Selective Attention and Interhemispheric Response Competition in the Split-Brain

Brain and Cognition 44, 511–546 (2000)

doi:10.1006/brcg.2000.1214, available online at http://www.idealibrary.com on

Selective Attention and Interhemispheric ResponseCompetition in the Split-Brain

Anthony Lambert and Neelam Naikar

Research Centre for Cognitive Neuroscience and Department of Psychology, University ofAuckland, Auckland, New Zealand

Published online August 18, 2000

The interfering effect of an unattended stimulus on processing of an attendeditem was studied in a single split-brain participant (LB) and in normal controls.Pairs of letters were presented to the left visual field (LVF), right visual field (RVF),or bilaterally. Participants attended to the rightmost letter while attempting to ignorethe leftmost letter. Responses associated with the attended and to-be-ignored letterscould be compatible or incompatible. Manual response latencies were generallyslower on Response Incompatible compared to Response Compatible trials. Nota-bly, LB displayed this effect on Bilateral trials, where target and distractor werepresented to opposite visual fields. LB was unable to perform a same–differentmatching task with bilateral letter stimuli, but was able to name bilateral lettersaccurately. Hence, in the bilateral condition, the ability to cross-compare letters wasdissociated from attentional interference and from letter naming. Implications ofthese findings are discussed. 2000 Academic Press

Key Words: split-brain; commissurotomy; selective attention.

The twin notions of parallel processing and modular architecture are cur-rently enjoying great popularity as theoretical constructs in cognitive neuro-science. For example, Merigan and Maunsell (1995) discuss processingwithin the parvo and magno divisions of the visual system and the degreeof independence and interconnection between the two systems. Posner andPetersen (1990) propose that attentional functions involve the operation ofseveral subsystems, which they argue are anatomically modular and whichoperate in close functional interaction. This article is concerned with an issue

Address correspondence and reprint requests to Anthony Lambert, Research Centre for Cog-nitive Neuroscience and Department of Psychology, University of Auckland, Private Bag92019, Auckland, New Zealand. E-mail: [email protected].

The authors thank Michael Corballis for invaluable assistance with testing and for helpfuldiscussions concerning this work. We also express our thanks to Eran and Dahlia Zaidel fortheir generous hospitality and use of laboratory facilities at UCLA and to Dr. Joseph Bogenfor permission to test the commissurotomy patient. We also thank Jeff Miller for valuablediscussions.

5110278-2626/00 $35.00

Copyright 2000 by Academic PressAll rights of reproduction in any form reserved.

512 LAMBERT AND NAIKAR

of this kind, which has been long standing in the field of laterality research.Following commissurotomy the two hemispheres of the human brain appearto become distinctly separate cognitive modules, operating in parallel, andlargely independent of each other. As is well known, commissurotomy, per-formed for the relief of intractable epilepsy involves severing the cerebralcommissures, notably the corpus callosum, which provide direct interconnec-tion between the left and right cerebral hemispheres. The consequence ofthis is a radical disconnection of cognitive processing in the left and rightcerebral hemispheres, the split-brain syndrome, which has been an enduringsource of inspiration in cognitive neuroscience. In the words of Roger Sperry(1985), following this operation ‘‘. . . each surgically disconnected hemi-sphere appears to have a mind of its own, but each cut off from and obliviousto, conscious events in the partner hemisphere.’’

The cognitive isolation of the two hemispheres is clearly evident whenlateralized testing procedures are used to restrict stimulus input and/or re-sponse output to a single hemisphere (see Trevarthen, 1990 for reviews).For example Eviatar and Zaidel (1994) found that two split-brain patients(LB and AA) were unable to cross-compare two-letter stimuli when thesewere directed to separate hemispheres, while good performance was obtainedwhen both letters were presented to a single hemisphere. Both Corballis(1995) and Zaidel (1995) conclude that interhemispheric transfer of complexvisual information, required for tasks such as letter matching, appears to beimpossible following commissurotomy.

A long standing paradox of the split-brain condition concerns the contrastbetween their performance under lateralized testing conditions, as describedabove, and everyday behavior. In most respects, the everyday behavior ofindividuals who have undergone commissurotomy is well integrated and ap-parently unified. This contrasts with the disunity, which is clearly apparentwith lateralized testing procedures. While it is clear that coherence and inte-gration in the everyday behavior of commissurotomy patients is achievedthrough interhemispheric interaction and cooperation, the nature of these in-teractions remains mysterious. In recognition of this problem, many recentstudies have been directly concerned with interhemispheric interaction andintegration in the split-brain (see Corballis, 1995; and Zaidel, 1995 for re-views of this work).

A strategy employed by several researchers has been to present commis-surotomy patients with multiple sources of information and to assess theextent of any interactions between information processing in the two cerebralhemispheres. For example, in a series of investigations by Lambert(1991, 1993, 1996) pairs of word, letter, and digit stimuli were presented tothe left and right visual fields of commissurotomy patients. Subjects wereasked to attend and respond to verbal information directed to the RVF-lefthemisphere while ignoring information directed to the LVF-right hemi-sphere. In different conditions, both inhibitory (Lambert, 1991, 1993, 1996)

ATTENTION AND THE SPLIT-BRAIN 513

and facilitatory (Lambert, 1996) interactions were observed between infor-mation processing in the surgically divided cerebral hemispheres of split-brain patients. It was proposed that these effects arose from implicit, noncon-sciously mediated interactions between attended and nonattended informa-tion sources, mediated via subcortical interconnections between the twohemispheres which are untouched by commissurotomy. It was also suggestedthat these intact processes may play an important role in maintaining theapparent behavioral unity and coherence displayed by split-brain patients ineveryday settings. An important reason for favoring this kind of interpreta-tion came from the observation by Lambert (1993) of interhemispheric nega-tive priming effects. Negative priming refers to an inhibitory effect in whichresponding is slower when an attended target is identical or related to a previ-ously ignored distractor. Allport (1989) has suggested that negative primingarises from selective processes that occur late in information processing, afterperceptual encoding, at the stage of ‘‘action selection.’’ Within this theoreti-cal framework, the observation of intact interhemispheric negative priming(Lambert, 1993) was nicely consistent with preservation of behavioral coher-ence following commissurotomy.

More recent studies by Pashler et al. (1994) and Reuter-Lorenz et al.(1995) have also suggested that attentional limitations at the stage of re-sponse production are an important locus for interhemispheric interaction inthe split-brain. Pashler et al. (1994) studied dual-task interference: On eachtrial subjects first made a choice response to a LVF stimulus (S1) with theleft hand and then made a choice response to a RVF (S2) stimulus with theright hand. When the delay between S1 and S2 was brief (50 or 150 ms)responses to S2 were clearly slower, for both commissurotomy patients andnormal control subjects. The authors interpreted this in terms of an atten-tional bottleneck operating at the stage of response selection. Since the effectwas similar in both subject groups the authors concluded that this responseselection bottleneck is unitary for commissurotomy patients as well as forthe neurologically intact controls.

Reuter-Lorenz et al. (1995) used a rather different paradigm, but also con-cluded that the stage of response production is an important locus of inter-hemispheric interaction in the split-brain. These authors report a carefullydesigned series of six experiments examining redundant target effects in asingle split-brain patient, JW. In this paradigm, subjects execute a simpleresponse to target stimuli. The redundancy effect refers to the gain in re-sponse speed that occurs when a subject is presented with more than oneinstance of the target stimulus. Reuter-Lorenz et al. found that JW showeda clear redundancy gain on trials where targets were presented bilaterally,one to the LVF and one to the RVF, and, second, that the magnitude of thisredundancy gain appeared significantly larger than that displayed by normalcontrol subjects. This pattern was not present on unilateral trials, where bothtargets were presented within the same visual field. Paradoxically, JW was


clearly impaired on tasks that required explicit detection of bilateral stimulusevents. With bilateral presentation, JW tended to neglect the LVF stimulus.

To explain these findings, Reuter-Lorenz and colleagues proposed thateach hemisphere of the split-brain competes for control of a unitary responseproduction process, similar to the response selection bottleneck identified byPashler et al. (1994). They suggested that following section of the corpuscallosum, interhemispheric competition for the control of action leads to atonic effect whereby each hemisphere inhibits response production by itscerebral partner. This process was invoked to explain the relatively slowresponse times on trials where a single target is presented to LVF or RVF.On bilateral trials, where two targets are presented, one to each visual field,they suggest that this inhibition is released as each hemisphere participatesin response preparation. This release from inhibition, they argue, is responsi-ble for the marked and abnormally large redundancy gain that JW displayedacross several experiments.

Thus, Lambert (1993), Pashler et al. (1994), and Reuter-Lorenz et al.(1995) all interpret interhemispheric interaction in split-brain subjects interms of selective processes at a relatively late stage of information pro-cessing, close to the stage of selecting and producing an action. However,it is worth noting that an important difference between these studies is thatwhile Lambert (1991, 1993, 1996) and Pashler et al. (1994) observed inter-hemispheric interaction effects that were comparable in magnitude to thosedisplayed by normal subjects, the interhemispheric effects observed by Reu-ter-Lorenz et al. (1995) appeared to be abnormally large, leading to a some-what different theoretical interpretation, as indicated above. The presentstudy had three aims. The first aim was to test for the presence of interhemi-spheric interaction effects in the split-brain, using a task which, classically,has been interpreted in terms of response competition. This task was a variantof the interference task devised by Eriksen and Eriksen (1974), which hasbeen studied extensively in the attention literature (see Broadbent, 1982,Eriksen, 1995). In this experimental design subjects are required to attendto target letters, while attempting to ignore distractor letters. Performancecan be compared across two main conditions. In the Response-Compatiblecondition, target and distractor letters are associated with the same response.In the Response-Incompatible condition, target and distractors are associatedwith different responses. Typically, it is found that response times are slowerin the Incompatible, relative to Compatible, condition, and this has beeninterpreted in terms of competitive interaction at the stage of response pro-duction (Broadbent, 1982; Eriksen, 1995). In the present experiment subjectswere presented with two letters simultaneously and were required to attendand respond to the rightmost letter, while ignoring the leftmost letter. Thistask was performed under three conditions: in the LVF-Right Hemispherecondition both letters were presented to the LVF; in the RVF-Left Hemi-sphere condition both letters were presented to the RVF; in the Bilateral


condition the target letter was presented to the RVF and the distractor letterwas presented to the LVF.1

The second aim was to provide a more direct test of the hypothesis thatimplicit attentional interactions remain intact, while explicit interhemispherictransfer of complex visual information is impossible following commissuro-tomy. In the earlier studies of Lambert (1991, 1993, 1996) explicit integra-tion of information directed to the two visual fields was not required—theabsence of explicit interhemispheric transfer was assumed rather than dem-onstrated directly. Therefore, the experiment reported below included botha selective-attention task, as described above, and a control task that requiredexplicit integration of information presented to each visual field. This in-volved deciding whether the two simultaneously presented letters were thesame or different. A single commissurotomy patient (LB) from the Californiaseries of Bogen and Vogel and 12 normal control subjects were tested. Forthe selective attention task it was predicted that processing of an attendedobject presented to the RVF-left hemisphere would be influenced by re-sponse competition, due to processing of the unattended distractor object bythe LVF-right hemisphere. It was predicted that this effect would be presentin the performance of both the commissurotomy patient and the controls. Incontrast, it was predicted that the split-brain patient LB would be unable toexplicitly integrate information presented simultaneously to the LVF andRVF, while the normal control subjects should have little difficulty with thistask. While previous reports have shown that LB is unable to cross-compareletter stimuli presented to the left and right visual fields (Eviatar & Zaidel,1994), it has also been reported that LB is able to name letters presented tothe LVF and appears to perform especially well when the stimulus set issmall. Johnson (1984) found that with a stimulus set size of two LB named100% of letters presented to his left visual field (see also Zaidel, 1995). Inview of this, a further control task was included in which LB was requiredto name letter pairs presented to LVF, RVF, and bilaterally.

In a recent report Zaidel (1995) has described a series of experiments thatalso tested the hypothesis that interhemispheric attentional interaction effects

1 Note that this design does not allow conclusions to be drawn based on within-subjectdifferences in distractor interference across the LVF-Right Hemisphere, Bilateral, and RVF-Left Hemisphere conditions. Such comparisons necessarily involve confounding visual fieldwith the retinal eccentricity of either target or distractor stimulus. However, this issue is notrelevant to our experimental aim of testing for the presence of response competition in the split-brain participant and comparing the amount of response competition shown by this subject withthat shown by the control subjects. In addition, the element of spatial uncertainty providedby the three conditions was desirable for two reasons: (1) uncertainty concerning the locationof the next stimulus is generally seen as a good way of encouraging central fixation in lateralityresearch and (2) spatial certainty is likely to be accompanied by a task strategy in whichspatial attention is strongly focused upon the target location. Under these conditions, distractorinterference effects are likely to be attenuated or absent (LaBerge et al., 1991).


are present following commissurotomy. In these experiments interhemi-spheric interaction effects were either absent or statistically fragile, leadingZaidel to conclude that evidence for implicit interhemispheric interactioneffects is weak (p. 525). In addition, Lambert (1996) reported that interhemi-spheric interaction effects, assessed via performance of selective attentiontasks, can show striking variation across different sessions of testing. In viewof these mixed results, and since the conclusion that interhemispheric atten-tional interaction effects are present in the split-brain is of some theoreticalimportance, a third aim of the present study was to subject the basic hypothe-sis, that interhemispheric attentional interaction effects can occur in the ab-sence of cerebral commissures, to further empirical test.

EXPERIMENT 1

Method

Subjects

A single split-brain patient (LB) was tested. LB was 42 years old at the time of testing andunderwent complete section of the corpus callosum and anterior and hippocampal commissuresin 1965. Detailed information concerning the surgery and neurological status of LB can befound in Bogen and Vogel (1975). Bogen, Schultz, and Vogel (1988) confirmed that the fore-brain commissures were completely sectioned in LB, using magnetic resonance imaging(MRI). Twelve adult right-handed male volunteers also took part as control subjects.

Apparatus

An IBM compatible personal computer with a VGA resolution screen (640 3 350 pixels)was used for presenting stimuli and recording responses. Timing and display routines werewritten in Turbo Pascal 6.0. A chinrest was used to control viewing distance at approximately53 cm.

Display stimuli

A small fixation cross subtending 0.6 3 0.6° was always present on the screen. Experimentalstimuli were the letters A and B, presented for 133 ms. Each letter subtended approximately5.4 (height) 3 3.5° (width). Letters were presented with the inner edge either 1 or 6.7° fromfixation.

Procedure

Subjects were seated in front of the display screen and informed that on each trial of theexperiment, two letters would appear. The letters appeared in one of three spatial arrangements.In the LVF-Right Hemisphere condition both letters appeared in the LVF. In the RVF-LeftHemisphere condition both letters appeared in the RVF. In the Bilateral condition, one letterappeared in the LVF and one in the RVF. Subjects were instructed that on each trial of theexperiment it was extremely important to maintain central fixation.

Interference task. LB performed this task before the Matching task or the Naming task.Twelve control subjects also performed this task. Subjects were instructed that on each trialtheir task was to decide whether the rightmost member of the letter pair was A or B and to


indicate their choice by pressing one of two keys. The keys ‘‘D’’ and ‘‘K’’ were coveredwith sticky labels marked with an ‘‘A’’; the keys ‘‘M’’ and ‘‘C’’ were covered with labelsmarked ‘‘B’’. When responding with the left hand subjects used the ‘‘D’’ key to indicate‘‘A’’ and the ‘‘C’’ key to indicate ‘‘B.’’ When responding with the right hand, subjects pressedthe ‘‘K’’ key to indicate ‘‘A’’ and ‘‘M’’ key to indicate ‘‘B’’. Subjects were instructed toignore the leftmost member of each letter pair and to respond to the target (rightmost) letteras quickly as possible, while making as few errors as possible.

Each trial of the experiment began when the subject indicated that they were fixating cen-trally and ready. The experimenter then pressed the key on the top right of the keyboard (‘‘/’’) to initiate the trial. One second later a letter pair was presented for 133 ms in one of thethree spatial arrangements described above. The subject then responded to the target letter.Incorrect responses were followed by a 200-ms warning ‘‘beep.’’ The next trial then began.

Each session of testing on the Interference Task comprised the following: 8 practice trialsfollowed by four blocks of 24 trials while responding with the right hand; and 8 practice trialsfollowed by four blocks of 24 trials while responding with the left hand. The computer pausedafter each trial block, and subjects were advised to take a brief rest. Within each block oftrials there was an equal number of response compatible and response incompatible trials; anequal number of trials where the target letter was ‘‘A’’ and where the target letter was ‘‘B’’;an equal number of trials where the distractor letter was ‘‘A’’ and where the distractor was‘‘B’’; and equal numbers of LVF-Right Hemisphere, RVF-Left Hemisphere, and Bilateraltrials. Each of these conditions varied pseudorandomly from trial to trial within each block.

LB took part in two sessions of testing. In the first he performed four trial blocks with left-hand responding followed by four blocks with right-hand responding. In the second sessionorder of hand use was reversed. Each of the 12 control subjects also took part in two testingsessions following the same procedure as for LB, with one proviso. For 6 subjects the orderof response-hand use across the two sessions was Left–Right (Session 1), followed by Right–Left (Session 2); for the other six subjects the order was Right–Left followed by Left–Right.

Matching task. LB performed this after performing the Interference task. Four control sub-jects also performed this task. Subjects were instructed that on each trial their task was todecide whether two letters presented on the screen were the same (AA or BB) or different(AB or BA). ‘‘Same’’ responses were indicated by pressing the ‘‘N’’ key on the keyboard;different responses were indicated by pressing the ‘‘B’’ key. Subjects were instructed thataccuracy rather than speed of responding was of primary importance. Display characteristics,timing, and stimulus randomization were identical with the Interference task.

LB performed 8 practice trials followed by two blocks of 48 experimental trials respondingwith the right hand. This was followed by 8 practice trials and two blocks of 48 experimentaltrials with left-hand responding.

Four control subjects performed the Matching task, following the same procedure as forLB with one proviso. Two subjects performed the task with right-hand responding followedby left-hand responding; this was reversed for the other two subjects.

Naming task. LB performed this task several months after completing the Interference andMatching tasks. LB was instructed to maintain central fixation and name the left and then theright member of each letter pair presented on the screen. Display characteristics, timing, andstimulus randomization were identical with the Interference and Matching tasks. Control datawere not collected, since this task was trivially easy for neurologically intact individuals.

Results

Interference Task

Response latencies. Since the experimental design involved assessing re-sponse latency effects within individual subjects, two measures were usedto minimize error variance in the latency data. Within each session, the first


FIG. 1. Response-latency results for split-brain participant LB in the Interference task ofExperiment 1.

24 trials of responding with each hand were treated as a warm-up periodand were not included in the main analysis. In addition, correct responseswith a latency of less than 100 ms or more than 2 standard deviations awayfrom the mean correct response time for that subject were excluded fromanalysis. This accounted for 3.5% of experimental trials for LB and 3.7%of control subjects’ trials. Exclusion of these outliers did not influence theoverall pattern of results. Both outlier exclusion and use of a warm-up periodare relatively common in studies of mental chronometry (e.g., see Miller,1987, 1992). Response latency data for LB and for the control subjects areshown in Figs. 1 and 2 and in Table 1.

Participant LB. Individual response latencies for participant LB were en-tered into an analysis of variance, with trials entered as a random variable.There were three factors: Response Compatibility (Compatible vs Incompati-ble), Visual Field-Hemisphere (LVF-Right Hemisphere, Bilateral, and RVF-Left Hemisphere), and Responding Hand (Left Hand vs Right Hand). Therewas a significant main effect of Response Compatibility F(1, 248) 5 18.44,p , .001. Responses were slower on incompatible compared to compatibletrials (669 ms vs 572 ms). This effect is illustrated in Fig. 1. None of theinteraction terms involving this response compatibility effect approached sig-


FIG. 2. Response-latency results for control subjects in the Interference task of Experi-ment 1. (A) Mean response times for all 12 control subjects. (B–E) Individual results for fourselected control subjects who were matched with LB with respect to mean response latency.

nificance (all F , 1). There was a significant main effect of visual field,F(2, 248) 5 11.21, p , .001 (see Fig. 1). Post hoc analysis of this effectusing Tukey’s studentized range (HSD) test (α 5 .05) showed that LB re-sponded more slowly in the LVF-Right Hemisphere condition (681 ms) rela-tive to either the Bilateral (556 ms) or RVF-Left Hemisphere condition (615ms). Response latencies in the Bilateral and RVF-Left Hemisphere condi-tions did not differ. There was a significant main effect of Responding Hand,


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F(1, 248) 5 14.91, p , .001. LB responded more rapidly when using hisright compared to his left hand (574 ms vs 659 ms). A full breakdown of LB’sperformance across responding hand, visual field, and response compatibilityconditions is shown in Table 1.

Since our experimental hypothesis was crucially concerned with LB’s per-formance in the Bilateral condition, a planned comparison evaluated the ef-fect of Response Compatibility within the Bilateral condition. This con-firmed that within the Bilateral condition LB’s responses were significantlyslower on Response-Incompatible (593 ms) relative to Response-Compatible(517 ms) trials, F(1, 248) 5 4.18, p , .05 (see Fig. 1). When data from theBilateral condition were considered on their own, in a separate analysis, therewas no evidence of any interaction between the Response-Compatibility ef-fect and Responding Hand, F , 1 (see Table 1).

Control subjects. Response latency results for the 12 control subjects areshown in Fig. 2A. Mean response times were calculated within each condi-tion, for each control subject. These data were entered into analysis of vari-ance with the same three factors as for patient LB. There was a significantmain effect of Response Compatibility, F(1, 11) 5 15.32, p , .005. Re-sponses were slower on incompatible relative to compatible trials (501 msvs 476 ms; see Fig. 2A). There was also a significant main effect of VisualField-Hemisphere, F(2, 22) 5 25.51, p , .001. Post hoc analysis of thiseffect using Tukey’s studentized range (HSD) test (α 5 .05) showed thatthe control subjects responded more slowly on RVF-Left Hemisphere trials(522 ms), relative to both Bilateral (470 ms) and LVF-Right Hemisphere(474 ms) trials (see Fig 2A). Response times in the Bilateral and LVF-RightHemisphere conditions did not differ. There was a significant interactionbetween Response Compatibility and Visual Field, F(2, 22) 5 5.69, p ,.025. Pairwise comparisons of the Response-Compatibility effect across Vi-sual Field-Hemisphere conditions revealed the following. In the RVF-LeftHemisphere condition there was a 51-ms difference between the Response-Incompatible (548 ms) and Response-Compatible (497 ms) conditions thatwas significantly greater than the 17-ms effect in the Bilateral condition (In-compatible 478 ms; Compatible 461 ms; F(1, 11) 5 4.94, p , .05) andsignificantly greater than the 7-ms effect in the LVF-Right Hemisphere con-dition (Incompatible 478 ms; Compatible 471 ms; F(1, 11) 5 16.85, p ,.005; see Fig. 2A). Finally, there was a significant three-way interaction be-tween Response Compatibility, Visual Field, and Responding Hand, F(2,22) 5 3.80, p , .05, which is illustrated in Table 1. As this table shows,there is a fairly complex pattern of variation in the magnitude of the responsecompatibility effect as a function of both visual field and responding hand.In the RVF-Left Hemisphere condition, response compatibility effects ofsimilar magnitude were obtained with left-hand and right-hand responding(47 and 53 ms respectively). In the Bilateral condition the difference in meanresponse time between incompatible and compatible trials was somewhat


larger with left-hand than with right-hand responding (30 ms vs 5 ms), whilein the LVF-Right Hemisphere condition this difference was present withright-hand but not with left-hand responding (16 ms vs 0 ms).

Again, since performance in the Bilateral condition was of particular inter-est, a planned comparison evaluated the effect of Response Compatibilitywithin this condition. This showed that for control subjects in the Bilateralcondition the difference in latency between Response-Compatible (461 ms)and Response-Incompatible trials (478 ms) did not attain significance, F(1,22) 5 3.52, p , .10.

Does LB show an abnormal pattern and amount of response competition?The above analyses show the following. Overall, LB shows a highly signifi-cant response competition effect that does not vary as a function of VisualField-Hemisphere or Responding Hand. Within the Bilateral condition, LBshows a significant response competition effect which does not vary ac-cording to Responding Hand. In contrast, normal control subjects show arather complex pattern of variation in the magnitude of response competitionacross Visual Field-Hemisphere and Responding Hand conditions. Overall,LB’s responses were 90 ms slower on response-incompatible trials. On aver-age, control subjects’ responses were 24 ms slower on response-incompatibletrials. These results prompt two questions which are closely related to theissues that Reuter-Lorenz et al. (1995) considered in relation to patient JW.First, do the present results demonstrate that commissurotomy patient LBshows an abnormal pattern of response competition effects across the differ-ent conditions of the experiment? Second, is the magnitude of response com-petition effect shown by LB abnormally large in some or all conditions ofthe experiment? In considering these questions and comparing the perfor-mance of LB with that of the controls, it should be remembered that the datashown in Fig. 1 (and Table 1) represent the performance of a single individ-ual, LB, while those shown in Fig. 2A (and Table 1) represent the averagedperformance of 12 individuals. In order to answer the question of whetherLB is different, further direct comparisons of his performance with that ofthe control subjects were undertaken. This was done in two ways.

1. The performance of each control subject was analyzed individually,with trials entered as a random variable, as in the analysis of LB’s data.Eight of 12 subjects showed significant main effects of response competition.One of these individuals (MJ) resembled LB, in showing a significant maineffect of Response Compatibility, that did not interact with any other factor(all interaction F ratios ,1). These analyses also revealed striking differ-ences within the control group with respect to response competition. For 3of 12 control subjects the Response-Compatibility effect failed to approachsignificance either as a main effect (F , 1) or in interaction with other fac-tors. This shows that the pattern of response competition effects shown byLB is not abnormal. The same pattern of effects was displayed by one ofour control subjects (MJ).


2. In assessing whether the magnitude of response competition shown byLB was abnormally large it was necessary to control for the possibility thatthe magnitude of response competition may be related to overall responsespeed. For example, the size of the effect may tend to be smaller for thoseindividuals and experimental conditions with relatively fast response laten-cies (i.e., floor effects may be present in some conditions). Pashler et al.(1994) make a similar point. Hence, in comparing LB with control subjectsit was important to address the problem of how to scale the magnitude ofthe response competition effect with respect to overall response speed. Thiswas done by selecting a subset of control subjects who were matched asclosely as possible to LB with respect to mean response time. This groupcomprised the four slowest responding control subjects; the average responsetime of these individuals was identical with LB (617 ms). The performanceof these four subjects is shown in Figs. 2B–2E. Four separate analyses ofvariance were then carried out that compared the performance of each indi-vidual with LB. In performing these analyses, critical values were adjustedto take account of the fact that the same comparisons were made betweenLB’s performance and that of a control subject on four repeated occasions.These analyses showed that (1) LB showed significantly more response com-petition than control subjects JW (p , .05) and HK (p , .002) (compareFig. 1 with Figs. 2D and 2E) and (2) in contrast, there was no difference inthe magnitude of response competition between LB and control subject JH(F , 1, compare Fig. 1 with Fig. 2B) or between LB and control subjectTM [F(1, 496) 5 1.41, n.s.; compare Fig. 1 with Fig. 2C]. None of the higherorder interaction terms that included Response Compatibility and the factorcomparing LB with JH or TM approached significance in these analyses.Within the Bilateral condition JH showed the largest mean difference be-tween Compatible RT and Incompatible RT, with both left-hand and right-hand responding. Hence, the size of the response compatibility effect shownby LB in the Bilateral conditions fell within the range observed for normalcontrols.

These detailed comparisons between LB and the control group highlightseveral important points. First, within a group of normal neurologically intactindividuals there are striking differences with respect to response competi-tion effects in this task. LB resembles some of the control subjects withrespect to the pattern of response competition across different experimentalconditions and with respect to the magnitude of the effects. Although thesize of the response competition effect shown by LB is large in relation tothe average performance of the control group, it is not abnormally large incomparison with other individuals with relatively long response times.

Errors in the interference task. The percentage of errors made by LB andby the 12 control subjects on the Interference task is shown in Table 2. Thenumber of errors made by each control subject was entered into analysis ofvariance with the same factors as for the RT data. The main effects of Visual


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FIG. 3. Performance accuracy in the Matching task of Experiment 1. The left panel showsresults for LB; the right panel shows results for the control subjects.

Field-Hemisphere [F(2, 22) 5 3.17, n.s.] and of Response Compatibility[F(1, 11) 5 2.98, n.s.] both fell short of significance, as did the interactionbetween them [F(2, 22) 5 2.59, n.s.]. As Table 2 shows, in general thepattern of errors made by LB resembled that of the control subjects.

Matching Task

Percentage correct. The percentage of correct responses made by LB andby the four control subjects on the Matching task is shown in Fig. 3. Regard-less of responding hand, LB’s performance was clearly better than chancein both LVF-Right Hemisphere conditions and both RVF-Left Hemisphereconditions (Binomial test, p , .001 in all four cases). In contrast, in thebilateral condition, LB’s performance was no better than chance on the bino-mial test when responding with either the left hand (47% correct) or theright hand (62% correct). The four control subjects all attained near-perfectperformance in all of the experimental conditions.

Naming Task

Percentage correct. The percentage of correct responses made by LB onthe Naming task is shown in Table 3. As this table shows, LB attained ahigh level of accuracy on this task. As noted above, control data were not

TABLE 3Letter-Naming Accuracy (%) of LB in Experiment 1

Visual field—location

LVF LVF RVF RVF

A A 1 A 1 AA A 1 & & 1 A A

A 1 A 1 A ANamingaccuracy 91.7 95.8 97.9 95.8(%) of LB


collected for this task, which was trivially easy for neurologically intact sub-jects.

Discussion

On the interference task LB’s performance was essentially normal. Whenhe attended to a letter in the RVF and ignored an LVF letter he showed asignificant and normal amount of response competition. Although on aver-age, LB showed considerably more response competition than the controlsubjects (90 ms vs 24 ms, see Table 1), the magnitude of this effect wasnot abnormally large, especially in comparison with control subjects whoresponded with latencies comparable to LB (see Figs. 1 and 2). In the crucialBilateral conditions, the response competition effect shown by LB fell withinthe normal range. These data extend the findings reviewed earlier in theIntroduction and provide further support for the conclusion that attentionalinteractions remain intact following commissurotomy. Effects of this kindhave now been reported for tasks in which commissurotomy patients:(i) attended to an RVF word while ignoring an LVF word (Lambert, 1991),(ii) attended to an RVF digit while ignoring an LVF digit (Lambert, 1993,1996), and (iii) attended to an RVF letter while ignoring an LVF letter (Lam-bert, 1996). At a broad level, the findings are also consistent with the findingsand theoretical proposals of Pashler et al. (1994) and Reuter-Lorenz et al.(1995) concerning the presence of interhemispheric response competition inthe split-brain. The results also find a parallel with those of Reuter-Lorenzand colleagues in that interhemispheric response competition was clearlydissociated from performance of one of our tests of explicit processing(Same–Different matching), though not from the other (Naming—seebelow).

At a more specific level, the present results prompt a somewhat differentinterpretation from that offered by Reuter-Lorenz et al. (1995), since themagnitude of interhemispheric response competition in the split-brain wasessentially normal in the present study, but apparently abnormal in the exper-iments of Reuter-Lorenz and colleagues. How then, should one interpret theintervisual field response competition effects observed here? Since the ef-fects displayed by LB appeared normal, the most parsimonious option isto invoke the same explanation as that traditionally applied to this kind ofattentional effect—namely that the effect arises from competitive interfer-ence at the stage of response production. According to this view, on bilateraltrials LB’s left hemisphere processed the RVF (target) letter, while simulta-neously and in parallel the right hemisphere processed the LVF distractorletter. Each hemisphere then processed its respective letter through to thelevel of response production. Clearly the RVF-Left Hemisphere letter ulti-mately controls the response on the vast majority of trials for LB, and pre-sumably for the control subjects also (see Table 2). However, a significantamount of competition for control of the response comes from the LVF-


Right Hemisphere, resulting in longer latencies on response incompatibletrials. In the absence of cerebral commissures, this effect is presumably medi-ated via interaction between processing at the cortical level and subcorticallinks, which remain undivided by commissurotomy (see General Discus-sion).

In performing the two control tasks (Matching and Naming) LB’s perfor-mance was paradoxical, but consistent with previous observations. In theNaming task, LB achieved a high level of accuracy for LVF, RVF, and bilat-eral letters (see Table 3). This is consistent with work reported by Johnson(1984) in which LB was able to name LVF letters accurately. Johnson re-ported that LVF letter-naming accuracy was especially high with small stim-ulus sets: with a set size of two, LB achieved 100% accuracy. Clearly, thisfigure agrees well with the data shown in Table 3. In contrast, when per-forming the Matching task, LB showed the classic disconnection syndrome.In agreement with earlier findings (Eviatar and Zaidel, 1994), he was ableto match letters when these were directed to a single hemisphere, but wasunable to perform the task when one letter was directed to each surgicallydisconnected hemisphere. Zaidel (1995) also noted that while LB was ableto name letters and colors presented to either visual field, he was unable tocross-compare these stimuli, even with a small set size comprising just threecolors.

While LB’s performance in the Naming and Matching tasks is consistentwith earlier observations, this pattern of results makes it difficult to providea simple answer to the question posed in the Introduction as to whetherinterhemispheric attentional interactions are mediated by implicit processes.Naming performance (which is often used as the test of explicit access toinformation) was clearly dissociated from matching performance. Why is itthat LVF information can be named, but cannot be used when LB is per-forming the Same–Different Matching task? Two explanations appear feasi-ble. One is that LB’s right hemisphere is capable of initiating speech directly.That is, commissurotomy makes it impossible for LB to cross-compare LVFand RVF information, but LVF naming can be achieved by direct controlof the speech apparatus from the right hemisphere using a mechanism thatis distinct and independent from RVF naming controlled by the left hemi-sphere. The issue of the extent to which LB and other commisurotomy pa-tients of the California series possess right-hemisphere speech has been amatter of some controversy (Gazzaniga, 1983; Zaidel, 1983). A second possi-bility, which may be more attractive and parsimonious in not requiring right-hemisphere speech, involves proposing that different representations areused by LB for naming and same–different judgments. With a set size ofjust two, accurate left-hemisphere naming of stimuli processed by the righthemisphere could be achieved by subcortical transfer of a simple binarycode. Reuter-Lorenz et al. (1995) suggest that such transfer may take theform of a simple response readiness signal, such as ‘‘respond A’’ or ‘‘re-


spond B.’’ It may be difficult or impossible for split-brain subjects to drawupon response-level representations when performing same–different com-parisons. That is, in both the naming and the distractor-interference task,response-code representations may be input directly to response-productionprocesses, producing the observed performance effects. However, an accu-rate matching response requires a further transformation involving a compar-ative decision concerning LVF and RVF codes. This decision process mayoccupy a more central locus, in information processing terms, and may re-quire perceptual representations that cannot be transferred if the corpus callo-sum has been sectioned. Lambert (1996) also reported evidence that is con-sistent with the proposal that interhemispheric attentional interactions(Lambert, 1991, 1993) may have been mediated by a simple binary coderepresenting the response category of unattended LVF stimuli.

In common with Reuter-Lorenz et al. (1995) and Pashler et al. (1994),our interpretation of LB’s performance in the Bilateral condition involvesproposing a subcortical mechanism whereby response processes initiated byeither hemisphere interact with each other. It seems reasonable to suggestthat such a mechanism may serve an important function in maintaining theintegration and unity of purpose apparent in the everyday behavior of split-brain individuals. Since subjects responded with a single hand within eachsession, this interpretation requires one to assume that each hemisphere ofLB is able to control the responses of the ipsilateral hand as well as thecontralateral hand. That is, when LB responded to an RVF letter (in theBilateral condition) there was competition for control of the response fromthe LVF letter processed by the right hemisphere. As noted earlier, this effectshowed no variation according to whether LB responded with the left orright hand. The presence of ipsilateral motor control for tasks involving sim-ple button-press responses has been proposed in previous research in whichLB has participated (Clarke & Zaidel, 1989) and in research involving othersplit-brain individuals (Trope et al., 1987). The presence of ipsilateral re-sponse control is also consistent with two further aspects of LB’s perfor-mance in this study. First, although LB performed rather less accurately inthe crossed visual field–response-hand conditions (i.e., LVF–right hand,RVF–left hand), compared with same visual field–hand combinations(10.4% error vs 3.1% error), his performance in the crossed conditions wassubstantially better than chance. With respect to response latency, althoughon average LB responded 15 ms more slowly in the crossed visual field–response-hand combinations, there was no consistent interaction between vi-sual field and responding hand [F(2, 248) 5 1.03, n.s.].

It is of particular interest than in the bilateral condition where subjectsresponded to a RVF target, LB was clearly influenced by the LVF distractorwhen responding with his right hand. Indeed, the response competition effectremained significant when data from the Bilateral, Right Hand conditionwere analysed separately [F(1, 42) 5 4.84, p , .05]. One would generally


expect that in the absence of cerebral commissures, responding to a RVFtarget using the right hand would maximally ‘‘encapsulate’’ cognitive pro-cessing within the left hemisphere. In discussing split-brain performanceZaidel (1995) and Eviatar and Zaidel (1994) refer to ipsilateral visual field–response-hand combinations as ‘‘pure hemisphere’’ processing conditions.Accordingly, an interpretation of the effect in terms of a subcortical mecha-nism that integrates the control of action, is especially plausible (see GeneralDiscussion below).

EXPERIMENT 2

Experiment 2 had two aims. The first was to replicate the effect observedin Experiment 1 in which LB’s response latencies were significantly sloweron Incompatible relative to Compatible trials in the Bilateral condition.

The second aim arose from an ambiguity in the present results concerningan appropriate interpretation for the overall difference in latency betweenthe Compatible and Incompatible conditions. According to the interpretationoffered above, the effect is attributed to a relative slowing of response latencyon incompatible trials due to response competition. Previous research onattentional interference in normal individuals (see Broadbent, 1982; Eriksen,1995 for reviews) certainly suggests that this provides the most likely expla-nation for performance of the neurologically intact control subjects. Never-theless, it remains conceivable that the effect arose either partially or wholly,due to a relative quickening on response compatible trials. An effect of thiskind could arise from priming that influences perceptual analysis of the targetletter. Perhaps more plausibly, it could reflect a coactivation effect operatingat the stage of response production.

Accordingly, Experiment 2 aimed to assess the contribution of responsecoactivation/priming to the effect observed in Experiment 1, in which laten-cies were slower on Incompatible relative to Compatible trials. This wasdone by including a third experimental condition. In addition to Response-Compatible and Response-Incompatible conditions, a Neutral condition wasincluded, where subjects were presented with a distractor item (the letter‘‘X’’) that was not associated with any response. If responses are faster in theCompatible condition, relative to Neutral, it can be concluded that responsecoactivation/priming is present and contributes to the difference in latencybetween Response Compatible and Incompatible conditions.

At first sight, the interpretation offered by Reuter-Lorenz et al. (1995) forJW’s performance in the redundant-targets task appears to predict that oursplit-brain subject should display an abnormally large amount of responsecoactivation in the bilateral condition. However, a close reading of their re-port shows that this is not a necessary prediction of their model. Althoughthey do propose that JW shows an abnormal amount of response coactiva-tion, this is explained in terms of a release of tonic response inhibition that


operates mutually and interhemispherically in split-brain individuals. Thismutual inhibition was seen as an adaptive consequence of callosotomy thatmay have arisen from interhemispheric competition for the control of re-sponse mechanisms. Hence, for Reuter-Lorenz et al. the absence of responsecompetition is an important component of the coactivation effect they ob-served in JW’s performance. In view of this, it is not entirely clear whethertheir model predicts an abnormal amount of response competition, an abnor-mal amount of response coactivation, or both, in LB’s performance in theBilateral condition of Experiment 2.

Method

Subjects

A single split-brain patient (LB) was tested, as in Experiment 1. Twelve adult right-handedmale volunteers also took part as control subjects. The ages of the control subjects rangedfrom 35 to 52 years with a mean of 43.5 years.

Apparatus

This was the same as for Experiment 1.

Display Stimuli

In general, this was the same as for Experiment 1. The Response-Compatible conditionaccounted for 1/3 of trials; 1/3 of trials were Response-Incompatible. The Neutral conditionaccounted for the remaining 1/3 of trials, in which the distractor stimulus was the letter ‘X.’’

Procedure

In general, this was the same as for the Interference task of Experiment 1. Each session oftesting on the Interference task comprised the following: 8 practice trials followed by fourblocks of 36 trials while responding with the right (or left) hand, then 8 practice trials followedby four blocks of 36 trials while responding with the left (or right) hand. The computer pausedafter each trial block, and subjects were advised to take a brief rest. Within each block oftrials there was an equal number of Response-Compatible (AA or BB), Response-Incompatible(AB or BA), and Neutral trials (XA, or XB); an equal number of trials where the target letterwas ‘‘A’’ and where the target letter was ‘‘B’’; and equal numbers of LVF-Right Hemisphere,RVF-Left Hemisphere, and Bilateral trials. Each of these conditions varied pseudorandomlyfrom trial to trial within each block.

LB took part in three sessions of testing. In the first he performed four trial blocks withleft hand responding, followed by four blocks with right hand responding. In the second andthird sessions order of hand use was reversed. Each of the 12 control subjects also took partin three testing sessions following the same procedure as for LB.

Results

Response Latencies

Response latencies were analyzed in the same way as for Experiment 1.That is, the first 24 trials of responding with each hand were treated as a


FIG. 4. Response-latency results for split-brain participant LB in Experiment 2.

warm-up period, and correct responses with a latency of less than 100 msor more than 2 standard deviations away from the mean correct responsetime for that subject were not included in the analysis.2 This accounted for2.3% of trials for LB and 3.4% of trials for the control subjects. Exclusionof these outlier responses did not alter the overall pattern of results. Responselatency data for LB and for the control subjects are shown in Figs. 4 and 5.

Participant LB. Response latency results for LB are shown in Fig. 4. Indi-vidual correct response latencies for patient LB were entered into an analysisof variance with trials entered as a random variable. There were three factors:Response Compatibility (Compatible, Incompatible, Neutral); Visual Field-Hemisphere (LVF-Right Hemisphere, Bilateral, RVF-Left Hemisphere); andResponding Hand (Left Hand vs Right Hand). There was a significant maineffect of Response Compatibility [F (2, 304) 5 4.22, p , .02]. This effectis shown in Fig. 4. Post hoc analysis of this effect using Tukey’s studentized

2 Although each experimental session comprised 144 trials, due to a programming error,data were recorded for the first 96 trials only (i.e., the number of trials per session employedin Experiment 1).


FIG. 5. Response-latency results for control subjects in Experiment 2. (A) Mean responsetimes for all 12 control subjects. (B–E) Individual results for the four selected control subjects(see text).

range (HSD) test (α 5 .05) showed that LB responded more slowly in theResponse-Incompatible condition (1104 ms) relative to the Response-Com-patible condition (937 ms). Response latencies in the Neutral condition (1020ms) did not differ significantly from either the Compatible or Incompatibleconditions. There were no interactions between the Response Compatibilityeffect and Visual Field or Responding Hand (all F , 1). A full breakdown


of LB’s performance across all conditions of the experiment is shown inTable 4. There was a significant main effect of Visual Field [F(2, 304) 54.03, p , .02], which is shown in Fig. 4. However, in post hoc analysis ofthis effect using Tukey’s test none of the between-condition comparisonsattained significance. As in Experiment 1, LB responded more rapidly withhis right hand (919 ms) than his left hand (1120 ms) [F(1, 304) 5 20.66,p , .001]. The interaction between Visual Field and Responding Hand ap-proached significance [F(2, 304) 5 2.35, p , .10] (see Table 4: When LBresponded with his right hand, his responses tended to be faster in the RVFcondition compared to the LVF or Bilateral conditions; when LB respondedwith his left hand, his responses tended to be faster in the LVF than in theRVF or Bilateral conditions).

Planned comparisons were used to evaluate differences in latency betweenthe Response-Compatible-Incompatible, and Neutral conditions within theBilateral condition. The difference between the Response-Incompatible and-Compatible conditions was significant on a directional test [F(1, 304) 53.49, p , .05]. Hence, the interhemispheric response compatibility effectobserved in Experiment 1 was replicated. There was no difference in latencybetween the Compatible and Neutral conditions (F , 1). Accordingly, therewas no evidence of a response coactivation effect in the Bilateral condition.

Control subjects. Response latency results for the 12 control subjects areshown in Fig. 5A. Mean response times were calculated within each condi-tion for each control subject. These data were entered into an analysis ofvariance with the same three factors as for patient LB. There was a significantmain effect of Response Compatibility [F(2, 22) 5 19.91, p , .001]. Thiseffect is shown in Fig. 5A. Post hoc analysis of this effect using Tukey’sstudentized range (HSD) test (α 5 .05) showed that responses were sloweron Incompatible trials (430 ms) than on Compatible (399 ms) or Neutral(404 ms) trials. Response latencies on Compatible and Neutral trials did notdiffer. Hence, the control subjects showed no evidence of responsecoactivation/priming in their performance of this task. There was a signifi-cant main effect of Visual Field-Hemisphere [F(2, 22) 5 33.15, p , .001](see Fig. 5A). Post hoc analysis of this effect using Tukey’s studentizedrange (HSD) test (α 5 .05) showed that the control subjects responded moreslowly on RVF-Left Hemisphere trials (427 ms) than on LVF-Right Hemi-sphere trials (408 ms). Responses on LVF trials were in turn slower than onBilateral trials (399 ms). There was a significant interaction between Re-sponse Compatibility and Visual Field [F(4, 44) 5 3.65, p , .025]. Thisinteraction is also illustrated in Fig. 5A. This figure shows that, as in Experi-ment 1, the overall difference between Compatible and Incompatible trialswas somewhat larger in the RVF condition (41 ms) than in the Bilateral (22ms) and LVF (28 ms) conditions.

Again, since performance in the Bilateral condition was of particular inter-est, planned comparisons evaluated differences in latency between the Re-


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sponse-Incompatible, -Compatible, and Neutral conditions. These showedthat in the Bilateral condition control subjects responded more slowly onIncompatible (413 ms) than on Compatible (390 ms) trials [F(1, 44) 5 17.00,p , .001]. Latencies in the Compatible (390 ms) and Neutral (394 ms) condi-tions did not differ (F , 1).

Does LB show an abnormal pattern of response competition? As before,further analyses were undertaken in order to compare LB’s performance withthose of the control subjects. The performances of 7/12 control subjects re-sembled LB in showing a significant main effect of Response Compatibilitywhich did not interact with any other factor. Hence the pattern of responsecompetition displayed by LB across the different conditions of Experiment 2is not abnormal.

Does LB show an abnormal amount of response competition? A notablefeature of LB’s performance in this experiment was that he responded muchmore slowly than the control subjects. In Experiment 1, LB’s mean responselatency was within the range displayed by the control subjects. However, inthis experiment, his mean response latency (1017 ms) was more than twice aslong as the slowest control subject (488 ms) and was more than 17 standarddeviations away from the mean of the control subjects. In the Compatibleand Incompatible conditions, LB’s latencies were also significantly slowerthan his own performance in Experiment 1 (p , .001).

In view of this, it was particularly important to take into considerationoverall differences in response speed when comparing the amount of re-sponse competition shown by LB and the control subjects. The possibilitythat the size of the response compatibility effect was related to general re-sponse speed was tested in this experiment by taking the mean response timeof each control subject in the Neutral condition, and correlating this valuewith the difference in response time between the Incompatible and Compati-ble conditions. The value of this correlation (Pearson’s r) was .405. A regres-sion function, expressing the relation between Neutral RT and the ResponseCompatibility effect (Incompatible RT 2 Compatible RT) was then fitted todata from the control subjects. This function was used to predict the magni-tude of the Response Compatibility effect, expected on the basis of LB’sresponse latency in the Neutral condition. The predicted difference betweenIncompatible and Compatible RT was 183 ms—a value which slightly ex-ceeds, but is close to, the observed difference in RT between Incompatibleand Compatible trials in LB’s performance—168 ms. Hence, although thesize of LB’s response compatibility effect was much larger than the averagevalue shown by the control subjects (29 ms), it was no larger than wouldbe expected, given his generally long response latencies.

Although it was not possible in this experiment to match LB with a subsetof control subjects, with respect to general response speed, it was possibleto compare his performance with those control subjects who tended to re-spond more slowly than average. Accordingly, the 12 control subjects were


ranked with respect to response latency in the Neutral condition. The perfor-mance of the four slowest subjects was then compared with LB, as in Experi-ment 1. As before, critical values were adjusted to take account of the factthat these analyses involved making the same comparisons between LB’sperformance and that of a control subject on four repeated occasions. Inthese analyses, the response-compatibility effect shown by LB did not differsignificantly from that shown by any of the four selected control subjects (allp . .10). None of the higher order interaction terms that included ResponseCompatibility and the factor comparing LB with each control subject ap-proached significance in these analyses. Hence, these analyses provided nosupport for the hypothesis that LB would show an abnormal amount of re-sponse competition, or response coactivation, either in the experiment as awhole or specifically within the Bilateral condition.

Errors

The number of errors made by LB and the 12 control subjects are shownin Table 5. This table shows that in the Bilateral and RVF-Left Hemisphereconditions, LB’s error rates fell within the normal range. However, in theLVF-Right Hemisphere condition his response accuracy was very poor, par-ticularly in the Incompatible condition, and fell outside the range shownby the control subjects. LB’s low level of performance in the LVF-Righthemisphere condition occurred with both left-hand responding (40.3% error)and right-hand responding (48.3% error).

The number of errors made by each control subject was entered into ananalysis of variance with the same factors as for the latency data. There wasa significant main effect of Visual Field-Hemisphere [F(2, 22) 5 7.21, p ,.005]. Post hoc analysis of this effect using Tukey’s studentized range (HSD)test (α 5 .05) showed that error rates were higher in the RVF-Left Hemi-sphere condition (6.7% error) than in the Bilateral condition (4.2% error).Error rates in the LVF-Right Hemisphere condition (5.4%) were not signifi-cantly different from either Bilateral or RVF conditions. There was a signifi-cant main effect of Response Compatibility [F(2, 22) 5 4.23, p , .05]. Posthoc analysis of this effect using Tukey’s studentized range (HSD) test (α 5.05) showed that error rates were higher in the Incompatible condition(6.93%) than in the Compatible condition (4.67%). Error rates in the Neutralcondition (4.72%) did not differ from either the Compatible or the Incompati-ble conditions.

Discussion

As in Experiment 1, LB’s responses were slower on Incompatible relativeto Compatible trials, and this effect showed no significant variation as afunction of visual field or responding hand. The performance data of 7 (of12) control subjects produced a similar pattern in showing a main effect of


response compatibility which did not interact with any other factor. In thecrucial Bilateral condition, LB’s response latencies in the Incompatible con-dition were again slower than in the Compatible condition, replicating theeffect observed in Experiment 1.

The magnitude of LB’s response competition effect (i.e., IncompatibleRT-Compatible RT) was compared with that of the control subjects. How-ever, within the control group there was evidence that the magnitude of re-sponse competition was related to overall response speed, indexed by NeutralRT. In comparisons that controlled for this relationship, there was no evi-dence that the amount of response competition shown by LB was abnormal,either in the dataset as a whole or specifically within the Bilateral condition.Hence, although the mean size of the response competition effect was consid-erably larger for LB than for the control subjects, it was not abnormally largein light of his generally slow response latencies.

There was no evidence of response coactivation or priming in the perfor-mance of either LB or the control subjects: No significant differences inresponse latency were observed between the Compatible and Neutral condi-tions. In view of this, it is appropriate to interpret LB’s performance and thatof the control subjects in terms of the same theoretical mechanism, namelycompetitive interference at the stage of response production.

While the results of Experiment 2 are very similar to those of Experiment 1with respect to the response compatibility effects that are directly relevant toour research question, it is also true that in other respects, LB’sperformance differed in the two experiments. LB’s performance in the twoexperiments differed in several ways.

As noted earlier, LB’s responses were markedly slower in Experiment 2relative to Experiment 1. In addition, while in Experiment 1, the accuracyof LB’s performance did not deviate markedly from the range observed inthe control group (see Table 2), in Experiment 2 his performance was verypoor in the LVF-Right Hemisphere condition (see Table 5). Indeed, on In-compatible trials his performance accuracy was at chance level. Without fur-ther data it is difficult to furnish a clear explanation for this deterioration inLB’s performance. One of us (AJL) gained the impression that his difficultywith LVF trials arose from a problem in following the instruction ‘‘Respondto the letter on the right’’ on trials where both letters were on the left. InExperiment 1 LB had no problem in comprehending and following this in-struction. In view of this, it is possible that both the general slowing of re-sponse time, and the problem with LVF trials, may have arisen from a generaldeterioration in LB’s level of cognitive functioning.

In Experiment 1, LB showed a visual field effect in which response laten-cies were faster in the Bilateral condition relative to the LVF and RVF condi-tions. This was not abnormal, since the same pattern was present in the per-formance of one of the control subjects. However, in Experiment 2 LBshowed a visual-field effect in which responses in the Bilateral condition


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were somewhat slower in the Bilateral condition (see Fig. 4). This patternwas not apparent in the performance of any control subject. Interpretationof this tendency is problematic, since it was present in Experiment 2, butnot in Experiment 1. However, the effect may be a further reflection of theresponse competition experienced by LB when both hemispheres are pre-sented with a stimulus letter.

A final point is that these differences in LB’s performance across the twoexperiments perhaps serves to underline the similarity in the findings thatwere observed in relation to the response-compatibility effects that were ofcentral interest.

GENERAL DISCUSSION

The Split-Brain Syndrome

In many respects the present results are in agreement with previous re-search in the sense of replicating and extending earlier findings concerninginterhemispheric interaction and integration in the split-brain. In agreementwith earlier work (Eviatar & Zaidel, 1994; Zaidel, 1995), LB was unable tocross-compare letter pairs when one item was presented to LVF and the otherto RVF. In agreement with previous findings of the first author (Lambert,1991, 1993, 1996), the latency with which split-brain individual LB re-sponded to a RVF stimulus was influenced by the presence and nature of asimultaneously presented LVF stimulus. The findings of Reuter-Lorenz et al.(1995) with the split-brain individual JW also involved interactions betweensimultaneously presented LVF and RVF stimuli. And finally, the observationthat LB was able to name LVF and RVF letters accurately concurs withresults reported by Johnson (1984). On the other hand, the results appeardiscordant with the proposal of Reuter-Lorenz et al. (1995) that followingsurgical disconnection of the cerebral hemispheres, split-brain patients dis-play an abnormal amount and pattern of interhemispheric response competi-tion. In the paragraphs below we first offer an overall interpretation of thepresent results and earlier findings and then attempt to reconcile our resultsand conclusions and those of Reuter-Lorenz et al. (1995).

Our overall interpretation of the findings reported above can be expressedconcisely: The results are consistent with the view that in split-brain individ-uals, while the cerebral hemispheres are radically divided with respect tomost perceptual functions (see Corballis, 1995), they continue to show rapidand intimate interaction with respect to the generation of action. Lambert(1991, 1993) reported that when split-brain individuals performed relativelycomplex categorical judgements concerning stimuli directed to the RVF,their performance was influenced by the category of stimuli presented tothe LVF. The surprising feature of these results was that interhemisphericinteraction effects were observed in the context of complex tasks involvingprocessing information at a semantic level of representation. In view of this,


the interpretation that was offered emphasized the apparent transfer of se-mantic information, a possibility that was sharply at odds with traditionalviews of the split-brain syndrome. However, in principle it was possible thatthese effects could have been mediated by interhemispheric interactions atany one of several levels of information processing (perceptual features, se-mantic representation, or response production). For a number of reasons thatare outlined below, an interpretation in terms of interaction at the stage ofresponse programming now appears to offer the most plausible account ofthe interhemispheric effects observed both in the present experiments andin earlier reports (Lambert, 1991, 1993, 1996). An account in terms of infor-mation transfer at the level of perceptual features seems highly unlikely.Such an account appears inconsistent with LB’s inability to cross-compareBilateral letters in Experiment 1 and inconsistent with a substantial body ofprevious research showing that the left and right hemispheres are unable toexchange the detailed visual form information that would be necessary forspecifying alphanumeric characters or words (see Corballis, 1995). An inter-pretation in terms of transfer of semantic, rather than visual, attributes nowappears relatively implausible. If interhemispheric transfer of semantic infor-mation is possible in the split-brain, one would expect to observe two kindsof experimental effect. First, it should be possible for split-brain patients tomatch stimuli with respect to abstract–semantic attributes. Eviatar and Zaidel(1994) found that three split-brain patients were unable to match different-case letter pairs with respect to nominal identity. Although Sergent (1986,1987, 1990) has reported a variety of experimental findings suggesting thatsplit-brain patients are able to compare LVF and RVF stimuli with respectto abstract attributes, attempts to replicate these results have not met withsuccess (Seymour, Reuter-Lorenz, & Gazzaniga, 1994; Corballis & Trudel,1993; Corballis, 1995; see also Corballis, 1994). Second, if interhemispherictransfer of semantic attributes occurs following commissurotomy, one wouldexpect to observe interhemispheric semantic priming. For example, present-ing a prime stimulus (e.g., ‘‘SALT’’) to one visual field should prime alexical decision made to an associated word (‘‘PEPPER’’) presented to theother visual field. Effects of this kind have not been reported. Reuter-Lorenzand Baynes (1992) failed to observe interhemispheric priming of letter stim-uli by different-case letter primes in split-brain participant JW.

These considerations render an interpretation of LB’s performance in Ex-periments 1 and 2 in terms of interhemispheric transfer of visual or semanticattributes relatively unlikely. In contrast, an account in terms of interhemi-spheric interaction at the stage of response production appears plausible. Thiskind of explanation is attractive on two counts. First, a substantial body ofresearch carried out with normal subjects supports an interpretation of theinterference effects observed in this task in terms of response competition(see Broadbent, 1982; Eriksen, 1995 for reviews). Since the interference ef-fects displayed by LB did not differ from those of normal subjects in either


experiment, it is appropriate and parsimonious to invoke the same explana-tion in both cases: namely that the effect arises from competitive interactionoperating interhemispherically at the stage of response production. Second,this kind of explanation is consistent with a large body of split-brain research,suggesting that section of the cerebral commissures produces a disconnectionsyndrome in which perceptual and higher order cognitive processes are di-vided, while overall behavioral integration apparent in everyday situationsis achieved by interhemispheric interaction at the stage of producing actions(Corballis, 1995; Seymour et al., 1994; Pashler et al., 1994).

In the absence of cerebral commissures, such interactions are presumablymediated via intact subcortical connections between the cerebral hemi-spheres. Which subcortical pathways might mediate the interaction betweentarget and distractor processing observed in this study? A candidate pathwayis suggested by work recently reported by LaBerge (1990, 1995). LaBergedescribes a series of experiments involving a closely similar task to thatemployed here. As in the present case, the task was modeled on Eriksen andEriksen’s work (1974) and involved responding to a target visual object inthe presence of distractor items. Positron emission tomography was used toestimate relative blood flow and hence metabolic activity in two midbrainareas and one cortical area: the pulvinar nucleus of the thalamus, the medio-dorsal thalamus, and area V1 of occipital cortex, respectively. The resultssuggested that attending to a target object in the presence of distractors in-volved heightened activity in the pulvinar nucleus of the thalamus. Anatomi-cally this midbrain area is a particularly promising site for the effect sinceit enjoys rich and reciprocal interconnections with several cortical areas.

Response competition in the split-brain—Normal or abnormal? As notedearlier, at a broad level the results are consistent with previous work byReuter-Lorenz et al. (1995) and by Pashler et al. (1994) in demonstratingthe presence of interhemispheric response-competition effects. However, ata more specific level our interpretation appears very different from that of-fered by Reuter-Lorenz et al. According to our interpretation, processes ofresponse competition in the split-brain are essentially normal, since the mag-nitude and pattern of interference effects in LB’s performance did not differsignificantly from those of neurologically intact subjects. In contrast, Reuter-Lorenz et al. (1995) propose that JW’s performance on the redundant targetstask is abnormal and requires a qualitatively different model of responsecompetition from that used to explain normal performance.

Two ways of resolving the conflict present themselves. One possibility isto appeal to intersubject differences between LB and JW in relation to atten-tion and response competition. Although it is possible that the findings re-ported here, based on LB’s performance, may fail to generalize to other split-brain individuals, previous research provides few grounds for supposing thatcallosotomy (the procedure undergone by JW, in which the corpus callosumwas sectioned) and commisssurotomy (the procedure undergone by LB, in


which the corpus callosum, together with the anterior and hippocampal com-missures were sectioned) have different consequences for processes of atten-tion and response competition. On the contrary, in the study of responsecompetition carried out by Pashler et al. (1994) the performance of JW andLB was very similar. Inspection of their Fig. 2 (p. 2382) shows that JW’sSession 2 performance and LB’s performance displayed a closely similaramount of interference in a task where each hemisphere performed a choicereaction-time task.

A second way of resolving the conflict involves appealing to task differ-ences between the interference paradigm employed here and the redundanttargets paradigm employed by Reuter-Lorenz et al. (1995). Although, theconcept of response competition plays a central role in the interpretationsoffered for both sets of results, the cognitive demands of the interferencetask and the redundant targets task are clearly very different. The former isa selective-attention task in which the subject avoids responding to an irrele-vant stimulus. The latter is a divided-attention task in which the subject re-sponds to target(s) that can appear at different locations. The former is achoice RT task in which responses were dependent on the visual form oftarget stimuli, while the latter is a simple RT task where the mere presenceof a stimulus is sufficient to trigger a response. It is entirely conceivable thatthe cognitive processes involved in each task have a distinct relation withthe split-brain syndrome and require different kinds of explanation. Hence,the conflict between our findings and those of Reuter-Lorenz et al. (1995)may be more apparent than real. That is, the response competition processinvoked by Reuter-Lorenz and colleagues in interpreting JW’s performancemay be quite distinct from the classic notion of response competition thoughtto underlie response compatibility effects, such as those observed in thisstudy. Clearly, further research is needed to clarify the relationship betweenperformance of the interference task and the redundant targets task by bothsplit-brain subjects and normal individuals.

Conceptions of Selective Attention

LB’s performance data in the Bilateral conditions of the interference taskmay also have more general implications for views of selective attention. Aclassic interpretation of interference effects of the kind described here andby Eriksen and Eriksen (1974) appeals to the notion of a limited-capacityprocessing channel in explaining the phenomena of selective attention. Thatis, selectively attending involves aligning a channel, of limited capacity, withthe desired object(s). In this way, object(s) in the attended processing channelgain privileged access to full perceptual processing, while objects outsidethe beam of attention receive more rudimentary perceptual analysis. Thiskind of view appears in many accounts of attentional phenomena, rangingfrom Broadbent’s filter theory developed in the early 1950s to more recent


work (Broadbent, 1958, 1982; Treisman, 1988; Mangun, Hillyard, & Luck,1993; Mangun & Hillyard, 1995). On this kind of view problems of interfer-ence arise when there is insufficient perceptual separation between attendedand distractor items. Interference is attributed to the notion that attended anddistractor items are both selected and processed within the same attendedchannel of limited capacity. The fact that interference effects reduce or disap-pear entirely with increasing spatial separation between attended and unat-tended objects is often cited as support for this account (Broadbent, 1982).However, the present results strongly suggest that this interpretation is incor-rect. In the bilateral condition, channel separation between attended and unat-tended items could hardly be more complete—each letter is processed, atleast initially, within a different surgically isolated hemisphere! Hence, inter-ference from the unattended object cannot be due to a problem of segregatingattended and unattended objects into distinct processing channels. An alter-native interpretation is that selective processing phenomena, such as interfer-ence effects, are driven not by the elusive capacity limitations of neural pro-cesses, but by the design requirements of a system that can respond rapidlyand appropriately to environmental events. At present the nature of thesedesign requirements is poorly understood, highlighting the need for a de-scription of attentional functions at the level of computational theory (cf.Marr, 1982). Allport (1989, 1993) identified the need to maintain behavioralcoherence as one such requirement. Clearly the ability to respond selectivelyto some environmental stimuli, while avoiding responding to others mayhave had powerful biological advantages during evolution. As others havenoted, split-brain individuals display radical disconnection of many cognitiveprocesses, but appear remarkably intact with respect to the coherence andintegration of everyday actions. Within Allport’s overall approach, the be-havioral integration displayed by split-brain individuals is consistent withobserving intact attentional interactions between objects appearing in oppo-site visual fields, such as the effects described above, and those describedin other reports (Lambert, 1991, 1993, 1996; Pashler et al., 1994).

SUMMARY AND CONCLUSIONS

Results from the two experiments reported in this article show that whena split-brain individual (LB) responded to a target object in the right visualfield, his response times were influenced by the nature of an unattended dis-tractor presented to the left visual field. Since the size of the effect did notdiffer from that shown by neurologically intact control subjects, it was con-cluded that interhemispheric response competition effects are essentially nor-mal following commissurotomy. This response competition effect and nam-ing performance were clearly dissociated from the ability to cross-compareLVF and RVF stimuli. Performance of the former may involve response-level representations, which can be transferred subcortically following com-


missurotomy, while the latter may require perceptual representations whichrequire intact forebrain commissures for interhemispheric transfer.

Second, it was concluded that distractor interference effects of the kinddescribed by Eriksen and Eriksen (1974) and studied by many others sincecannot be explained in terms of difficulty in segregating perceptual pro-cessing of target and distractor objects. LB showed normal levels of dis-tractor interference, even when target and distractor objects were directedto different hemispheres which have been surgically disconnected by com-missurotomy. Following Allport (1993) it is suggested that phenomena suchas flanker interference may arise from the computational design characteris-tics of attentional–perceptual systems that operate rapidly and flexibly in acomplex world. These design features may be driven by factors such as theneed to maintain behavioral coherence rather than by problems of limitedchannel capacity, which have proved difficult to characterize in neuroscien-tific terms.

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Selective Attention and Interhemispheric Response Competition in the Split-Brain

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