Individual Differences in Stroop and Local-Global Processing: A Possible Role of Interhemispheric Interaction

Brain and Cognition 45, 97–118 (2001) doi:10.1006/brcg.2000.1259, available online at http://www.idealibrary.com on

Individual Differences in Stroop and Local-Global Processing: A Possible Role of Interhemispheric Interaction Stephen D. Christman University of Toledo Published online January 17, 2001

Two experiments are reported examining individual differences in the processing of centrally presented stimuli containing two dimensions of information lateralized to opposite cerebral hemispheres. Left-handers, arising from (a) their lesser degree of functional lateralization and (b) their greater degree of callosal connectivity, were hypothesized to exhibit greater interdimensional (and presumably interhemispheric) interaction. Experiment 1 utilized localglobal stimuli, and left-handers were found to be impaired at keeping the two dimensions independent and superior at integrating the two dimensions . Experiment 2 used Stroop stimuli, and left-handers again were impaired at keeping the two dimensions independent (i.e., showed greater Stroop interference). Correlational analyses indicated that the mechanisms of interdimensional integration versus independence are at least partially independent from one another. Results suggest that aspects of interhemispheric interaction can be addressed via the use of nonlateralized input.  2001 Academic Press

The use of Stroop-like stimuli, that is, stimuli composed of two (or more) dimensions that can convey congruent or conflicting information, has proven to be a useful tool in studying patterns of facilitation and interference among concurrent cognitive processes (i.e., MacLeod, 1991). A typical finding is that the processing of the target dimension is slower when the irrelevant dimension carries incongruent information. Early accounts of such effects concentrated on the relative speed and/or processing priority of one dimension relative to the other. For example, traditional accounts of the Stroop effect have been couched in terms of the greater speed and/or automaticity of reading relative to color naming. Similarly, experiments employing so-called ‘‘local-global’’ stimuli, in which a large global letter or form is made up from smaller, local letters or forms, have reported a phenomenon of global interference, in which incongruent information at the global level interferes with the processing of the local dimension. Initial accounts of this effect argued that it arose from a sequential pattern of information processing, in which processing of the global dimension either preceded or was finished sooner than processing of the local dimension (e.g., Navon, 1977). However, current theories of these effects emphasize the parallel and concurrent processing of the different dimensions of Stroop and local-global stimuli. In his rePreparation of this article was supported in part by a Faculty Research Fellowship from the University of Toledo. I gratefully acknowledge the assistance of Susan Mooney and Robyn Gandy in data collection. I also thank Wendy Shore for helpful comments on an earlier version of this article. Address correspondence and reprint requests to Stephen D. Christman, Department of Psychology, University of Toledo, Toledo, OH 43606. E-mail: [email protected]. 97 0278-2626/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.

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view of 50 years of research on the Stroop effect, MacLeod (1991) argued that accounts of the Stroop effect in terms of the greater speed and/or automaticity of reading relative to color naming were insufficient and proposed a model that emphasized the parallel processing of the two dimensions, with facilitation and interference arising from the strength of connections between processing units comprising the different pathways associated with each dimension. Similar accounts have been offered for global interference, the idea being that local and global dimensions are processed in parallel (e.g., Hoffman, 1980; Miller, 1981), with global interference arising from the inhibition of local pathways by pathways carrying global information (e.g., Hughes, 1986; Kitterle, Christman, & Conesa, 1993). The emphasis of such theories on parallel pathways and the interaction between concurrent processes may provide valuable insights into reports of individual differences in Stroop and global interference. That is, individual differences in patterns of interdimensional interaction may reflect differences in the facilitative and inhibitory connections between these pathways. For example, Yee and Hunt (1991) reported stable individual differences in Stroop interference, with some subjects being strongly influenced by the (irrelevant) color word and others not; in particular, they suggested that these effects may reflect individual differences in the degree of inhibition of irrelevant information. A similar finding was reported by Ward (1985) in a study employing local-global type information. He reported stable individual differences in the ability to process the two dimensions in separable versus integral manners. Furthermore, he suggested that these individual differences may reflect more general individual differences in selective attention, as subjects exhibiting greater interaction between the dimensions were also more field dependent as measured by the Embedded Figures Test. A possible framework for interpreting and synthesizing these patterns of individual differences lies in the role of interhemispheric interaction in the processing of bidimensional stimuli. Given evidence that the two dimensions of Stroop and localglobal stimuli are processed in parallel, a possible neural substrate for such processing could involve a between-hemisphere division of labor, with one cerebral hemisphere assuming processing control of one dimension and the other hemisphere controlling the processing of the other dimension. Given the use of central, foveal presentation of input in most Stroop and local-global experiments, combined with the fact that foveal input projects to both hemispheres, this framework is at least consistent with the physiology of the visual system. Such a scheme seems particularly plausible for the processing of local-global stimuli, given robust evidence for left-hemisphere (LH) versus right-hemisphere (RH) processing of local versus global dimensions, respectively (see reviews by Robertson & Lamb, 1991; Van Kleeck, 1989). With regards to Stroop stimuli, LH control over reading the word is a reasonable assumption, and evidence tentatively indicates that color perception may be lateralized to the RH (e.g., Capitani, Scotti, & Spinnler, 1978; Davidoff, 1976; Njemanze, Gomez, & Horenstein, 1992; Pennal, 1977; Pirot, Pulton, & Sutker, 1977). While the spoken naming of colors appears to be lateralized to the LH (e.g., Malone & Hannay, 1978; McKeever & Jackson, 1979), reflecting LH control of speech, the present experiments involved manual keypress responses in an attempt to ensure RH processing of the color dimension. Furthermore, assuming the LH does control reading of the word, it is possible that the RH might assume control over color perception by default of not being involved in the reading of the word. Within this framework, then, individual differences in the amount of interaction between the two dimensions of Stroop and local-global stimuli could at least partially reflect individual differences in the degree of interhemispheric interaction. To the

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extent that the two dimensions of Stroop and local-global stimuli are processed in separate hemispheres, greater versus lesser degrees of interhemispheric interaction should result in greater versus lesser degrees of interdimensional interaction, respectively. Unfortunately, little experimental attention has been paid to the question of possible individual differences in interhemispheric interaction. One possibility, however, is that left-handers may exhibit greater degrees of interhemispheric interaction than right-handers, arising from the presence of (i) a larger corpus callosum in lefthanders (e.g., Witelson, 1985, 1989) and (ii) lesser degrees of hemispheric asymmetry in left-handers (e.g., Hellige, 1993). These findings suggest that, in left-handers, the LH and RH may be more richly interconnected and/or more similar in their modes of processing and hence more likely to interact. Thus, left-handers might be expected to exhibit greater degrees of Stroop and global interference, arising from their relative inability to keep LH word reading and local processing isolated from RH color perception and global processing. To date, few studies have explicitly looked at the role of handedness in mediating interhemispheric interaction, with some studies reporting greater interhemispheric interaction in left-handers (e.g., Christman, 1993; Dimond & Beaumont, 1972; Honda, 1982; McKeever & Hoff, 1983; Moscovitch & Smith, 1979; Potter & Graves, 1988; Verillo, 1983), some reporting greater interaction in right-handers (e.g., Eviatar, Hellige, & Zaidel, 1992; Miller, 1983, but see Christman, 1995, for an alternative account of Miller’s results), and others reporting no differences (e.g., Banich, Goering, Stolar, & Belger, 1990; Beaumont & Dimond, 1973, 1975; Christman, 1991; Piccirilli, Finali, & Sciarma, 1990). Banich et al. (1990) point out that evidence for greater interhemispheric interaction among left-handers is obtained primarily with tasks requiring visuomotor integration; studies requiring interhemispheric integration of more abstract, conceptual information have been less likely to yield handedness differences. However, virtually all of the aforementioned studies investigated interhemispheric interaction by presenting separate input items to the LH and RH and examining how the stimuli are compared or integrated. While clearly a valuable technique in studying interhemispheric interaction, the use of such bilateral stimulus displays imposes three important constraints. First, the necessary use of peripheral presentation of input in bilateral displays means that processing is almost always data limited to some degree; to the extent that both quantitative and qualitative differences exist between peripheral and foveal vision, the possibility is raised that aspects of interhemispheric interaction under conditions of peripheral input may not fully generalize to how the hemispheres interact under conditions of central presentation. Second, use of bilateral displays creates a forced (and perhaps artificial) division of labor between the hemispheres: that is, presentation of separate input to each visual field constrains each hemisphere to process the input it receives. The use of centrally presented input may allow a more natural division of labor between the hemispheres, constrained not by location of presentation but by higher level information processing differences between the hemispheres. With central presentation, input is equally available to each hemisphere, and potential hemispheric division of labor may reflect more directly the competencies of each hemisphere. Finally, and perhaps most importantly, there is tentative evidence for qualitative differences in interhemispheric interaction when (i) comparing two separate input items versus (ii) dealing with unified object representations. Sergent (1986, 1990) has shown that, while split-brain patients are incapable of comparing two objects presented to separate hemispheres (e.g., determining whether two digits flashed to opposite visual fields are the same or different), they are capable of making judgments

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based on an integrated representation of such objects (e.g., determining whether the sum of the two digits is greater or less than 10). This raises the possibility that aspects of interhemispheric interaction that have been noted using the paradigm of comparisons between separate items presented to different hemispheres may be different in situations where the hemispheres are operating on unified input. Thus, findings from studies requiring comparisons among separate inputs may not be wholly applicable to studies involving interactions among dimensions comprising unified input. Similarly, characteristics of interhemispheric interaction under conditions where two input dimensions are presented independently to separate visual fields may not fully generalize to conditions where different input dimensions are presented in an integrated manner to the fovea and are hence equally available to both hemispheres. In this sense, previous studies have addressed interhemispheric interaction by directly examining how the hemispheres compare their separate contents, while the current studies attempt to address interhemispheric interaction by examining the interactions among dimensions comprising central, unified input and inferring patterns of interhemispheric interaction from individual differences in interdimensional interaction. Two studies employing neurological subject populations provide indirect support for the hypothesis that interhemispheric interaction at least partly underlies patterns of interdimensional interaction in the processing of Stroop and local-global stimuli. First, Robertson, Lamb, and Zaidel (1993) investigated the processing of local-global stimuli in normal and commisurotomized subjects. Although both groups exhibited the usual finding of global precedence (as well as the expected RH versus LH advantages for global versus local processing, respectively), only normal subjects exhibited interference from incongruent information at the global level when judging the identity of the local letter, suggesting that the lack of interhemispheric interaction in commisurotomized subjects reduced or eliminated certain forms of interaction between local and global stimulus dimensions. Interestingly, for global judgments, commisurotomy patients still exhibited interference from the irrelevant information at the local level, suggesting that the mechanisms underlying global interference are different than those underlying local interference and that only the former may be dependent on intact callosal transfer. Second, there is evidence that, relative to normal subjects, subjects with agenesis of the corpus callosum exhibit reduced Stroop interference for centrally presented stimuli (David, 1992). Echoing Robertson, Lamb, and Zaidel’s (1993) line of reasoning that intact interhemispheric relations underlie interference between local and global levels of form, David interpreted his results as reflecting the importance of intact interhemispheric relations for producing interference between the verbal and chromatic dimensions of input. In other words, the acallosal subjects, who exhibit lesser degrees of interhemispheric interaction, also exhibit lesser interference between levels. The current framework proposes the converse effect: Left-handers, who exhibit greater degrees of interhemispheric interaction, should exhibit greater interference. This approach represents an extension of ideas concerning ‘‘functional cerebral distance’’ developed by Kinsbourne and Hicks (1978), who proposed that the extent to which two concurrent processes would show facilitation versus interference with each other was a function of (a) the functional distance between the different cerebral regions mediating the two processes and (b) the extent to which the two processes are concordant versus discordant. Specifically, Kinsbourne and Hicks argued that the lateralization of different functions to the two cerebral hemispheres allows for the reduction of interference between such tasks when performed concurrently; that is, if

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subjects have to perform two tasks at once, to the extent to which the neural substrates underlying the processing demands of the separate tasks are relatively isolated from each other, there will be less intertask interference. Kinsbourne’s approach has been applied primarily to dual-task situations, with the general finding that interference between tasks is maximal versus minimal when the two processes must be carried out in the same versus different hemispheres, respectively. However, if concurrent processes are highly concordant and strongly lateralized (e.g., naming of multiple, simultaneous words), a within-hemisphere advantage may be found (e.g., Liederman, 1986). The current approach involves two modifications to Kinsbourne’s paradigm. First, rather than varying functional cerebral distance by employing within versus between hemisphere processing, the current experiments compare the performance of leftversus right-handers under conditions of between-hemisphere processing. The assumption is that, owing to the greater degree of interhemispheric callosal connectivity in left-handers (e.g., Witelson, 1985, 1989), there is less functional distance between right- and left-hemisphere processes in left-handers than in right-handers. It should be stressed that this is a working hypothesis; a thicker corpus callosum is not a necessary or unambiguous indicator of greater interhemispheric interaction. For example, a larger corpus callosum in one group may still contain the same number of axons, with the larger size resulting from gross structural differences that have little functional import. Conversely, Juraska and Kopcik (1988) reported no gross differences in callosal size between male and female rats; however, a fine-grained analysis revealed more axons of smaller size in females. Nonetheless, the current study will attempt to ascertain if the apparently larger corpus callosum in left-handers can be indirectly related to the magnitude of interhemispheric interaction. Accordingly, following from Kinsbourne’s hypothesis, it is predicted that left-handers will exhibit a relative performance advantage over right-handers in situations where LH and RH processing is concordant and/or needs to be integrated, while left-handers should show a relative disadvantage in situations where LH and RH processing is discordant and/or needs to be kept separate. The second modification of Kinsbourne’s paradigm entails the use of a nonlateralized, single-task paradigm employing unitary stimuli containing different levels of information; most previous tests of Kinsbourne’s hypothesis have utilized pairs of tasks, at least one of which is lateralized, such as unimanual dowel balancing or finger tapping (e.g., Hellige & Longstreth, 1981; Kinsbourne & Cook, 1971). In the current studies, the focus is on interactions between the two levels of information within the context of performance on a single, nonlateralized task rather than on the interaction between two independent tasks. It is worth noting that the current framework also bears similarity to a proposal by Friedman and Polson (1981), who argued that the two cerebral hemispheres could be viewed as possessing separate, noninteracting pools of attentional resources. Their model explicitly applied only to right-handers and, in a manner akin to Kinsbourne’s model, predicted that concurrent tasks lateralized to opposite hemispheres should exhibit little or no interference, while tasks lateralized to the same hemisphere should exhibit substantial interference, at least under conditions of sufficient processing load. The current framework may be viewed as an extension of Friedman and Polson’s model, with the supposition that the two hemispheres exhibit greater sharing and/or lesser independence of LH and RH attentional resources in left-handers relative to right-handers. Direct support for the current hypothesis was reported by Christman (1995), who employed bidimensional stimuli of the sort discussed above. In addition to Stroop and local-global stimuli, facial stimuli consisting of an expressive dimension (i.e., a

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smile or a frown) and a verbal dimension (i.e., the word ‘‘happy’’ or ‘‘sad’’ written across the center of the face) were used. Also, abstract geometric forms consisting of left and right halves that either were or were not left–right symmetric were employed. Subjects’ task was to indicate whether the two dimensions of each stimulus were congruent or, in the case of the geometric forms, whether the left and right halves were symmetric. The underlying reasoning was the converse of that discussed above in relation to Stroop and global interference; that is, judgments of whether the two dimensions are congruent requires the dimensions to be integrated (as opposed to being kept separate, as in the standard versions of Stroop and local-global tasks). Therefore, it was hypothesized that greater interhemispheric interaction would be beneficial in this task. The results were supportive: left-handers exhibited RT advantages for the Stroop, local-global, and face stimuli; there were no handedness differences on the symmetry task. The failure of left-handers to exhibit an advantage in the Symmetry task may have arisen from two factors. First, the symmetry task required interhemispheric integration of perceptual information (i.e., the shape of the left and right halves of the stimuli), whereas the other three tasks required the transfer of higher level cognitive information (i.e., the nature of information occurring at different levels of the stimulus). This is consistent with the observation that the regions of the corpus callosum that are larger in left-handers (e.g., midsaggital regions) are those regions involved in the transfer of more abstract information; those callosal regions (e.g., the splenium and genu) involved in transfer of sensory and motor information do not differ between handedness groups (Habib, Gayraud, Oliva, Regis, Salamon, & Khalal, 1991; Witelson, 1989). Second, the relative disadvantage shown by left-handed subjects may be related to findings of greater left–right confusion among left-handers (e.g., Hannay, Ciaccia, Kerr, & Barrett, 1990). The fact that sinistrality was not associated with a performance advantage in the symmetry task is useful in ruling out the possibility that sinistrality was simply associated with a general RT advantage, regardless of the specific nature of the task. Data from a second experiment of the same study provides additional support for ruling out an explanation of the left-handers’ advantage in terms of an overall RT advantage, above and beyond any assumed role of interhemispheric processing. Christman (1995) had left- and right-handers perform foveal versions of the LocalGlobal and Face tasks from the aforementioned experiment as well as versions of the physical identity and name identity letter matching tasks developed by Posner and colleagues (e.g., Posner, Boies, Eichelman, & Taylor, 1969). While the RT advantage for left-handers was partially replicated for the Local-Global (p ⬍ .065) and Face ( p ⬍ .05) tasks, there were no differences as a function of handedness for either of the letter-matching tasks, Fs ⬍ 1. While the letter-matching tasks require same– different judgments, such stimuli do not contain qualitatively distinct dimensions of information and, thus, presumably do not require explicit interhemispheric integration of information. The focus of the current experiments involves a more systematic investigation of individual differences in processing Stroop and local-global stimuli, utilizing various tasks that require either interdimensional integration or independence. These stimuli are common tools in psychological studies and represent useful and appropriate stimuli for investigating interdimensional interaction. Furthermore, to the extent that handedness differences are obtained, and, in turn, these differences can be related to aspects of interhemispheric interaction, a useful bridge between the often disparate methods and theories of mainstream cognition versus laterality research will be possible.

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EXPERIMENT 1

Experiment 1 examined how individual differences in the nature of the interaction between LH and RH processes (as indexed by handedness) influence the interaction between the local and global dimensions of hierarchical letter stimuli. Since leftversus right-handed individuals are hypothesized to exhibit greater versus lesser degrees of interhemispheric interaction, respectively, left-handers should show a relative advantage on tasks requiring the integration of local and global information, whereas right-handers should show a relative advantage on tasks requiring local and global information to be kept independent. This prediction is the converse of the results reported by Robertson et al. (1993), who reported that split-brain patients who exhibit little or no interhemispheric interaction exhibited little or no interference from the global dimension. Methods Subjects. Sixty-four subjects participated in this study for course credit; 32 subjects were righthanded with no left-handed relatives in their immediate family and 32 were left-handed. A number of recent reports have implicated gender as also being related to aspects of callosal morphology and interhemispheric interaction (Potter & Graves, 1988; Witelson, 1989). Consequently, gender was equally represented in both handedness groups. Handedness was assessed by use of a brief handedness questionnaire (Oldfield, 1971) administered at the start of the experimental session. All subjects had normal or corrected-to-normal vision. Apparatus. All stimuli were presented on a Macintosh IIfx computer with a high-resolution color monitor and an Apple 8-24GC graphics accelerator card. Control of stimulus presentation and response timing was under the control of the Reaction Time module of the MacLaboratory program v.2.1 (Chute, 1988). Stimuli. The hierarchical letter stimuli consisted of 14-point bold Helvetica versions of the letters ‘‘H,’’ ‘‘T,’’ ‘‘F,’’ and ‘‘L.’’ These letters were chosen following the procedure employed by Sergent (1982). Global letters were constructed out of the local letters arranged in appropriate positions within a matrix five letters wide by seven letters high. Stimuli appeared as black letters on a white background, and subtended 2.0° ⫻ 2.9° of visual angle. Procedure. All subjects participated in each of three tasks, described below. Subjects were run in two sessions approximately 1 week apart. The divided attention task was always run in one session, and the focused attention and same-different tasks were run in the other session. The order of sessions, order of tasks within a session, and assignment of hand of response was counterbalanced across all combinations of handedness and gender groups. For all three tasks, trials began with a warning tone of 200 ms duration that preceded the stimulus by 500 ms. Stimulus exposure duration was 100 ms and the interstimulus interval was 2 s for all three tasks. The divided attention task required subjects to report whether a specified target letter occurred in the display. Subjects responded by pressing a key with the index finger of one hand if a target was present and pressing a key with the index finger of the other hand if no targets were present in the display. The letters ‘‘H’’ and ‘‘T’’ were designated as targets, and ‘‘F’’ and ‘‘L’’ were nontargets. There were four different stimulus types: stimuli where both the large (global) and small (local) letters were targets (L⫹S⫹), stimuli where the target occurred at the global level only (L⫹S⫺), stimuli where the target occurred at the local level only (L⫺S⫹), and stimuli where no target letters were present at either level (L⫺S⫺). All stimulus types did not occur with the same frequency, since this would have led to a situation where 75% of the stimuli had targets present, which could potentially induce a response bias. To roughly balance the number of target versus nontarget trials, nontarget trials were overrepresented by a factor of 2, yielding blocks where 60% of the stimuli had targets present. Stimuli for this task were presented to the left and right visual fields (LVF and RVF) at eccentricities of 2.5°. This task provided a measure of the lateralization of local and global processes to the two cerebral hemispheres in rightversus left-handers, following the procedures employed by Sergent (1982). Subjects participated in two blocks of 160 trials each, yielding a minimum of 32 replications per trial type. The focused attention task required subjects to identify a letter at a specified level (i.e., local or global); the identity of the letter at the other level was irrelevant. A common dependent variable for this task

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is the amount of interference (RT for stimuli where the letters at the two levels are incongruent minus RT for stimuli where the two levels are congruent). Stimuli for this task were presented foveally. This task was run in two separate blocks: one requiring the identification at the local level and the other at the global level. Each block consisted of 64 trials, equally balanced between congruent and incongruent trials. Since this task requires the information at the local and global levels to be kept separate, it was hypothesized that right-handers should exhibit less interference between the two levels, arising from the presumed greater independence of LH-local processing and RH-global processing. In particular, greater interference on local judgments from the irrelevant global level was predicted, following the aforementioned results of Robertson, Lamb, and Zaidel (1993). Finally, the same-different task was an exact replication of the same-different local-global task employed by Christman (1995), requiring subjects to determine whether the letters occurring at the local and global levels were the same or different (i.e., was the large letter the same as or different than the small letters?). Stimuli for this task were presented foveally in a single block of 72 trials with equal numbers of ‘‘same’’ and ‘‘different’’ trials.

Results and Discussion The overall error rate for the divided attention task was 2.4%, and an analysis of variance revealed only two significant effects: a main effect for condition, F(3, 186) ⫽ 29.57, p ⬍ .001, arising from lower error rates in the L⫹S⫹ conditions relative to the other three conditions, and a main effect of visual field, F(1, 62) ⫽ 19.9, p ⬍ .02, arising from a lower error rate for LVF trials (2.2%) than for RVF trials (2.6%). The RT data for the divided attention task are shown in Fig. 1. The only main effect was for condition, F(3, 186) ⫽ 185.1, p ⬍ .001 (L⫹S⫹ RT ⬍ L⫹S⫺ RT ⫽ L⫺S⫹ RT ⬍ L⫺S⫺ RT). A significant interaction between visual field and condition was obtained, F(3, 186) ⫽ 16.94, p ⬍ .01, arising from the presence of a LVF-RH advantage for global processing (i.e., L⫹S⫺), F(1, 63) ⫽ 17.55, p ⬍ .001, and a RVF-LH advantage for local processing (i.e., L⫺S⫹), F(1, 63) ⫽ 8.82, p ⬍ .005. The lack of any effects involving handedness (all Fs ⬍ 1) indicates that left- and right-handers did not differ in the direction or degree of lateralization of local versus global processing to the LH versus RH, respectively.

FIG. 1. Reaction-time data (in milliseconds) for divided attention task as a function of stimulus type, visual field, and subject handedness in Experiment 1 (error bars indicate mean standard error).

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FIG. 2. Interference data (Incongruent RT minus Congruent RT) for the focused attention task as a function of target level and subject handedness in Experiment 1 (error bars indicate mean standard error).

The RT data (Incongruent RT minus Congruent RT) for the focused attention task are shown in Fig. 2. Error rates were again very low (⬍2%), and preliminary analyses indicated that the only main effect was for congruency, F(1, 62) ⫽ 9.33, p ⬍ .005 (with incongruent stimuli yielding higher error rates), so analyses concentrated on the RT data. There were main effects of congruency, F(1, 62) ⫽ 141.3, p ⬍ .001, and target level, F(1, 62) ⫽ 6.99, p ⬍ .01, with congruent and global judgments being made more quickly. The only interaction involved handedness and congruency, F(1, 62) ⫽ 12.48, p ⬍ .001, arising from the fact that left-handers showed a larger difference between congruent and incongruent conditions (i.e., exhibited more interference between the local and global levels). Interestingly, the lack of a significant three-way interaction between handedness, congruency, and target level (F ⬍ 1 for both error and RT data) indicates that left-handers exhibited greater degrees of both local and global interference. This result is somewhat contrary to that reported by Robertson et al. (1993), who found that commisurotomy patients, who exhibit a lack of interhemispheric interaction, exhibited diminished global, but not local, interference. Extrapolating from their results, left-handers might have been expected to show greater interference in the locally directed condition only. However, the fact that Robertson et al. employed parafoveal stimuli, whereas the current study employed foveal stimuli, limits direct comparison. The error rate for the same-different task was 4.6%. Analysis of variance indicated no significant main effects or interactions for the error data. The RT data for the same-different task are shown in Fig. 3. There was a main effect of decision, F(1, 62) ⫽ 164.6, p ⬍ .001, with ‘‘different’’ judgments taking longer than ‘‘same’’ judgments. A main effect of handedness, reflecting an advantage for left-handers, missed significance, F(1, 62) ⫽ 2.71, p ⬍ .11. However, unpublished data from Christman (1995) involved exact replications of the present same-different task. When the present data were combined with data from two previous experiments (for a total sample size of 160), a highly significant advantage emerged for left-handers on the same-different task, F(1, 158) ⫽ 7.53, p ⬍ .007, indicating an advantage

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FIG. 3. Reaction-time data for the same-different task as a function of decision and subject handedness in Experiment 1 (error bars indicate mean standard error).

for left-handed subjects in integrating LH and RH levels of information in foveally presented input. However, the fact that only one of the three individual studies yielded a significant effect suggests that the left-handers’ advantage in interlevel integration may not be particularly robust. Finally, correlational analyses were used to examine relationships between performance on the three different tasks. Correlations were computed using the entire sample of 64 subjects (preliminary analyses indicated no difference in the nature or strength of obtained correlations for left- versus right-handers considered separately) and are shown in Table 1. Four significant correlations were obtained. A significant positive correlation between global and local laterality quotients (LVF-RT minus RVF-RT for the locally and globally directed conditions of the focused attention task), r ⫽ .29, p ⬍ .05, indicated that subjects who exhibited relatively greater lateralization of local processing to the LH tended to show relatively lesser lateralization of global processing to the RH (and vice versa). This is similar to other reports of positive correlations between laterality quotients for tasks lateralized to opposite TABLE 1 Correlation Coefficients for Experiment 1 a

Local LQ Global interf. Local interf. Same RT Diff. RT

Global LQ

Local LQ

Global interf.

Local interf.

Same RT

.29* ⫺.07 .17 ⫺.04 ⫺.06

⫺.17 .18 ⫺.36** ⫺.32*

.15 ⫺.03 ⫺.04

⫺.14 ⫺.11

.85**

LQ ⫽ LVF RT minus RVF RT (divided attention task); INTERF ⫽ Incongruent RT minus Congruent RT (focused attention task). * p ⬍ .05. ** p ⬍ .01. a

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hemispheres (e.g., Kim & Levine, 1991; Levy, Heller, Banich, & Burton, 1983). Such findings have been interpreted as reflecting individual variations in hemispheric arousal or activation, superimposed upon differing patterns of hemispheric asymmetry as a function of the task. Significant negative correlations were obtained between the local laterality quotient and both Same (r ⫽ ⫺.36, p ⬍ .01) and Different RTs (r ⫽ ⫺.32, p ⬍ .02) for the same-different task. These indicate that subjects who exhibited greater LH lateralization of local processing were slower in the same-different task. This is consistent with the current hypothetical framework, indicating that greater degrees of lateralization of function may be associated with relative impairment at tasks requiring integration of LH and RH processes. Finally, a significant positive correlation was obtained between Same and Different RTs (r ⫽ .85, p ⬍ .01). Interestingly, the lack of significant correlations (⫺0.14 ⬍ all r’s ⬍ ⫺.03) between measures of local and global interference, on the one hand, and the Same and Different RTs, on the other, suggests that the ability to keep LH and RH processing independent (e.g., as indexed by smaller interference scores) is not related to the ability to integrate LH and RH processing (e.g., as indexed by faster RTs on the same-different task). In other words, although left-handers as a group exhibited both greater interference in the focused attention task and faster RTs in the same-different task, it was not the same individuals who necessarily exhibited both effects. This is intriguing, as it suggests that, although left-handers may indeed exhibit both greater interhemispheric integration and lesser interhemispheric independence, these effects reflect separate and independent mechanisms of integrative versus inhibitory interhemispheric interaction. EXPERIMENT 2

Experiment 2 serves as an extension of Experiment 1 by applying the same theoretical framework of integrated versus independent bidimensional/bihemispheric processing to performance on variants of the Stroop task. The equivalent of the focused attention and same-different tasks from Experiment 1 were employed. In the focused attention task (which is the standard version of the Stroop task), Ss are required to name the color in which the stimulus is printed, ignoring the identity of the word. The same-different task will be an exact replication of the Stroop task from Christman (1995). As in Experiment 1, it is predicted that, for centrally presented input, lefthanders will exhibit a relative disadvantage in keeping the two dimensions separate in the focused attention task (and will consequently exhibit greater Stroop interference) and a relative advantage at integrating the two dimensions in the same-different task. Methods Subjects. Sixty-four subjects (16 right-handed females, 16 right-handed males, 16 left-handed females, and 16 left-handed males) participated in this study for course credit as part of their enrollment in an introductory psychology course. Right-handed subjects had no left-handed relatives in their immediate family. Apparatus. All stimuli were presented on a Macintosh IIfx computer with a high-resolution color monitor and an Apple 8-24GC graphics accelerator card. Control of stimulus presentation and response timing were under the control of the MindLab program (Bharucha, Meike, & Baird, 1987). Stimuli and procedure. The Stroop stimuli consisted of the words ‘‘BLUE,’’ ‘‘GREEN,’’ ‘‘YELLOW,’’ and ‘‘RED,’’ along with a control condition consisting of a string of three to six ‘‘X’’s; the actual colors that these stimuli were printed in were either blue, green, yellow, or red. All possible

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combinations of word and color were used. Stimuli for this task subtended a range from .41° ⫻ .82° (for the word ‘‘RED’’) to .41° ⫻ 1.4° (for the word ‘‘YELLOW’’). In order to increase their contrast and visibility, these stimuli were presented on a dark background with a luminance of 1.1 cd/m 2. The luminance values for the stimuli were 17 cd/m 2 for the color red, 43 cd/m 2 for green, 10 cd/m 2 for blue, and 68 cd/m 2 for yellow. These stimuli were presented to left, central, and right visual field locations; lateralized stimuli were presented 2.6° from fixation. To reduce potential confounds between visual field and eccentricity (i.e., the beginning of words being farther from fixation for LVF trials), stimuli were presented in a vertical orientation. All subjects participated in two tasks. For both tasks, stimuli appeared randomly for 150 ms in the LVF, RVF, or central visual field (CVF). Trials began with a warning tone lasting 200 ms that preceded the stimulus by 500 ms. Task order and hand of response were counterbalanced across subjects. One task was a variant of the Stroop task: Subjects were to indicate the color of a letter string flashed on a computer screen. Subjects responded by pressing one key with the index finger of one hand if the stimulus color was blue or yellow and by pressing another key with the index finger of their other hand if the stimulus color was red or green. Response assignment was counterbalanced across subjects. There were three stimulus conditions: (1) Congruent, in which the letter string was a color word corresponding to the stimulus’ actual color; (2) Incongruent, in which the letter string was a color word that did not correspond to the stimulus’ actual color nor the other color in the relevant response set (e.g., the word ‘‘GREEN’’ never appeared in the color red); and (3) Neutral, in which the letter string consisted of a series of Xs. This task was run in two blocks of 96 trials each for a total of 192 trials. There were 12 replications per stimulus color per visual field for the Congruent and Incongruent stimulus types (yielding 24 replications per response set) and 16 replications for the Neutral stimuli. Inclusion of Neutral stimuli allows explicit examination of facilitation effects, defined as the RT advantage for Congruent stimuli relative to Neutral stimuli. Some researchers have suggested that Stroop interference and Stroop facilitation reflect independent processes (e.g., MacLeod, 1991). Lindsay and Jacoby (1994), however, have recently argued that Stroop interference and Stroop facilitation may not reflect separate mechanisms, but rather may both reflect conjoint contributions of word-reading and color-naming processes. However, they point out that as the influence on performance of color-naming processes decreases (relative to the influence of word-reading processes), facilitation increases relative to interference. Thus, while comparison of interference and facilitation measures cannot be used in a simple or direct way to separately estimate handedness differences in bihemispheric independence versus integration, the presence of such differences in the relative magnitudes of interference and facilitation may shed light on possible individual differences in the contribution of color-naming versus word-reading processing to performance on Stroop stimuli. The second task involved subjects judging whether the color word was the same as or different than the actual color in which the word was printed (the stimuli for this task consisted of the Congruent and Incongruent stimuli described above). This task consisted of one block of 96 trials, yielded by the factorial combination of 3 visual fields ⫻ 2 judgments (same versus different) ⫻ 4 colors ⫻ 4 replications.

Results and Discussion The average error rate across conditions of the Stroop task was 7.4%. Analyses of variance for the error data revealed main effects of condition, F(2, 480) ⫽ 18.54, p ⬍ .001, and of visual field, F(2, 120) ⫽ 3.61, p ⬍ .03, with Congruent trials and CVF trials yielding lower error rates. There were no other significant main effects or interactions. Two RT measures were analyzed for the Stroop task: the amount of interference (RT for Incongruent minus Neutral trials) and the amount of facilitation (RT for Neutral minus Congruent trials); the Interference and Facilitation RT data are shown in Figs. 4 and 5 respectively. A three-factor ANOVA (Handedness ⫻ Sex ⫻ Visual Field) for the interference data yielded only one significant effect: an interaction between Handedness and Visual Field, F(2, 120) ⫽ 3.44, p ⬍ .035. This interaction arose from the fact that, while left- and right-handers did not differ for LVF or RVF trials, Fs ⬍ 1, left-handers exhibited significantly more interference than right-handers for CVF presentations (90.4 vs 39.8 ms), F(1, 62) ⫽ 4.80, p ⬍ .04. This suggests that for lateral presentations, where the processing of the verbal and chromatic stimulus dimensions were presumably confined to a single hemisphere, both handedness

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FIG. 4. Interference data (Incongruent RT minus Neutral RT) as a function of handedness and sex in Experiment 2 (error bars indicate mean standard error).

groups exhibited equal amounts of interference; this in turn suggests that left- and right-handers may not differ significantly in characteristics of intrahemispheric processing. For CVF trials, however, where the opportunity is present for the hemispheres to each process their own preferred dimension, left-handers exhibited greater interference, presumably reflecting a relative inability to keep LH and RH processes functionally isolated. An alternative way to view these results is to note that, relative to lateralized trials, right-handers exhibited the least interference for CVF trials while left-handers exhib-

FIG. 5. Facilitation data (Neutral RT minus Congruent RT) as a function of handedness and sex in Experiment 2 (error bars indicate mean standard error).

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ited the greatest interference for CVF trials. This suggests that right-handers were benefited by central presentation, presumably because this allowed the processing of the verbal and chromatic dimensions to be segregated to opposite hemispheres which exhibit minimal interaction. Conversely, left-handers were particularly impaired by central, relative to lateral, presentation. When both stimulus dimensions are projected to a single hemisphere via lateral presentation, it may be the case that the single hemisphere cannot adequately process both dimensions simultaneously; to the extent that the processing of the verbal dimension is diminished, interference would decrease. For central presentations, however, the greater ability of each hemisphere to process its preferred dimension, along with greater interhemispheric integration, leads to increased magnitude of interference in left-handers. Although the primary focus of the current experiment is on handedness differences for central presentations, the results for LVF and RVF trials can be compared to previous studies of hemispheric asymmetry in Stroop effects. The lack of differences between left- and right-handers for lateralized trials mirrors the results of Franzon and Hugdahl (1986); similarly, the lack of RT differences between LVF and RVF trials mirrors the findings of Dyer (1973), Tsao, Feustel, and Soseos (1979) and Warren and Marsh (1981). On the other hand, some researchers have reported greater interference for RVF trials (Guiard, 1981; Schmit & Davis, 1974), while others have reported greater interference for LVF trials (Long & Lyman, 1987). The origin of these discrepancies is unclear. The only significant effect for the facilitation data was a three-way interaction between Handedness, Sex, and Visual Field, F(2, 120) ⫽ 3.19, p ⬍ .05. This effect arose from a trend for right-handed males to display more facilitation than righthanded females for RVF trials only, while left-handed males displayed more facilitation than left-handed females for LVF trials only. There were no other systematic differences between handedness groups for the facilitation data, although left-handers exhibited nominally greater facilitation than right-handers (27.4 vs 8.3 ms), F(1, 120) ⫽ 1.75 p ⬍ .19. Thus, under conditions of bihemispheric access to foveal input, left-handers exhibit greater interaction between the verbal and chromatic dimensions of Stroop stimuli, displaying significantly greater interference and marginally greater facilitation. These effects presumably reflect the greater degree of interaction in left-handers between LH and RH processing of the verbal and chromatic dimensions of Stroop stimuli. Following the reasoning of Lindsay and Jacoby (1994), the fact that the magnitude of interference versus facilitation in the LVF and RVF was equivalent for both handedness groups suggests that word-reading and color-naming processes contributed equally to the production of Stroop effects. Similarly, the fact that the only condition where the magnitude of facilitation exceeded that of interference was for LVF trials among left-handed males suggests that the RH of left-handed males may be relatively impaired at color-naming and/or efficient at word reading. The overall error rate for the same-different task was 8.3%. Analysis of variance indicated a significant main effect of visual field, arising from lower error rates for CVF relative to LVF and RVF trials, F(2, 120) ⫽ 17.82, p ⬍ .001. There were also two significant interactions: Decision ⫻ Visual Field ⫻ Handedness, F(2, 120) ⫽ 4.64, p ⬍ .02, and Decision ⫻ Visual Field ⫻ Handedness ⫻ Sex, F(2, 120) ⫽ 8.16, p ⬍ .01. The four-way interaction arose from the fact that, while there were no differences for CVF trials as a function of handedness or sex, performance on RVF versus LVF trials reflected a complex influence of decision, handedness, and sex variables. In particular, right-handed females exhibited no visual field difference for ‘‘same’’ trials and a RVF advantage for ‘‘different’’ trials; right-handed males exhibited a RVF advantage for ‘‘same’’ trials and no visual field difference for ‘‘dif-

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ferent’’ trials; left-handed females exhibited a RVF advantage for ‘‘same’’ trials and a LVF advantage for ‘‘different’’ trials; and left-handed males exhibited no visual field difference for ‘‘same’’ trials and a RVF advantage for ‘‘different’’ trials. This complex set of effects is not readily interpretable within the current framework; however, it can be noted that left-handers did not exhibit any overall performance advantage on this task, in contrast to the results reported by Christman (1995). A four-factor ANOVA (Handedness ⫻ Sex ⫻ Decision ⫻ Visual Field) for the RT data from the same-different task yielded main effects for Decision (Same RT faster than Different RT), F(1, 300) ⫽ 97.2, p ⬍ .001, and Visual Field (CVF RT faster than LVF and RVF RT), F(2, 300) ⫽ 9.05, p ⬍ .001. A significant interaction between Handedness, Sex, and Decision was also obtained, F(1, 300) ⫽ 4.22, p ⬍ .05, arising from the fact that the difference between Different and Same RTs (collapsed across visual field) was larger for left-handed females (92 ms) and righthanded males (100 ms) than it was for right-handed females (62 ms) and left-handed males (63 ms). The absence of any main effect for handedness suggests that lefthanders were not superior at comparing and contrasting the verbal and chromatic levels of Stroop stimuli. This result also stands in contrast to that obtained by Christman (1995), where left-handers displayed an overall RT advantage for CVF trials. This lack of replication may have arisen from methodological differences between the two experiments: Christman (1995) employed longer exposure durations that did not terminate until the subject responded, and the stimuli were horizontally, not vertically, oriented. A final set of analyses focused on intra- and intertask correlations (collapsed across both subject groups), which are shown in Table 2. To simplify the presentation of these data, correlations are presented separately for each visual field. With regard to cross-field correlations, all same and different RTs correlated with each other; the only other significant cross-field correlations were between RVF facilitation and LVF facilitation (r ⫽ .310, p ⬍ .05), between RVF facilitation and CVF facilitation (r ⫽ .251, p ⬍ .05), between LVF facilitation and CVF facilitation (r ⫽ .367, p ⬍ .01), and between CVF interference and RVF interference (r ⫽ .328, p ⬍ .01). Of particular concern are within-field correlations between measures of facilitation/interference, on one hand, and same-different RTs, on the other. Such correla-

TABLE 2 Within-Field Correlations for Experiment 2 a Interference LVF Facilitation Same Different CVF Facilitation Same Different RVF Facilitation Same Different

Facilitation

Same

⫺0.610** 0.079 ⫺0.034

0.149 0.264*

0.853**

⫺0.646** 0.299* 0.274*

⫺0.312* ⫺0.192

0.821**

⫺0.312* 0.149 0.185

⫺0.023 0.023

0.928**

Interference ⫽ Incongruent RT minus Neutral RT; Facilitation ⫽ Neutral RT minus Congruent RT. * p ⬍ .05. ** p ⬍ .01. a

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tions indicate possible relations among measures of interhemispheric independence (as indexed by the standard Stroop task) and integration (as indexed by the samedifferent task). Interestingly, significant intertask interactions were obtained primarily for CVF trials (where bihemispheric processing was most likely). Specifically, for CVF trials, Stroop interference scores were positively correlated with both Same and Different RTs (p ⬍ .05), indicating that subjects who could not keep the two stimulus levels independent during the Stroop task (i.e., showed greater interference) were impaired at comparing the two levels during the same-different task. Additionally, CVF facilitation scores were negatively correlated with CVF Same RTs ( p ⬍ .01), indicating that subjects who tended to integrate information between the two levels during the Stroop task (i.e., showed greater facilitation) were also faster at comparing the information at the two levels during the same-different task. The only other crosstask, within-field correlation was between LVF facilitation and LVF same RT. These correlational analyses indicate that, independent of handedness, Ss who showed greater interaction between the two stimulus levels in one task tended to show related types of interlevel interaction in the other task, but primarily for CVF trials only. It is hypothesized that such interlevel interactions reflect characteristics of interhemispheric interaction under conditions of foveal input, allowing for bihemispheric access to input. This set of results stands in contrast to the correlational analyses for Experiment 1, which indicated no relation between CVF measures of interlevel interference (i.e. in the focused attention task) and interlevel integration (i.e., same-different task). The source of this disparity is unclear. Logically, one might expect that greater interhemispheric integration (as indexed by faster RTs on the same-different task) would be associated with lesser interhemispheric independence (as indexed by interference measures). The results of Experiment 1, however, indicated no such relation. Furthermore, the correlation between CVF interference and CVF same-different RTs in the Experiment 2 suggests the opposite: namely greater interhemispheric independence (i.e., less interlevel interference) was associated with greater interhemispheric integration (i.e., faster same-different RTs). Conversely, the current negative correlation between CVF facilitation and CVF Different RT indicates a relation between lesser interhemispheric independence (as indexed by greater Stroop facilitation) and greater interhemispheric integration (as indexed by faster same RTs). At the very least, the varied and complex nature of these sets of correlations suggests that the relation between interhemispheric integration versus independence is complex, varying as a function of task and input variables. Turning to intratask correlations, the magnitude of interference and facilitation within each visual field on the Stroop task were negatively correlated with each other. Similar effects were reported by Lindsay and Jacoby (1994), who suggest that such negative correlations arise when color-naming processes exhibit greater variability than word-naming processes in their contribution to Stroop performance. Finally, the amounts of facilitation for LVF, CVF, and RVF trials were all positively correlated with each other. Following the reasoning of Lindsay and Jacoby (1994), this may reflect the fact that the relative contribution of color- and word-naming processes remains constant within a subject across different viewing conditions.

GENERAL DISCUSSION

The present experiments present evidence for individual differences in the nature of interdimensional interaction in the processing of Stroop and local-global stimuli. In particular, left- versus right-handers exhibited greater degrees of interdimensional

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integration versus independence, respectively. In turn, these results were tentatively interpreted as reflecting (i) the fact that the processing of the two dimensions of such stimuli are lateralized to opposite hemispheres and (ii) left- versus right-handers exhibit greater degrees of interhemispheric integration versus independence, respectively. The current results extend the ideas of Kinsbourne and Hicks (1978) regarding functional cerebral distance in two ways. First of all, there appears to be lesser functional cerebral distance between the LH and RH in left-handers than in right-handers. This can sometimes lead to a performance advantage for left-handers when LH and RH processes need to be explicitly integrated. Conversely, when the two dimensions of unitary input represent discordant information (e.g., incongruent Stroop and localglobal stimuli), the lesser functional distance in left-handers leads to greater interference between the two dimensions. The presence of a varied set of correlations across tasks and between measures of interference versus facilitation indicates that mechanisms of interdimensional (and presumably interhemispheric) interaction vary as a function of task, input, and subject variables. For example, the results of Experiment 1 suggest that processes of interdimensional integration versus independence are dissociable, implying that the ability to integrate local and global processing does not simply mirror an inability to keep local and global processing separate: while left-handers exhibited both greater interdimensional integration and lesser interdimensional independence, these effects were not correlated. Thus, mechanisms underlying local-global integration are separate from those underlying local-global interference. On the other hand, the fact that measures of interdimensional integration and independence in Experiment 2 were related suggests that, for at least some tasks, common mechanisms can underlie both interdimensional integration and independence. Future work should further examine the unique versus shared mechanisms underlying interdimensional integration versus independence. The current results clearly indicate that these phenomena are, at least in part, separate. One intriguing, although speculative, possibility is that interhemispheric integration is related to the degree of functional cerebral lateralization, while interhemispheric independence is related to the degree of callosal connectivity. For example, the finding from Experiment 1 that lesser functional lateralization was related to greater interhemispheric integration but not to lesser interhemispheric independence suggests that, to the extent to which the LH and RH share common functions, they can exhibit greater integration of information. In contrast, and in line with the interpretations offered by David (1992) and Robertson, Lamb, and Zaidel (1993) for the lack of interdimensional interference in callosal agenesis and split-brain patients, interhemispheric independence may be callosally mediated. From a methodological viewpoint, the current experiments offer an example of how mechanisms of interhemispheric interaction might be studied without relying on lateralized input presentation. The use of bilateral displays, allowing for projection of input to both hemispheres, provides a useful tool for studying interhemispheric processing, as they allow for direct control over which hemisphere processes which dimensions of input (e.g., Banich & Belger, 1990; Banich & Karol, 1992). As the hemispheric division of labor in the current experiments is only inferred, not directly controlled, the current approach should be viewed as complementary to the use of bilateral displays in the study of interhemispheric interaction. Nonetheless, bilateral displays are constrained by input degradation, and the use of separate inputs limits direct comparability with studies employing unitary input. Indeed, the relative lack of contact between theories of normal human cognition and theories of hemispheric asymmetry stems in large part from the disparate methodologies employed in these

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areas; typical cognitive research employs centrally presented input with relatively long exposure durations, whereas laterality research employs parafoveal input presentation with short exposure durations. It should be pointed out in this context that Hellige’s use of qualitative error patterns to infer hemispheric metacontrol has also been successfully adapted to incorporate the use of central presentation of input (e.g., Hellige, Taylor, & Eng, 1989); however, such an approach is limited to tasks for which qualitative error measures are readily available. The ability to study interhemispheric processing via the use of central input offers a valuable bridge between the areas of cognitive and laterality research. In particular, the presence of systematic individual differences (as a function of handedness) in the processing of central input offers important implications for cognitive researchers who may not be explicitly interested in neuropsychological variables. At the very least, researchers may consider controlling for subject handedness (which can be readily measured) in their designs as a way of accounting for extra variance (as opposed to simply treating intersubject differences as random error variance). At best, the inclusion of handedness as an explicit theoretical factor may provide important insights for models of human perception and performance. For example, the individual differences reported by Yee and Hunt (1991) for Stroop interference were suggested to reflect temporal differences in activating word meanings and/or differences in the ability to suppress irrelevant information; the current results suggest that the latter factor may be more important. Similarly, Ward’s (1985) findings of individual differences in local-global interference were interpreted in terms of the integrality versus separability of dimensional processing (cf. Garner, 1976); the current results suggest that left-handers may exhibit lesser separable and greater integral dimensional processing. The possibility that left-handers may have indeed been overrepresented in the integrated processing group in Ward’s study is strengthened by his finding that those subjects who processed the stimuli in an integral manner exhibited greater field dependence than subjects who processed the stimulus dimensions in a more separable manner. Numerous studies have reported that lefthanders are more field dependent than right-handers (e.g., O’Connor & Shaw, 1978; Silverman, Adevai, & McGough, 1966). Finally, a combination of preferential interhemispheric transfer of lower spatial frequencies (e.g., Berardi & Fiorentini, 1987; Kitterle, Christman, & Conesa, 1995) and the presence of greater interhemispheric integration in left-handers could potentially account for evidence that left-handers are superior in the foveal processing of lower spatial frequency information (Christman, 1989). Examination of the kinds of stimuli that yield interdimensional interference offers implications concerning possible evolutionary advantages bestowed by lateralization of complementary and/or incongruent processes to opposite hemispheres. For example, in his extensive review of the Stroop interference literature, MacLeod (1991) cites the following paradigms as yielding Stroop-like interdimensional interference: (a) the standard Stroop paradigm, involving interference between verbal and chromatic dimensions; (b) picture–word interference tasks, involving interference between verbal and pictorial dimensions; (c) auditory analogs of the Stroop task, involving interference between verbal and intonation dimensions; (d) font judgment tasks (in which subjects identify the type font of words printed in either the congruent or incongruent font, such as the word ‘‘bold’’ printed in bold or normal font), involving interference between verbal and orthographic dimensions; (e) numerosity tasks (in which subjects count the number of digits in a display, with the number of digits being the same as or different than the identity of the digits), involving interference between alphanumeric and numerosity dimensions; and (f ) local-global tasks, involving interference between local and global dimensions. In each case, interference is

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obtained between dimensions lateralized to opposite hemispheres; that is, the LH is specialized for processing verbal and local dimensions, whereas the RH is specialized for chromatic, spatial, intonational (Blumstein & Cooper, 1974; Hartje, Willmes, & Weniger, 1985), orthographic (Bryden & Allard, 1976), numerosity (Boles, 1986; Kimura, 1966), and global dimensions. Thus, the lateralization of such dimensions to opposite hemispheres may confer an evolutionary advantage of reduced interdimensional interference. Presumably, the greater interference exhibited by left-handers is offset by an advantage in interdimensional integration. In conclusion, the current experiments provide an initial report of a novel approach to studying interhemispheric interaction using methods commonly employed in mainstream research. In addition, the use of multitask studies and consideration of individual differences offers a useful analytic tool for obtaining converging evidence concerning interrelationships among various subcomponents of complex cognitive processes. Previous attempts to relate individual differences in cognition to neuropsychological variables have suffered from simplistic reasoning. For example, Bogen, De Zure, Tenhouten, and Marsh (1972) devised the term ‘‘hemisphericity’’ to refer to the hypothesized tendency for people to rely more on the mode of processing of one hemisphere than the other, leading to systematic differences in cognitive style. The idea of hemisphericity has come under conceptual attack (Beaumont, Young, & McManus, 1984; Hardyck & Haapanen, 1979). One of the main critiques that Beaumont and colleagues (1984) leveled against the theoretical basis of hemisphericity is that ‘‘it seems to ignore the fact that the cerebral hemispheres work together in normal people, forming a single integrated system’’ (p. 205). The hypotheses developed in the current article suggest that what is important may not be a tendency to prefer the use of one hemisphere over another (a simplistic notion to begin with, since most complex tasks require the resources of both hemispheres); rather, what may be an important factor in determining individual differences in cognitive style could be the extent to which the processing abilities of the left and right cerebral hemispheres operate in relatively integrated versus independent manners. REFERENCES Banich, M., & Belger, A. (1990). Interhemispheric interaction: How do the hemispheres divide and conquer a task? Cortex, 26, 77–94. Banich, M., Goering, S., Stolar, N., & Belger, A. (1990). Interhemispheric processing in left- and righthanders. International Journal of Neuroscience, 54, 197–208. Banich, M., & Karol, D. (1992). The sum of the parts does not equal the whole: Evidence from bihemispheric processing. Journal of Experimental Psychology: Human Perception and Performance, 18, 763–784. Beaumont, G., & Dimond, S. (1973). Transfer between the cerebral hemispheres in human learning. Acta Psychologica, 37, 87–91. Beaumont, G., & Dimond, S. (1975). Interhemispheric transfer of figural information in right- and nonright-handed subjects. Acta Psychologica, 39, 97–104. Beaumont, J. G., Young, A., & McManus, I. (1984). Hemisphericity: A critical review. Cognitive Neuropsychology, 1, 191–212. Berardi, N., & Fiorentini, N. (1987). Interhemispheric transfer of visual information in humans: Spatial characteristics. Journal of Physiology (London), 384, 633–647. Bharucha, J., Meike, B., & Baird, J. (1987). The Macintosh as a user-friendly laboratory for perception and cognition. Behavior Research Methods, Instruments, & Computers, 19, 131–134. Blumstein, S., & Cooper, W. (1974). Hemispheric processing of intonation contours. Cortex, 10, 146– 158. Bogen, J., DeZure, R., Tenhouten, W., & Marsh, J. (1972). The other side of the brain IV: The A/P ratio. Bulletin of the Los Angeles Neurological Societies, 37, 49–61.

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