respectively[F(2,18) = 11.32; p < .001], whileRTs for right and left hand did notdiffer significantly [t(9) = 1.005, n.s.]."Same" responses were faster than"different" responses, 1,042.5 and1,114.7 msec, respectively[F(l,9) = 15.36; p < .01]. TheResponse Mode by Responseinteraction was also significant

358 Perception &£ Psychophysics, 1972, Vol. 12 (4)

Table 1Overall Mean Reaction Time (MWiMconds) for Each Field by Respon..

Mode by Respon.. Combination: Verbal T"

ResponseField

Mode Response Ri&ht Left

Right Hand Same 981.6 1023.8 1048.5Different 1081.1 1107.5

Left Hand Same 979.6 1011.1 1042.2Different 1068.2 1110.1

Verbal Same 1115.8 1142.8 1145.0Different 1153.2 1168.1

1063.2 1093.9

Table 2Percent Errors for Each Field by Response

Mode Combination: Verbal Task

ResultsMean RTs for each visual field (right

or left), response mode (verbal, right

5.24.65.0

4.43.34.4

Field

Right Left"(Percent) (Percent)

ResponseMode

Right HandLeft HandVerbal

matrices differed from each other bytwo black cells and had one black cellin common; Group 3 matrices had noblack cells in common. The matricesused are shown in Fig. 1.

Using the 12 matrices, 48 stimulusconfigurations, 12 "same" and 36"different," were designed. Each wascomposed of either two of the samematrix, one above the other ("same"),or of two matrices from the samegroup, one above the other("different"). Each of the 12 possible"same" configurations appeared threetimes to both the right and left offixation. Each matrix in each groupwas paired with all other matrices inthat group in both the top and bottomposition to form the "different"stimuli. Each different configuratiunappeared once to both the left andright of fixation. The 144 stimuluspresentations were divided into fourblocks of 36 stimuli; both the right-and left-field presentations of astimulus appeared within a block. Theassigned block order for each sessionfor each S in Experiment 2 matchedan order assigned to an S inExperiment 1.

Multiple copies of the 12 matriceswere made by an offset printingprocess. The 144 stimuli were made bycutting out individual matrices andaffixing them to white constructionpaper. The matrices appeared,respectively, 1 mm above and below afixation dot that appeared in thepreexposure field, centered 36 mm tothe left or right of the fixation dot(3 deg 28 min at a viewing distance of23 in.). The matrices were1% x 1% in., subtending a visual angleof 3 deg 8 min x 3 deg 8 min at aviewing distance of 23 in. The closestapproach to the fixation dot was,therefore, approximately 2 deg.

Procedure. The apparatus andexperimental procedure were identicalto those of Experiment 1. Each Sserved in eight seuion, during whichRT measures for the 144 test stimuliwere obtained. For each S, thesequence of the nature of responseused in a given session was matched toa sequence assigned to an S inExperiment 1.

EXPERIMENT 2: SPATIAL TASKIn this experiment, as in the verbal

experiment, Ss made a"same"-"different" decision regardingthe stimulus presentation. In this case,the decision was whether two 16-cellmatrices with 3 blackened cells wereidentical (all 3 blackened cells incorrespondence) or not ("different").It was felt that this task would beappropriate for the spatial condition inpart because Sekuler and Abrams(1968) reported that Ss presented witha similar task seemed to perform a"template match." In an effort toemphasize the spatial "Gestalt" natureof the task, Ss were instructed to "tryto take into account the total stimulusconfiguration" when making theirresponses.

summed across Ss, between the handby field combinations of the motorresponse sessions, [x 2 (3) = 5.84, n.s.]or between the response mode by fieldcombinations, summing across allsessions [x 2 (3) = 4.77, n.s.]. Thepresent results indicate that evidencefor hemispheric asymmetry, asindicated by RT differences, may befound even under conditions in whichno significant difference in error rate isfound. This suggests that RT mayprove to be a more sensitive measureof hemispheric specialization than isthe more traditional percent correctmeasure.

MethodSubjects. The Ss were five male and

five female members of the Stanfordcommunity between the ages of 18and 27, who were paid $1.75 persession. All Ss met the selectioncriteria (vision, right-handedness, andno speech defects) described inExperiment 1. The 10 Ss scored anaverage of 20.8 on the Crovitz andZener (1962) handedness scale. Noneof the Ss had served in Experiment 1.

Stimuli. Three groups of four16-cell black-and-white matrices with3 cells blackened' were designed.Group 1 matrices differed from eachother by one black cell and had twoblack cells in common; Group 2

[ F ( 2 ,1 8) = 4.0 1 ; P < .05 ] ; the"different" response took relativelylonger than "same" for the motorresponse modes than for the verbal(93 rnsec VB 31 msec). This difference,however, is significant at only the .05level, one-tailed, and it does notappear to yield any informationrelevant to hemispheric specialization.

No other interactions weresignificant. The Field by ResponseMode interaction failed to reachsignificance, indicating no differentialfield effect depending upon presumedhemispheric locus of response (forexample, if the RT difference for theright VB the left visual field had beengreater for the verbal response andright-hand response than for theleft-hand response, a statisticallysignificant difference would have beenobtained). A t test performed on RTdifferences for RVF vs L VF usingmotor responses alone (motorresponse field difference) showed nodifferential field effect for the right vsleft hand [t(9) = .13, n.s.}. A t testbetween combined right-hand andleft-hand motor field differences andverbal field differences was alsononsignificant [t(9) =.29, n.s.},indicating that the right-field RTadvantage is the same with both motorand verbal responses. The Field byResponse interaction also failed toreach significance, indicating that theright-field superiority is maintainedregardless of whether S is making a"same" or a "different" response.

Because the traditional measure ofthe laterality effect has beendifference in percent correct reportbetween fields, several analysesregarding error rate for differentconditions were performed. Percenterrors summed across Ss for eachcondition is shown in Table 2. A t testfor correlated means was performedon the differences between thenumber of errors made in the RVF vsthe LVF for each S (mean error rate of4.1% in the RVF VB 5% in the LVF).This difference between fields did notreach significance (t(9) =1.86, n.s.},In addition, chi-square testa showed nosignificant difference in errors,

Perception & Psychophysics, 1972, Vol. 12 (4) 359

Field

Right Left

843.0 827.1 866.6910.6 886.8

837.7 830.6 869.7912.4 898.1

1007.0 980.4 1020.91068.8 1037.4

928.2 909.9

9

II

10

12

Response

SameDifferent

SameDifferent

SameDifferent

GROUP 3

ResponseMode

Right Hand

Left Hand

Verbal

Experiment 1, a t test indicated nodifference in the magnitude of theleft-field advantage obtained with rightvs left hand [t(9) =.83, n.s.] or forverbal vs motor response [t(9) = 1.13,n.s, ). These data, comparable to theresults obtained in the verbalexperiment, support the hypothesisthat at least some part of all spatialprocessing must take place in the rightcerebral hemisphere, since the locus ofoutput did not affect the magnitude ofthe field difference obtained. TheField by Response interaction alsofailed to reach significance, indicatingthat the left-field superiority ismaintained regardless of whether S ismaking a "same" or a "different"response.

Several analyses regarding error ratefor different conditions wereperformed. Percent errors summedacross Ss for each condition are shownin Table 4. A t test for correlatedmeans was performed on thedifference between the number oferrors made in the LVF vs R VF foreach S (mean error rate of 5.3% in theLVF vs 5.5% in the RVF). Thisdifference between fields did not reachsignificance [t(9) = .47, n.s.},

In addition, as for Experiment 1,chi-square tests showed no significantdifferences in errors summed acr088 Ssfor right and left visual fields and rightand left hands [x' (3) -= 1.12, n.s.] orfor right and left visual fields andverbal and motor response [x' (3) =2.70, n.s.). As in Experiment I, then,evidence for hemispheric asymmetry,as indicated by RT, was found in theabsence of significant differences in

differ significantly [t(9) = .27, n.s.}. error rate.As in the verbal task, "same" A secondary analysis was performedresponses were faster than "different" in order to examine the notion (LeVY,responses, 887.6 msec and 950.5 msec, 1969) that it is the Gestalt nature ofrespectively [F(I,9) = 6.98; p < .05). the task for which the right

No interactions were significant. hemisphere is specialized. If this wereThe Field by Response Mode so, the number of blackened cells ininteraction failed to reach significance, register should not significantly affectindicating no differential field effect the RT for the "same"-"different"depending upon presumed hemispheric judgment, since simultaneous checkinglocus of response (e.g., if verbal and of all cells would be made in aright-hand response behaved similarly "template match." The average RT forand showed a smaller left-field effect each type of "different" judgmentthan did left-hand response). As with (zero, one, or two blackened cells inthe right-field advantage found in register) and for the "same" judgment

Table SOverall Mean Reaction Time (Milliseconds) for Each Field by Response

Mode by Response Combmation: Spatial Task

7

5

6

8

GROUP 2

Fig. 1. Matrices used in spatial task.

2

3

4

GROUP I

hand, or left hand), and response("same" or "different") werecomputed for each session for each S.As for Experiment 1, these meansexcluded error trials. Mean error ratewas 5.3%; the range over Ss was 2.9%to 10.1%. Extreme RTs were droppedaccording to the same criterion used inExperiment 1. Means so obtained foreach session were averaged acrosssessions, yielding 12 means for each S.The pattern of means obtainedaveraged across Ss is shown in Table 3.

An analysis of variance of thesemean RTs (Field by Response Modeby Response, within Ss)was performed. The analysis ofvariance showed that RT for thisspatial task was faster for the LVFthan for the RVF, 909.9 and928.2 msec, respectively[F(1,9) =16.83; P < .01). All of the10 Sa showed faster RT to LVFstimuli, a "left-field effect."

Verbal response was slower thanthat of either the right or the lefthand, 1,020.9 msec vs 866.6 and869.7 m s e c , respectively[F(2,18) = 15.98; P < .001), whileRTs for right and left hand did not

360 Perception & Psychophysics, 1972, Vol. 12 (4)

Table 4.Percent Enors for Each Field by Resporue

Mode COlDblnation: Spatial T"

Comparison ofVerbal and Spatial Tasks

In order to determine whether themagnitudes of the field differencesobtained were the same for both theverbal and spatial tasks, the average ofthe motor response (average of rightand left hands) and verbal responsefield differences was obtained for eachS in each experiment. A two-samplet test was performed on the absolutemagnitude of field differences for thetwo tasks (28.3 msec for the verbaltask, 19.8 rnsec for the spatial task).The absolute magnitude of the fielddifference did not differ significantlyaccording to the nature of the task[t(18) =1.02, n.s.},

In general, then, the magnitude offield differences obtained for each taskwas the same for verbal or manualresponse modes and "same" or"different" responses. The field

was obtained for each visual field foreach session for each S. These scoreswere then averaged across sessions.The relationship of RT (averaged over10 Ss) to number of cells in registerfor each visual field is shown in Fig. 2.

As was indicated by the..same..·..different.. dichotomy in theanalysis of variance for thisexperiment, "same" RT was leaa thanany of the "different" RTs. The slopeof the line for the "different" RTs wasfitted by the least-squares method foreach S for each visual field. Asignifican( linear component wasfound for increasing RT withincreasing number of cells in registerfor both the right visual field [slope =51.5; t(9) .. 6.20; p < .001] and forthe left visual field [slope = 47.4; t(9)=4.6; p < .01]. The slopes for right valeft visual fields, compared with acorrelated means t test, were notsignificantly different [t(9) .. .95,n.a ]. These results are not in accordwith those of Sekuler and Abrams(1968), since they did not obtain any"same"·"different" dichotomy, butobtained instead a linear trend (withsmall slope) for increasing RT withincreasing number of cells in register,regardless of whether the appropriateresponse was "same" or "different."The results of the present analyses donot suggest the employment of asolely Gestalt processor.

"SAMEllCELLS

more consistent with the time requiredfor a few synaptic transmisslons. Suchstudies measured the difference in RTfor simple visual stimuli (which could,presumably, be proceaaed in eitherhemisphere), which were presented tothe visual field ipsilateral (uncrossedconnections) or contralateral (crossedconnections) to the responding hand.RT differences on the order of2·6 msec were found. The magnitudeof RT differences obtained in thepresent experiment (an average of24 msec) is not consistent with thesemeasures of simple callosaltransmiaaion time.

Even if it were poaaible for theobtained latency differences to beconsistent with all previous measuresof callosal transmission time, however,it is neceaaary to account for the dataregarding inferior recognition of verbalmaterial in the LVF (Kimura, 1966;Bryden, 1965) and of spatial materialin the RVF (Kimura, 1969; Schell &Satz, 1970). A required callosaltransmission would cause longerlatency of response, but, if thetransmiaaion were complete in detail,should not cause inferior performancefor stimuli initially projected to thenonspecialized hemisphere. If,however, the corpus callosum had alimited channel capacity, this inferiorperformance would be explained, sinceinformation loaa during transmissioncould decrease accuracy. This effectwould be most pronounced in thoselituations where stimulus informationwas already minimal, as when stimuli

e----e RIGHT VISUAL FIELD.. - .. LEFT VISUAL FIELD

• 923.8• 905.1

2,

//

~{022.6/

//~Y: 47.4 X + 917.6

Y: 51.5 X +934.9///-964.2

// ·944.4/ 946.0

927.9

"DIFFERENT"NUMBER OF BLACKENED

IN REGISTER

o,

900

1050

UIVE1000UJ2F=z 950o

~IJJa:::

Fig. 2. Reaction time (spatial task) for each visual hemifield as a function ofnumber of blackened cells in register.

differences for the two tasks were inopposite directions (right-fieldsuperiority for the verbal task,left-field superiority for the spatialtask), but were not significantlydifferent in absolute magnitude. Thetwo field differences, then, canreasonably be attributed to the sameunderlying phenomenon. A summaryof the RTs obtained in the twoexperiments is shown in Table 5.

DISCUSSIONIt might be asked whether the

between-field RT differences obtainedin this study are consistent withmeasures of callosal transmission time.The magnitude of field differencesobtained in these experiments isconsistent with someelectrophysiological studies (Bremer,1968; Grafsstein, 1959; Teitelbaum,Sharpless, & Byck, 1968), which haveahown that excitation originatingexclusively in one hemisphere takesapproximately 10 msec (primarypositive wave) to 35 maec (secondarynegative wave) to cross the callosumand its related synapses to theopposite hemisphere. This consistencywould suggest that differences in RTfor verbal va spatial tasks for the twovisual hemifields are attributable tothe extra time required for a callosaltransmission of stimulus informationfor processing. Other behavioralstUdies (Poffenberger, 1912; Berlueehi,Heron, Hyman, Rizzolatti, & Umilt8,1971), however, indicated a muchshorter callosal transmission time,

4.85.15.6

Field

5.65.66.4

Right Left(Percent> (Percent>

ResponseMode

Right HandLeft HandVerbal

Perception & Psychophysics, 1972, Vol. 12 (4) 361

Table 5Overall Mean Reaction Time (Milliseconds): Comparison of Verbal and Spatial Tasks

Resjronse .fodeField by

Task Field Motor* Verbal Task Means

Right 1027.6 1134.5 1081.0Verbal Left 1063.1 1155.5 1109.3

Verbal Means 1045.3 1145.0 1095.2

Right 875.9 1032.9 954.4Spatial Left 860.4 1008.9 934.6

Spatial Means 868.2 1020.9 944.5

*Mean of the right and left hand

are presented at very short durations.Such a callosal transmissionexplanation, then, would explain bothdifferences in error rate and latencydifferences between visual hemifields.

Such an inherent limitation inchannel capacity is indicated bybehavioral (Myers, 1962) andelectrophysiological (Berlucchi &Rizzolatti, 1968) studies in cats. Theseexperiments suggest that the effect ofstimuli projected via the corpuscallosum is not identical to that of thesame stimuli projected to a givenhemisphere via geniculocorticalpathways.

Furthermore, Buchsbaum and Fedio(1970) report a greater stability (tworesponses to the same stimulus have ~higher correlation with each Ot}l":l."~ .,.evoked response activity in humans tostimuli projected via the direct visualpathways to each hemisphere than tostimuli taking secondary, indirectpathways. It seems very likely,therefore, that the latency differencesobserved in the present tasks areattributable to both callosaltransmission time and to increasedlatency of response accompanyinguncertainty attendant upon a"degenerate" stimulus transmission. 1

Geffen et al (1971) have suggestedthat processing of a given stimulusmay be performed, although moreslowly, by the hemisphere which is notspecialized for that stimulus type.They found a shorter RT in the LVFfor perception of faces with a motorresponse, but not with a verbalresponse. Geffen et al interpretedthese data as indicative of aright-hemisphere superiority(quickness) for this spatial task, whichwas "cancelled" because of thenecessity for verbal response (initiatedsolely by the left hemisphere ). Thepresent experiment yielded aright-hemisphere superiority for thespatial task regardless of whetherverbal or manual response wasrequired; the magnitude of this fielddifference did not differ significantlyfor verbal vs motor response. The dataof the present experiment are not inaccord with Geffen's conclusion thatthe nonspatial hemisphere can performspatial processing, although more

slowly; rather, they indicate that all orat least some part of all spatialprocessing must take place in the righthemisphere. It is possible that thematrix task in the present experimentwas a more difficult spatial task thanthe "faces" task or was more strictlydependent upon spatial cues, such thatprocessing had to take place in theright hemisphere, while processing ofthe faces might take place in the lefthemisphere as well.

The present data can be mostparsimoniously accounted for bymodels that postulate that all or partof the processing of verbal stimulimust take place in the left cerebralhemisphere, while all or part of theprocessing of spatial stimuli must takeplace in the right cerebral hemisphere.It might then be expected that, inaccordance with the results ofBradshaw and Perriment (1970), theright hand would be faster for a verbaltask (left hemisphere), while the lefthand would be faster for a spatial task(right hemisphere). (This "handeffect" hypothesis is distinct from aHand by Field interaction, theimplications of which have beenpreviously discussed.) It is interesting,however, to note that the differencesin RTs for the two hands in thepresent experiment, althoughnonsignificant, were exactly theopposite of that proposed by a modelthat assumes motor output directedexclusively from the hemispheredominant for a given task (in order toavoid an additional callosaltransmission for output). The lefthand was faster for the verbal task,while the right hand was faster for thespatial task. The total number oferrors for all Ss for each hand in eachtask were in the same direction as thereaction time differences, althougherror differences are also notsignificant. Rizzolatti et al (1971 )reported RT differences and error ratedifferences for the two hands that arein the same direction as those in thepresent experiment, although theseauthors did not discuss this trend.Klatzky and Atkinson (1971) found ageneral left-hand superiority in a RTtask, which was lessened if thenecessary processing was spatial rather

than verbal. Although none of theresults described are statisticallysignificant, consistent results such asthese might suggest that motor output(possibly for both hands) is beingdirected by the hemisphere that is notperforming the processing, but is,instead, somehow monitoring for theresult (a trme-sharing system, of sorts).

The results of this experiment alsoprovide some suggestions regarding thenature of a "spatial" task. Rizzolattie t al (1971) mentioned thatright-hemisphere superiority for therecogni tion of faces (their spatial task)might be related to some peculiarity ofphysiognomy as a visual pattern. Inthe present experiment, aright-hemisphere superiority wasfound for processing of complex,nonface spatial stimuli, suggesting thatit is the necessary spatial coding(nonlinguistic nature of the task) thatis the relevant factor.

Levy (1969) has suggested that theright hemisphere may be specializedfor Gestalt processing. Bradshaw andWallace (1971), however, foundevidence for a serial model for theprocessing of faces, and facial stimulihave been shown to yield aright-hemisphere advantage. Similarly,in the present spatial task, no evidencewas found to support the hypothesisof the employment of a Gestaltprocessor, even though, under otherexperimental conditions (Sekuler &Abrams, 1968), a similar matrices taskwas found to be Gestalt in nature. (Itshould be noted, however, thatSekuler and Abrams got better"Gestalt" results with two-cell stimulithan with four-cell stimuli. It may bethat the three-cell stimuli of thepresent experiment are too difficult tobe processed in an entirely Gestaltmanner.) In the present experiment,the highly significant linear trend forincreasing RT with increasing numberof cells in register for "different"stimuli is not suggestive of a Gestaltprocess. In spite of mitigating factorsthat might serve to explain whyprocessing was not Gestalt in nature inthe present matrices task, it is stillvalid to say that the data obtainedprovide no evidence to indicate thatright-hemisphere superiority must bedependent upon the Gestalt nature ofthe task. Rather, it seems sufficientthat processing for the task utilizesspatial cues. It is, however, possiblethat further experiments will indicatethat Gestalt tasks produce a morepronounced laterality effect.

The results of the presentexperiment, then, definitely support amodel of hemispheric specializationfor processing (as distinct from justoutput), the left hemispherespecialized for verbal tasks and theright hemisphere specialized for spatial

362 Perception & Psychophysics, 1972, Vol. 12 (4)

tasks. The results also indicate nosignificant effect of lateralization ofright-hand vs left-hand motor output,but there are some aspects of the datathat suggest, although they are notstatistically significant, that the locusof motor output may vary with taskand output demands.

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NOTE1. Support for the idea that the latency