Bugs and Faces in the Two Visual Fields: The Analytic/Holistic Processing Dichotomy and Task Sequencing

BUGS AND FACES IN THE TWO VISUAL FIELDS: THE ANALYTIC/HOLISTIC PROCESSING DICHOTOMY AND

TASK SEQUENCING

John L. Bradshaw and Del Sherlock

(Monash University, Clayton, Victoria 3168, Australia)

A left visual field (LVF, right hemisphere) superiority is typically reported when normal subjects process a wide range of visuospatial stimuli (see e.g., Moscovich, 1979, for review), with analogous effects occurring in the clinical literature (see recent review by Benton, 1979). Face stimuli demonstrate such effects par excellence, with the result that some would claim that there is a specialist mechanism dedicated to the processing of faces in the right hemisphere (Carey and Diamond, 1977), though as Benton (1980) notes in his clinical review under certain circumstances bilateral or even left hemisphere processing of faces may be apparent. With normal subjects a RVF (left hemisphere) superiority may occur with the recognition of well known faces (Marzi, Brizzolara, Rizzolatti, Umilta and Berlucchi, 1974; Marzi and Berlucchi, 1977), possibly as a result of verbal mediation, though Moscovitch, Scullion and Christie (1976) when their subjects matched caricatures of faces to photographs obtained a L VF superiority even for well known faces. Independently of naming, a RVF superiority may appear with increased familiarity with hitherto unknown faces (Umilta, Brizzolara, Tabossi and Fairweather, 1978). A RVF superiority, again independent of naming, was found when nontarget faces were very similar to targets, differing from them by only a single feature, thus requiring detailed analytic processing (Patterson, 1976; Patterson and Bradshaw, 1975; Fairweather, Brizzolara, Tabossi and Umilta, unpublished manuscript), a finding which is compatible in broad terms with an analytic/holistic distinction.

Findings such as those reviewed above have led to questions (Bradshaw and Nettleton, 1981) about the adequacy of the old (e.g. Kimura, 1961) verbal/visuospatial dichotomy. With search for a more fundamental antecedent mode of specialization, several versions of an analytic/ holistic distinction (see e.g., Nebes, 1978) have often independently surfaced, e.g., serial/parallel (Cohen, 1973), focal/diffuse (Semmes, 1968), similarity judgements versus difference detection (Egeth and Epstein, 1972), and a view that the left hemisphere may be pre-eminently specialized for the analysis of temporal sequences (e.g. Efron, 1963) and the

Cortex (1982) 18, 211-226

212 John L Bradshaw and Del Sherlock

sequential control of finger, hand, limb and articulatory musculature (Kimura, 1977). According to the basic concept of the analytic/holistic separation of function, the right hemisphere is superior at perceiving the relationships between component parts and the whole configuration, in performing spatial transformations of the visual input and in making rapid identity matches, while the left is concerned with isolating discrete features or elements within the entire configuration, and possibly with difference detection (see e.g. Bradshaw, Gates and Patterson, 1976). Indeed there is now considerable evidence for a unique left hemisphere specialization in terms of temporal order, sequencing and segmentation, whether manifested in terms of a right ear advantage for complex, rhythmic, sequential series of acoustic stimuli (Carmon, 1978; Efron, 1963; Halperin, 'Nachshon and Carmon, 1973; Mills and Rollman, 1979; Pap~un, Krashen, Terbeek, Remington and Harshman, 1974; Robinson and Solomon, 1974), or in terms of the control of limb, hand, finger or articulator positioning (Kimura, 1977; Sussman and Westbury, 1978). Left hemisphere mediation of language accordingly may possibly depend upon its involvement in such analytic, time dependent and sequential functions.

Notwithstanding the above conclusions, it is curious that no-one has seen fit or been able to demonstrate while employing the same visual stimuli, reversed field asymmetries as a function of imposed processing set. Galper and Costa (1980) came near to doing this when their subjects were required to learn the target faces in terms either of perceived overall personality characteristics (holistic mode), or of individual component features (analytic mode). Unfortunately their results failed fully to support their hypothesis. Similarly most studies have manipulated the individual features or elements themselves, rather than the spatial patterns of interrelationships between and otherwise independent of the actual elements. Moreover, no studies have employed and compared the processing of face-like stimuli and similarly complex and meaningful naturalistic patterns such as outline bugs in the same experiment. Finally, such a comparison itself permits an investigation of the effects of task order, sequencing and practice upon lateral asymmetries. According to some (Hellige, 1976; Bradshaw and Gates, 1978; Goldberg and Costa, 1981) more robust or typical effects may be more characteristic later in a stimulus sequence.

The three experiments we report aimed specifically to address the above issues. Instead of manipulating processing set itself (Galper and Costa, 1980), we required subjects either to attend to the overall configuration, or to specified feature elements, and in the various experimental conditions, analytic or holistic, we were able to compare the effects of using outline face or bug stimuli. Indeed, real life objects may be recog

213 The analytic/holistic processing dichotomy

nized not only by characteristic variations in their component features (e.g. shape of nose in a face, gross structure of thorax or abdomen in a bug) but also by the distance between eye axis and nose in faces, and the relative sizes of head and thorax or thorax and abdomen, irrespective of their absolute values, in the case of bugs.

GENERAL METHOD

Stimuli and Tasks

Face stimuli consisted of 16 schematic outlines wherein all3 features (eyes, nose and mouth) and the distance between these features could adopt one of two possible values. Eyes were represented by squares or diamonds, and the nose (a small triangle) was either apex up or down (i.e. upright or inverted). The mouth (a diamond) was either closed (i.e. the diamond was transected by a horizontal line) or open (without the transect). With the position of the nose fixed, the eyes and mouth could adopt two possible positions relative to the nose, that is close to or distant from the nose. Ears and hairlines were not varied. Suberi and McKeever (1977) suggest that the emotional content of schematic faces which we have previously used (Patterson and Bradshaw, 1975) may be sufficient in itself to induce a L VF superiority. The current series of faces were so designed as to be judged devoid of expression. Half of the 16 faces were targets, half nontargets. The tasks associated with these faces were either analytic or holistic.

In the Holistic Faces task, target faces (8) were readily distinguished from nontargets (8) in that all3 features were far apart in the former and close together in the latter. In the Analytic Faces task, target faces (8) had an upright (apex up) nose, while nontargets (8) had an inverted (apex down) nose. See Figure 1.

Upon presentation, the nearest feature of a face was 1.5° from fixation, and the midpoint of the face was 2.0°. Each face sub tended 2.5 o vertically, the subject sitting 57 em from the rear projection screen upon which the stimuli were subsequently projected.

With respect to bugs, each of the 16 schematic stimuli consisted of 4 elements, antennae, head, thorax and abdomen. In the Holistic Bugs task, there were 4 targets, of which 2 were large and 2 were small, in each case the relative sizes of the 4 elements being quite regular. In the remaining 12 stimuli (nontargets), 2 of the elements were large, 2 were small, i.e., the outlines were irregular.

It should be noted that regularity of outline (i.e. relative sizes of the various elements) was the more salient and useful cue to discriminate nontargets from targets, as the latter could be either large or small overall, with the former intermediate in size. In the Analytic Bugs task, the 8 targets had a pointed thorax, and the 8 nontargets had a curved thorax. (This thoracic variation was the reason for there being 4, not just 2 target bugs in the Holistic condition). See Figure 2.

From fixation to the midpoint of each bug was a distance of 2.0°, and to the near edge it was 1.5° (maximum) or 1.3° (minimum). Width was 1.4° (maximum) or 1.0° (minimum). Heightwas3.0° (maximum) or 1.0° (minimum). These values again relate to the stimuli as subsequently projected.


STIMULI FOR HOLISTIC FACE TASK

TARGETS

NON TARGETS Fig. 1 - Faces stimuli employed in the three experiments. In the Holistic Faces task, the 8

targets (top 2 rows) and 8 non targets (bottom 2 rows) are as shown, with the feature elements respectively far apart (targets) or close together (nontargets). In the Analytic Faces task, the targets are shown in the top and secondfrom bottom rows (noses apex up) and the nontargets are in the bottom and second from top rows (noses apex down).


STIMULI FOR HOLISTIC BUGS TASK

TARGETS

NONTARGETS Fig. 2- Bugs stimuli employed in the three experiments. In the Holistic Bugs task, the 4 targets (2 large and 2 small) are as shown in the top row, andpossess a regular configuration. The 12 nontargets in the remaining 3 rowspossess an irregular configuration with 2 large and 2 small elements. In the Analytic Bugs task, the 8 targets all possess a pointed thorax (second column from left, and right-most column) and the 8 nontargets all possess a curved thorax (left-most column and second column from the right).


Apparatus

Stimuli were drawn from prepared templates on to white cards and photographed. The resultant 35 mm negatives were mounted and projected by projection tachistoscope. The subject sat with head in a chin rest, facing the back projection screen. Each trial was preceded by a warning tone (1kHz for 1 sec.) during which the fixation point, a red light-emitting diode, flashed in the middle of the screen. The test item then appeared for 150 msec. The subjects responded by pressing buttons on a panel in front of them thus stopping an electronic msec. timer which had started at stimulus onset. The response panel consisted of four buttons, where subjects pressed either the two nearer buttons ("target" response) or the two buttons further away ("nontarget"). This relationship was reversed for half the subjects. Subjects were instructed to press the two buttons of an appropriate pair simultaneously, the first button depressed stopping the timer. This bimanual response was employed, as in all our previous studies, to avoid one hand or hemisphere leading. Each subject was required to respond as quickly and as accurately as possible, the best strategy being to fixate the flashing red light at the commencement of the warning tone. Trials where there were any errors or long reaction times which exceeded by an arbitrarily set criterion of 400 msec. the subject's average during the practice block, were subsequently replaced at the end of the block, subject to the same randomization constraints.

Subjects and procedure

Subjects were all students at Monash University who had normal or corrected-to-normal vision with not more than 2 points difference between eyes on the Bausch and Lomb Orthorater. Handedness was carefully assessed by our standard questionnaire, pegboard task and object manipulation procedures (Patterson and Bradshaw, 1975), and subjects were only accepted if strongly dextral. All had had previous experience in tasks of this general nature. In each of the three experiments a different group of 24 subjects were employed, 12 of either sex.

In all three experiments, the same general procedure was adopted. Subjects first studied for about 5 minutes a copy of the 16 stimuli relevant to the ensuing task (faces or bugs, holistic or analytic). They were then given a practice block (32 trials). If they reached a satisfactory criterion they immediately progressed to the test trials; otherwise they were given another practice block. If they still failed to reach criterion they were replaced. The criterion was not to make more than 1 error. A similar procedure was employed before the commencement of the second task, and if any subjects failed to reach a satisfactory criterion, their data for the first task were dropped from analysis, and replaced. Because of this stringent criterion for accurary, few errors were made in the experimental series, too few for statistical analyses, and only reaction time {RT) data will be considered. Both tasks were completed within 1 hour. The two faces tasks comprised 6 blocks of 32 trials, and the two bugs tasks 4 blocks of 48 trials, the difference in numbers of blocks per task being necessary to balance for equal numbers of trials in either visual field, for equal numbers of targets and nontargest. A pseudorandom sequence was employed with respect to targets and nontargets, and left and right field presentations. In all three experiments there were two order conditions, Order 1 and Order 2. Half the subjects encountered the tasks in the Order 1 condition (e.g., Analytic Faces preceding Holistic Bugs), and with the remainder


the order was reversed (Order 2, e.g., Holistic Bugs preceding Analytic Faces). Order was subsequently entered as a factor into the Analysis of Variance (ANOVA) in order to examine the possible effects of task sequencing upon laterality effects.

ExPERIMENT 1

In this experiment subjects performed either the Analytic Faces task followed by Analytic Bugs (Order 1, 6 males and 6 females), or the same two tasks in the opposite sequence (Order 2, the remaining 12 subjects). In terms of the analytic/holistic processing hypothesis (Bradshaw and Nettleton, 1981) and our earlier findings with outline face stimuli (Patterson and Bradshaw, 1975, Experiment 3), a RVF superiority might be predicted, more strongly perhaps for target stimuli (Suberi and McKeever, 1977). Apart from the single successful though unpublished study by Fairweather et al. (unpublished manuscript), this experiment would appear to be the first recorded attempt at a replication and extension of our earlier study with faces. It should be noted that the traditional verbal/visuospatial processing dichotomy would predict a L VF superiority for both tasks, particularly so perhaps for the faces task if the right hemisphere possesses an additional specialization for mediating faces (Carey and Diamond, 1977).

Results and discussion

A five-way ANOVA (Sex, Order, Task, Visual Field, Targets/Nontargets with repeated measures on the last three factors) was performed on the RT data (see Table 1).

TABLE I

Reaction Time Data for the Three Experiments, Split by Task, Target/ Nontarget Responses and Visual Field (left visual field: LVF, right visual field: R VF)

Experiment 1 Experiment 2 Experiment 3 Analytic Analytic Holistic Holistic Analytic Holistic

faces bugs faces bugs faces bugs

Target LVF 675 596 566 626 671 592 RVF 656 573 584 639 652 611 LVF-RVF 19 23 -18 -13 19 -19 Nontarget LVF 691 599 599 642 689 629 RVF 672 602 592 647 681 626 LVF-RVF 19 -3 7 -5 8 3 Overall LVF 683 597 583 634 680 610 RVF 664 588 588 643 667 618 LVF-RVF 19 9 -5 -9 13 -8

218 John L. Bradshaw and Del Sherlock

Analytic Faces (673 msec.) were significantly slower than Analytic Bugs (592 msec.), F = 37.93; d.f. = I, 20; p < .001. The LVF (640 msec.) was as predicted significantly slower than the RVF (626 msec.), F = 16.34; d.f. = I, 20; p < .001, and Nontargets (64I msec.) were significantly slower than Targets (625 msec.), F = I4.27; d.f. = I, 20; p < .01. The significant Task by Visual Field interaction, F = 4.81; d.f. = I, 20; p< .05, showed that the significant RVF superiority for Analytic Faces (I9 msec.), t = 3.97; d.f. = 23; p < .001, was rather larger than that for Analytic Bugs (9 msec.), t = 3.07; d.f. = 23; p < .01. The significant Visual Field by Targets/Nontargets interaction, F = 5.87; d.f. = I, 20; p < .05, showed that as predicted the RVF superiority was present for Targets, t = 5.63; d.f. = 23; p < .001, but not for Nontargets, t = 1.7. .

Four two-way ANOVAs (Visual Field, Half Task, i.e., first and second halves of Faces and Bugs tasks) were performed on the two tasks in the two order conditions (Orders 1 and 2), in order to determine whether or not processing strategies were consistent between the two halves of a given task. This procedure also permitted us further to examine the possible effects of task sequencing (i.e., the effect of one task, easier or more difficult, upon laterality patterns associated with the subsequent task). For Analytic Faces, a significant RVF superiority appeared in both Order 1(23 msec.), F = I2.55; d.f. = I, 11; p < .01, and Order2 (I6 msec.), F = 10.36; d.f. = I, 11; p < .01. For Analytic Bugs, the RVF superiority (9 msec.)wasonlysignificantinOrder I,F = 4.97; d.f. =I, 11; p < .05, that is when the more difficult Analytic Faces appeared as the first task. The Visual Field by half task interaction was not significant in any of the four analyses, indicating that subjects were consistent in their processing strategies throughout the course of a given task.

In summary, strong RVF superiorities were obtained as predicted in these analytic tasks, particularly so for the more difficult faces task and for the easier bugs task when it appeared as the second task, and significantly for targets but not for nontargets in either task. We can therefore conclude that there is a left hemisphere superiority for mediating analytic discriminations, thus supporting the analytic/holistic processing dichotomy, rather than the verbal!visuospatial. This therefore appears to be the first successful recorded replication of our original finding (Patterson and Bradshaw, I975, Experiment 3), extending it to another complex, meaningful set of organized patterns (bugs).

EXPERIMENT 2

In this experiment, in the Order I condition (i.e. for half the subjects), Holistic Faces preceded Holistic Bugs, while in the Order 2 condition, for

219 The analytic/ holistic processing dichotomy

the rest of the subjects, the sequence was reversed. A LVF superiority might therefore be predicted for both tasks, particularly perhaps for target stimuli. The main aim of the experiment was to determine whether this reversal of field superiority from the last experiment could be obtained while still employing the self -same stimuli as before, but now requiring the subjects instead to attend to a different set of features, ignoring those which had been chosen as discriminating in the previous experiment. To avoid the possibility of interference from a processing set adopted in the previous experiment, a new group of subjects were recruited.


A five-way ANOVA was performed as before on the RT data (see Table 1). Holistic Bugs (638 msec.) were significantly slower than Holistic Faces (585 msec.), F = 8.73; d.f. = 1, 20; p < .01. While no other main effects were significant, that of Visual Field only just failed to reach the .05 level of significance, F = 4.25; d.f. = 1, 20; the LVF (608 msec.) being somewhat faster than the RVF (615 msec.). However the Order by Visual Field interaction was significant, F = 4.48; d.f. = 1, 20; p < .05. The LVF superiority (15 msec.) was significant in the Order 2 condition (when the more difficult Holistic Bugs preceded the easier Holistic Faces), t = 2.79; d.f. = 11; p < .02, but not in the Order 1 condition, t = 0.05. Moreover the Visual Field by Targets/Nontargets interaction also was significant, F = 16.57; d.f. = 1, 20; p < .001. As predicted, the LVF superiority was presentforTargets, t = 4.35; d.f. = 23; p < .001, butnotforNontargets, t = 0.32. No other interactions were significant.

Four two-way ANOVAs (Visual Field, Half Task) were performed as in the last experiment on the Faces and Bugs tasks in the two order conditions. No LVF superiority was apparent for either task in Order 1, but was significant in Order 2 (when the more difficult Bugs task came first) both for Holistic Faces (15 msec.), F = 5.6; d.f. = 1, 11; p < .05 and for Holistic Bugs (15 msec.), F = 9.48; d.f. = 1, 11; p < .025. Moreover the Visual Field by Half Task interaction was significant for the Holistic Bugs task Order 1, F = 7.37; d.f. = 1, 11; p < .025. Only in the first half of this task was the L VF superiority significant, t = 2.23; d.f. = 11; p < .05, being nonsignificant in the second half, t = 1.40. The preceding task was Holistic Faces, suggesting that some effect from this holistic task might have carried over into the first half of the next task. The Visual Field by Half Task interaction was not present in the other task/order conditions.

In summary, the almost-significant predicted L VF superiority for these two holistic tasks reached significance for the target rather than the


nontarget stimuli, and for both tasks separately only when the more difficult task preceded. This was the case for exactly the same stimuli as had in the previous experiment produced RVF superiorities, when subjects had attended to a different set of critical features. Moreover the L VF superiorities now demonstrated in the two tasks were of the same order of magnitude. Finally, the L VF superiorities were here found in tasks where subjects attended to the patterns of interrelationships between, and otherwise independent of the component features themselves. In previous studies employing faces where right hemisphere superiorities have been found, whether Identikit, protographed or schematically drawn (Geffen, Bradshaw and Wallace, 1971; Rizzolatti, Umilta and Berlucchi, 1971; Hilliard, 1973; Patterson and Bradshaw, 1975; Klein, Moscovitch and Vigna, 1976; Moscovitch et al., 1976; Ley and Bryden, 1977), the actual features themselves have instead been manipulated. The present situation perhaps more truly stresses gestaltic or configurational relationships of the sort ascribed to right hemisphere holistic processing.

EXPERIMENT 3

In this experiment we sought to obtain unequivocal evidence of a L VF superiority for holistic processing with nonfacelike (i.e. bugs) materials, and to combine an analytic and a holistic task (faces and bugs) in the same experiment, while employing the same subjects. In the previous two experiments one group of subjects had experienced two analytic tasks (faces and bugs), and another had experienced two holistic tasks (faces and bugs). We wished now to determine whether the same group of subjects would show a reversed pattern of asymmetries between the two tasks. Accordingly, in this experiment in the Order 1 condition, Analytic Faces preceded Holistic Bugs. In the Order 2 condition, this sequence was reversed. A L VF superiority was predicted for Holistic Bugs, and a RVF superiority for Analytic Faces, on the basis of findings from the two previous experiments. ·


A five-way ANOVA was performed as before on the RT data (see Table 1). Only two main effects reached significance: Holistic Bugs (614 msec.) were significantly faster than Analytic Faces (673 msec.), F = 26.37; d.f. = 1, 20; p < .001. Targets (631 msec.) were significantly faster than Nontargets (656 msec.), F = 13.27; d.f. = 1, 20; p < .01. The value for the LVF (645 msec.) was almost identical to that of the RVF (642


msec.). However there was a significant Task by Visual Field interaction, F = 13.96; d.f. = 1, 20; p < .01. A significant RVF superiority (13 msec.) was present for the Analytic Faces task, as predicted, t = 2.36; d.f. = 23; p < .05, while the predicted LVF superiority (8 msec.) for the Holistic Bugs task just failed to reach significance, t = 1.84. However, as found in the significant Task by Visual Field by Target/Nontarget interaction, F = 10.33; d.f. = 1, 20; p < .01, Target stimuli determined field superiorities in both tasks, with a 19 msec. RVF superiority being present for Target Faces, t = 2.08; d.f. = 23; p < .05, and a 19 msec. LVF superiority for Target Bugs, t = 2.75; d.f. = 23; p < .02. RVF superiorities for Nontarget Faces (8 msec.) and Nontarget Bugs (3 msec.) were not significant, t = 1.56, and 0.52 respectively.

Four two-way ANOVAs (Visual Field, Half Task, i.e., first and second halves of each task) were performed as before on the Faces and Bugs tasks in the two order conditions. A significant RVF superiority for the Faces task (18 msec.), F = 7.84; d.f. = 1, 11; p < .025, and a significant LVF superiority (17 msec.) for the Bugs task, F = 8.91; d.f. = 1, 11; p < .025, was present when these tasks were encountered as the second task, but not when encountered as the first task. In no instance was the Visual Field by Half Task interaction significant, suggesting that within a given task subjects were consistent in their processing strategies.

In summary, experienced subjects demonstrated the predicted LVF superiority for the Holistic Bugs task, and the predicted RVF superiority for the Analytic Faces task, but these effects were largely confined to target stimuli and were strongest when the two tasks were encountered as the second in the series.

GENERAL DISCUSSION

The main findings can be summarized as follows. The predicted LVF superiorities generally appeared for the holistic tasks where stimuli were discriminable in terms of the gestalt patterns of interrelationships between and otherwise independent of individual component features. In previous studies with normal subjects, the individual features themselves generally had been varied, leading to a weaker test of the holistic hypothesis. Also for perhaps the first time, similarly complex and meaningful patterns (bugs) were employed, where past experiments had tended to rely on faces, or simple lines, curves and circles or polygons (see e.g. Moscovitch, 1979). The obtained L VF superiorities for the two tasks, faces and bugs, were of the same order of magnitude, thus perhaps tending to argue against the hypothesis of a specialist processor for faces located in the right hemisphere (Carey and Diamond, 1977), though it should be noted


that our stimuli were only schematic outline drawings rather than true face cartoons (Moscovitch et al., 1976). A RVF superiority was demonstrated for analytic discriminations, perhaps the first replication to be reported of the original demonstration with normal subjects (Patterson and Bradshaw, 1975). Moreover these effects (a L VF superiority for holistic and a RVF superiority for analytic tasks) were demonstrated using the same stimuli, the subjects in the two conditions merely attending to different features or aspects of the stimulus. We were thus able to successfully demonstrate an effect which had previously been attempted with only limited success, though with a somewhat different paradigm, by Galper and Costa (1980). Galper and Costa had tried to impose analytic and holistic processing strategies upon the same set of stimuli by manipulating how subjects initially leatnt target faces (in terms of discrete salient features, or of overall apparent personalities). Sherlock (1980) unsuccessfully attempted to increase the magnitude of L VF superiorities in a pattern-processing task by employing a speed rather than an accuracy set. Her failure may have stemmed from ceiling effects, as a prominent L VF superiority was already apparent even with the accuracy set.

In all three experiments, the predicted field effects were stronger for target than for nontarget stimuli. This seems to be a fairly common finding. As Suberi and McKeever (1977) note, subjects may adopt a set to respond to the target, responding to nontargets without much awareness of what actually was presented. Sequence effects were also apparent. While in all three experiments the general absence of a Visual Field by Half Task interaction within a given task showed that subjects were usually fairly consistent in their processing strategies during the course of that task, nevertheless we found that between tasks field differences in the predicted directions tended to be generally stronger for the more difficult task (Experiments 1 and 3), for both tasks when the more difficult one preceded (Experiments 1 and 2), and for any task appearing second (Experiments 1 and 3). While these three sets of effects are very probably closely interrelated and at least partially confounded, future research could carefully control the effects of task difficulty and sequencing, and attempt to document these effects more fully.

Indeed, a common theme underlying many of the studies which examine or report variability in the patterns of laterality effects is that such effects are more consistent when subjects must deeply code or analyze stimulus material, rather than respond merely at a perceptual or "precategorical" level (Moscovitch, 1979). Thus retention intervals must exceed a certain lower limit (Dee and Fontenot, 1973; Hilliard, 1973; Patterson and Bradshaw, 1975; Moscovitch et al., 1976; Cohen, 1976), or the stimuli must be subjected to some form of masking, degradation or abstraction (e.g., caricaturing) (Moscovitch et al., 1976), or the subject


must identify a test face which appears with changes in viewing angle (and possibly also in expression, age and health) as compared with the target (Bertelson, V anhaelen and Morais, 1979), or a memory load is imposed which is either intrinsic to the task (Kirsner, 1980) or extrinsic and concurrent (Hellige, Cox and Litvac, 1979; Wexler and Heninger, 1980).

This situation might account for reported changes in laterality effects as a function of previous tasks (Klein et al., 1976) or of practice, usually more robust or typical effects appearing later in a sequence (Patterson, 1976; Hellige, 1976; Bradshaw and Gates, 1978; Goldberg and Costa, 1981).

Finally, there was never the slightest evidence of a differential sex effect. McGlone (1980) reviews the evidence of a weaker pattern of cerebral asymmetry in females. We (Bradshaw, 1980) have repeatedly confirmed this in our verbal studies, but have never in any of our nonverbal tasks had even a hint of such an effect. This dissociation of differential asymmetries between the sexes with respect to verbal and nonverbal tasks also warrants further investigation.

ABSTRACT

We report three experiments employing outline face and bug stimuli. Subjects either attended to the spatial relationships between and otherwise independent of the individual feature elements (holistic processing condition), or instead attended (with the same stimuli) to the shape of certain discrete feature elements (analytic condition).

They were timed in performing discriminatory manual responses to laterally presented targets and nontargets. A left visual field superiority generally occurred under conditions of holistic processing both for faces and bugs, and a right field superiority when analytic processing was required; effects were rather more apparent in the second of a pair of sequential tasks, when the more difficult of the two tasks came first, for the more difficult of the two tasks, and with target rather than, nontarget stimuli. We conclude that those aspects of a stimulus to which a subject is set to attend can determine lateral asymmetries, as well as the actual stimuli themselves, that an analytic/holistic processing distinction is supported, rather than one based on verbal/visuospatial processing, and that task difficulty and order are important variables to be controlled and studied in this context in future research.

Acknowledgement. This work was supported by a grant from the Australian Research Grants Committee to the first author. We gratefully acknowledge all the technical help given by Vlad Kohout, Bob Wood, and Kevin Donahoo, and the advice from and helpful discussions with Norman Nettleton and Meredith Taylor.

Please address requests for reprints to John Bradshaw, Department of Psychology, Monash University, Clayton, Victoria, 3168, Australia.


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Dr. John Bradshaw and Del Sherlock, Department of Psychology, Monash University, Clayton, Victoria 3168, Australia.

Documents

Bugs and Faces in the Two Visual Fields: The Analytic/Holistic Processing Dichotomy and Task Sequencing