Hemispheric interactions: The Bilateral Advantage and Task Difficulty

HEMISPHERIC INTERACTIONS: THE BILATERAL ADVANTAGE AND TASK DIFFICULTY

w.n. Norman l , M.A. Jeeves2 , A. Milne3 and T. Ludwig4 (IRedeemer College, Ancaster, Ontario, Canada; 2University of St. Andrews, St. Andrews, U.K.; 3University College of Wales, Cardiff, U.K.; 4Hope College, Holland, MI, U.S.A.)

INTRODUCTION

Functional specialization of the cerebral hemispheres having been well established (Bradshaw, 1989) more recent investigations have focused on the equally important issue of how the two hemispheres interact. Several investigators (Bradshaw, 1989; Hellige, 1987, 1990; Jeeves, 1991) have noted different paradigms used for studying interhemispheric interactions and a variety of models to explain the data collected. Hellige (1987, 1990) has described three experimental methods for investigating interhemispheric interactions. With Dual-Task Interference, subjects are required to perform, concurrently, two interfering tasks. The question asked is whether performance is better when the processing load can be spread across the two hemispheres instead of restricted to one hemisphere (e.g. Umilta, Rizzolatti, Anzola, Luppino and Porro, 1985). In a second paradigm, Cross-Hemispheric integration, two stimuli are presented either both to the same visual field (and hence to the same hemisphere) or one to each visual field (with the result that each hemisphere receives half the information). Subjects judge whether the two stimuli are the same or different. Successful judgements in the across-field (or bilateral) condition require that information from the two hemispheres be integrated or shared (e.g. Davis and Schmit, 1971; Sergent, 1990). A third paradigm, Simultaneous Bihemispheric Presentation, involves presenting a target stimulus in a central location followed by a probe stimulus to only one or to both visual fields. Subjects judge whether the probe stimulus matches the target stimulus (e.g. Hellige, Johnson and Michimata, 1988). Earlier investigations reported that sending the stimuli initially to two hemispheres seemed to convey a performance advantage compared with when the same stimuli went to only one hemisphere (Davis and Schmit, 1971, 1973; Dimond, 1971; Dimond and Beaumont, 1972a, 1972b; Liederman, Merola and Hoffman, 1986; Merola and Liederman, 1985, 1990; Miller, 1981, 1983). There were, however, exceptions (Beaumont and Dimond, 1973, 1975; Berger and Perret, 1986; Bradshaw, Nettleton and Patterson, 1973; Dimond, 1969; Dimond and Beaumont, 1974; Dimond, Gibson and Gazzaniga, 1972; Kleinman and Little, 1973; Leiber, 1982; Liederman, 1986; Liederman, Merola and Martinez, 1985; Schmitz-Gielsdorf, Willmes, Vondenhoff, and Hartje, 1988). The matter remains unresolved and current research seeks to discover which factors are associated with a bilateral advantage (Ludwig, Jeeves, Norman and Cortex, (1992) 28, 623-642

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DeWitt, manuscript submitted for publication). Factors so far identified include (1) whether the two stimuli to be compared are presented successively or simultaneously (Banich, 1985; Dimond, Gibson and Gazzaniga, 1972; see also Berger and Landolt, 1990 for review of references), (2) stimulus configuration (Ludwig et al.; Schmitz-Gielsdorf et aI., 1988), (3) putative variations in the efficiency of interhemispheric pathways (Galin, Johnston, Nakell and Herron, 1979; Quinn and Geffen, 1986), (4) metacontrol issues (Hellige, Johnson and Michimata, 1988), (5) emotional state or mood of the subject (Banich, Stolar, Heller and Goldman, cited in Banich and Belger, 1990), (6) compatibility of stimulus input to supposed hemispheric specialization (Berger and Landolt, 1990; Berger, Perret and Zimmerman, 1988), (7) amount of practice on the task (Liederman, Merola and Martinez, 1985), and (8) task load (Banich and Belger, 1990; Merola and Liederman, 1990; see also discussion by Hellige, 1987, 1990). This report addresses the relationship between the bilateral advantage and task load. The latter may vary with changes in several of the factors listed above. Increased practice would be expected to render a task easier; stimulus configurations that mobilize particular scanning strategies may make responding to bilateral presentations easier; more mature and perhaps, therefore, more efficient interhemispheric pathways may influence the ease with which a task is performed. Thus, task difficulty, as defined by the experimenter may not necessarily be the same as that experienced by the subject. The purported finding of an increased bilateral advantage with increasing task difficulty is also of interest for methodological reason. Different individuals or groups of individuals may find a given task more or less difficult. If this produces varying degrees of bilateral advantage then combining individuals' scores within a group may mask the true effect of the independent variable under investigation or obscure differences between groups operating at different levels of difficulty (Le., children versus adults performing the same task). Several recent investigations have examined the role of task difficulty on the bilateral advantage. Banich and Belger (1990) varied the difficulty of the decision process (Le., physical versus name matches or summation versus ordinal tasks). They reported the presence of a bilateral advantage wHen the decision process was assumed to be more difficult but not under conditions of low task load. Merola and Liederman (1990) in a set of experiments representative of a series of investigations by them manipulated task difficulty by varying the number of items to be processed in a display and by projecting a visual mask 40 msec. after offset of the displays. They employed verbal stimuli in a naming task. In line with Banich and Belger, they reported the presence of a bilateral advantage at higher levels of task difficulty but not at the lowest levels employed. The methods used to manipulate task difficulty in the above studies whilst having face validity, remain largely empirical. The question arises as to whether it is possible to vary the difficulty of the task in a more systematic manner. Jeeves and Lamb (1988) in noting that some researchers invoked the concept of task difficulty post hoc in order to explain unexpected findings in laterality stu-

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dies, used an information theory metric to manipulate difficulty systematically (based on work by Posner, 1965; Posner and Keele, 1968, 1970; see also Klatzky, 1970; Klatzky and Atkinson, 1971). This involved learning to recognize families of dot patterns that had been distorted from a parent pattern by zero, four, or eight bits of information. Their definition of task difficulty was clearcut but the task was demanding and the subjects participating in the study were highly motivated university students. It is unlikely that the same task would be suitable for widespread use with, for example, patients or young children. Overall, response latencies were shortest when the stimuli for comparison were presented bilaterally (Le., a bilateral advantage). The present experiment was conducted with the above considerations in mind. In an attempt to manipulate task difficulty as unambiguously as possible we did so in a manner similar to that employed by Jeeves and Lamb and like them used visuospatial stimulus materials. Obtaining a pattern of results similar to that by Jeeves and Lamb would give the attempt to manipulate task difficulty a face validity. Difficulty varied by increasing the number of dots in a 3 x 3 matrix from two, to four, to six in the patterns to be compared. Thus, we assumed that when making "same"-"different" judgements it would be easier to compare two 2 Dot patterns presented briefly than to compare 6 Dot patterns and that 4 Dot patterns would fall between the two in terms of task difficulty. The procedure used here was, therefore, a perceptual matching task unlike the memory-scanning task employed by Jeeves and Lamb.·In addition, since most previous studies of the bilateral advantage had employed letter or digit stimuli we sought to discover whether the effect was material specific or would also happen when the stimulus materials were visuospatial patterns. We predicted that increasing the number of dots in the patterns to be compared would lead to an increasing performance advantage under the Bilateral presentation condition as compared with Unilateral presentations. Should this pattern of results be obtained this experimental paradigm could provide a technique for studying how hemispheric interaction changes if the main interhemispheric pathways have been damaged or are immature.

MATERIALS AND METHOD

Subjects Twelve woman and thirteen men aged 20 to 38 participated in the experiment. All individuals were right-handed by the Edinburgh Inventory (Oldfield, 1971).

Stimuli Stimulus presentations consisted of two dot patterns, each in a 3 x 3 matrix and composed of either two, four or six dots. On 500/0 of the trials the two presented patterns were different. They were the same on the remaining trials. Over the entire session there were tweive trials in each Presentation Condition (Unilateral Left Visual Field [L VFJ, Unilateral Right Visual Field [RVFJ, and Bilateral); Number of Dots (2, 4, and 6); and Response ("same" and "different") for a total of 216 trials. Sample patterns are presented in Figure 1.

626 Fig. I - Schematic representation for three hypothetical stimulus pattern presentations; Bilateral, Unilateral Left, and Unilateral Right Visual Field. Number of Dots and subject's correct response are indicated.

W.D. Norman and Others

BILATERAL PRESENTATION: 2 Dots; Response=Same

1••:1

UNILATERAL LEFT VISUAL FIELD PRESENTATION: 4 Dots; Response = Same

:1

1• •

UNILATERAL RIGHT VISUAL FIELD PRESENTATI ON: 6Dots; Response = Different

•• [lli] •••

I!el

l!!!J

Apparatus and Procedure Pairs of dot patterns were presented on a computer screen. Stimulus presentation and data collection were under computer control. For half the subjects the left hand was used to indicate a "same" judgement and the right hand for a "different" judgement. For the remaining subjects "same" and "different"' judgements were indicated by the opposite hands . Subjects were given the following instructions: "Two patterns of dots will briefly appear on the computer screen. Sometimes they will appear on the left side of the screen, sometimes on the right side, and sometimes one pattern will appear on each side of the screen. Your task is to decide whether the patterns of dots in the two boxes are identical or not and then to press a key on the computer keyboard as quickly as you can to indicate this. Sometimes the two patterns will be identical and sometimes they will not. When the warning tone sounds be sure you are looking at the circle in the center of the screen. You should be as accurate as possible in making your judgements and as fast as possible in responding." Subjects were first given a block of 24 practice trials during which response times and cumulative error rate were noted. If the cumulative error rate was greater than 20% additional blocks of practice trials were administered. In this experiment no subject required more than two blocks of practice trials. The actual test trials were presented in four blocks of 72 trials per block. Between blocks subjects rested for approximately three to five minutes. The patterns were presented in three locations on the computer screen. For Unilateral presentations the pairs of patterns appeared one above the other in either the left or the right

627

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Fig_ 2 - Mean RT scores for Left Visual Field, Right Visual Field, and Bilateral presentations as afunction of Number of Dots per pattern for "same" and "different" judgements_

Same Judgements

1100

~ 1000

g'"

5:

c:

900

_ _ RVF

01

(I)

-----0----

LVF _ _ Bilateral

~ 800

700 - ' - - - - , - - - - - r - - - - - - - , - 2 4 6 Number of Dots

Different Judgements

1100 ci1000 (I)

g'" III:

900

_..._. ___ RVF

c:

01 (I)

~

800

-----0----

LVF

--0-

Bilateral

700 2

4 Number of Dots

6

visual field, the inner edges of the matrices on which the patterns were located were 2.5 degrees from the fixation point, the outer edges were 7.5 degrees. For Bilateral presentations one of each pair of patterns appeared in each of the two visual fields, between 2.5 and 7.5 degrees from the fixation spot and on a horizontal axis through the fixation point. The stimuli appeared for 180 msecs.

RESULTS

Correctness of response and response latencies to "same" and "different" judgements were recorded and analyzed separately.

Overall Analyses of Reaction Time Scores The reaction time scores were submitted to a 2 Between (Gender) x 3 Within (Presentation Condition: L VF, RVF, and Bilateral) x 3 Within (Number of Dots: 2,4, and 6) x 2 Within (Response Type: "same" and "different") ANO-

628

W.D. Norman and Others TABLE!

Mean RT Scores for "Same" and "Different" judgements Dots

2 4 6

Combined

"Same" judgements

"Different" judgements

LVF

RVF

Bilat.

LVF

RVF

Bilat.

797 980 1001 926

808 1013 1092 971

751 876 932 853

865 979 1058 967

854 1005 1047 969

801 925 1025 917

VA. Significant main effects of Presentation Condition (F = 33.93; d.f. = 2,46; p
Analyses of Interactions: Response Type x Number of Dots x Presentation Condition Planned comparisons revealed a bilateral advantage relative to LVF and RVF presentations at each of the three Dot levels for "same" judgements (all comparisons p
629

Hemispheric interactions 40

Fig. 3 - Mean percent ere ror scores for Left Visual Field, Right Visual Field, and Bilateral presentations as a function of Number of Dots per pattern for "same" and "different" judgements.

Same Judgements

e 30 w Ul

'EQ) 20 ~

......,. .•. LVF .._....•._.• RVF

Q)

c..

10

--<>-

Bilateral

0 2

4 Number of Dots

6

Different Judgements

40 ......,. ... LVF

!!? 30

e

._......-

w

--<>-

RVF Bilateral

'EQ) 20 ~

Q)

c..

10 0 246 Number of Dots

Bilateral presentations, but no difference between "same" and "different" judgements for RVF responding.

Overall Analyses of Percent Error Scores Percent error scores were submitted to the same ANOVA design. Significant main effects were identified for Presentation Condition (F = 12.37; d.f. = 2, 46; p=O.OOOI; LVF= 13.76; RVF= 12.77; Bilateral=9.57), Number of Dots (F=71.46; d.f.=2, 46; p
630

W.D. Norman and Others TABLE II

Percent Error Scores for "Same" and "Different" Judgements Dots

"Same" judgements

2 4 6 Combined

"Different" judgements

LVF

RVF

Bilat.

LVF

RVF

Bilat.

3.90 18.85 30.50 17.75

3.31 16.07 30.43 16.60

2.56 13.68 13.68 9.97

4.87 10.15 14.27 9.76

3.29 9.24 14.30 8.94

2.29 8.88 16.30 9.16

fects between Presentation Condition and Number of Dots (F = 2.63; d.f. = 4, 92; p = 0.0385), Response Type x Number of Dots (F = 5.84; d.f. = 2,46; p = 0.0057), Response Type x Presentation Condition (F = 4.83; d.f. = 2, 46; p = 0.0124), and Response Type x Number of Dots x Presentation Condition (F = 6.93; d.f. = 4, 92; p = 0.0002) were also significant. Average percent error scores for each·condition are presented in Figure 3 and Table II.

Analyses of Interactions: Response Type x Number of Dots x Presentation Condition Planned comparisons revealed a significant bilateral advantage relative to LVF (F=30.67; d.f.= 1,23; p
Further Analyses Addressing the Bilateral Advantage Relative to Task Difficulty Analyses presented above address the question of whether a bilateral advantage was present relative to LVF or RVF presentations at each of the levels of task difficulty. However, they do not assess whether the size of the bilateral advantage changes as task difficulty increases. In order to investigate this latter relationship, one can compute difference scores; LVF minus Bilateral, to ex-

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631

amine the size of the bilateral advantage relative to unilateral presentations in LVF, and RVF minus Bilateral, to examine the size of the advantage relative to RVF presentations. These bilateral advantage scores were computed for each subject's RT and percent error data for 2, 4, and 6 Dot conditions. The RT data were submitted to a 2 Within (Presentation condition: L VF minus Bilateral and RVF minus Bilateral) x 3 Within (Number of Dots: 2, 4, and 6) x 2 Within (Response Type: "same" and "different") ANOVA. Significant main effects were identified for Presentation Condition (F = 7.50; d.f. = 1, 24; p = 0.0110: L VF minus Bilateral = 61.29; RVF minus Bilateral = 84.43) and Response Type (F = 12.09; d.f. = 1, 24; p = 0.0023: "same" = 95.39; "different" = 50.33). The main effect for Number of Dots just missed being significant at the 0.05 level (F = 3.01; d.f. = 2,48; p = 0.0573): 2 Dots = 55.05; 4 Dots = 92.59; 6 Dots = 70.93). These main effects were qualified as follows: (1) planned comparisons revealed a Number of Dots effect only for "same" judgements in the RVF minus Bilateral condition (F = 10.76; d.f. = 1, 24; p = 0.0034: 2 Dots = 57.92; 4 Dots = 136.08; 6 Dots = 159.72). (2) The main effect for Response Type was modified by a significant Response Type x Presentation Condition interaction (F = 6.82; d.f. = 1, 24; p = 0.0146). For RTs the bilateral advantage, relative to RVF presentations (RVF minus Bilateral), was larger than the bilateral advantage relative to LVF presentations (L VF minus Bilateral) only for "same" judgements (F = 16.16; d.f. = 1, 24; p = 0.0008). For "different" judgements the magnitude of the advantage did not differ with respect to LVF or RVF presentations. The same ANOV A design was applied to the bilateral advantage scores using the percent error data. Significant main effects were identified for Number of Dots (F = 4.17; d.f. = 2, 48; p = 0.0209: 2 Dots = 1.42; 4 Dots = 2.32; 6 Dots=7.22) and Response Type (F=8.11; d.f.=I, 24; p=0.0087: "same"=7.18; "different"=O.13). The Response Type x Number of Dots interaction was also significant (F = 10.42; d.f. = 2, 48; p = 0.0004). Planned comparisons showed that the magnitude of the bilateral advantage increased as the number of dots increased only for "same" judgements (F = 33.57; d.f. = 1, 24; p
Recapitulating the major findings for response latencies we see: (1) as the number of dots in the patterns to be compared increased, RTs increased for both "same" and "different" judgements; this applied to LVF, RVF, and Bilateral presentations; (2) an overall bilateral advantage occurred relative to L VF

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and RVF presentations, it was more reliable for "same" than "different" judgements; (3) responding was faster for "same" than "different" judgements for LVF and Bilateral presentations but not for RVF presentations; and (4) based on "same" judgements, the size of the bilateral advantage increased relative to RVF presentations as the number of dots increased. The major findings for percent error scores were as follows: (1) as the number of dots in the patterns increased accuracy decreased for both "same" and "different" judgements; this applied to L VF, RVF, and Bilateral presentations; (2) for L VF and RVF (but not Bilateral) presentations, responding was less accurate for "same" than "different" judgements; and (3) for "same" judgements only the bilateral advantage increased relative to LVF and RVF presentations as the number of dots increased.

Hemispheric Interactions and the Bilateral Presentation oj Inputs A number of investigators have adressed the question of how the two hemispheres might interact when stimuli are presented bilaterally as compared with unilaterally. For example, Sereno and Kosslyn (1991) argued that bilateral presentations result in faster responding because there is less competition for common processing structures within a single hemisphere (as on unilateral trials) and/or less intrahemispheric inhibition. Liederman and Cunningham (1987) proposed that between-hemisphere division of labor is only advantageous when the two hemispheres can perform the task at hand with approximately equal facility. Hellige, Taylor and Eng (1989) stated that it is not possible for the two hemispheres to simultaneously engage in mutually inconsistent modes of processing. Therefore, under certain circumstances metacontrol will be necessary with one hemisphere's processing mode dominating on bilateral trials. Arguing along lines similar to Liederman and Cunningham, Hellige et al. propose that the selected mode may be the one for which both hemispheres have some competence. Berger and Landolt (1990) presented different types of stimuli to one or both visual fields (Le., words, bargraphs, and dot clusters). They concluded that superior performance in the bilateral condition will occur when the stimulus in a given visual field is compatible with the predominant mode of processing of the hemisphere to which it is projected. They went on to argue that bilateral performance will be poorer the less the stimulus materials are accessible to verbal or sequential processing. The procedures used in the present study were different from those used by these investigators. However, cautious comparison of studies similar to ours may be helpful. Sereno and Kosslyn (1991) employed a visuospatial task but only reported combined unilateral and not separate LVF and RVF responding. They found faster responding to bilateral rather than unilateral presentations for both "same" and "different" judgements. In the condition in which only numberword stimuli were used, Berger and Landolt (1990) found the best performance on a summation task to be in the bilateral condition. However, when the information was in the form of dot clusters there was a trend toward a RVF superiority and when both inputs were bargraphs the trend was toward a L VF super-

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633

iority. Differences between their results and ours are probably related to the nature of the task employed (Le., summation versus comparison for identity).

The Bilateral Advantage and Task Difficulty A major aim of the present study was to examine the relationship between the bilateral advantage and task difficulty. Recently, several investigators have put forward the hypothesis that a bilateral advantage will occur when task difficulty is above a certain threshold. According to Hellige (1990, p. 70), " ... the advantage of spreading the processing load across both hemispheres (Le., the between hemisphere condition) becomes greater when the information processing task becomes more difficult." Merola and Liederman (1990) along with Banich and Belger (1990) also postulate that the superiority of bilateral performance depends on task difficulty. Task difficulty in these latter two studies was manipulated by varying the amount of information in a display to be processed or by varying the kind of processing required to perform the task. It is clear from our results that both task difficulty (in terms of the amount of information per display) and the type of response judgement (Le., "same" or "different") required are important factors to be considered when investigating bilateral versus unilateral presentations. As task difficulty increased the size of the bilateral advantage increased, as predicted by the above-mentioned investigators. However, this increase was observed only under certain conditions. Taking the RT scores first (see Figure 4), increases in the size of the bilateral advantage are most evident when Bilateral responding is compared to RVF responding. The fact that this effect is not evident when Bilateral and L VF responding are compared is most likely due to the manner in which subjects responded in the 6 Dot condition. It was only at 6 Dots that LVF responding was faster than RVF responding. Even though both hemispheres may have been able to process the dot patterns, differences in the type of processing used by each hemisphere may have resulted in the left hemisphere (LH) experiencing considerable difficulty with the complex 6 Dot stimulL Below that level of difficulty there is evidence that the size of the bilateral advantage did increase relative to LVF responding as task difficulty increased. Looking at percent error scores reveals an increasing bilateral advantage with respect to both visual fields, but again only for "same" judgements. Although it was not statistically significant, there was a trend for increasing difficulty to produce a bilateral disadvantage for "different" judgements. As noted above, our procedure differed from that of Merola and Liederman in that they used two conflicting tasks whilst we required subjects to compare two imputs for identity. They recorded the proportion of correctly named letters whilst we recorded RT and accuracy scores. In view of these differences the following comparisons can be made. In general, Merola and Liederman found that when task difficulty was low there was no benefit associated with dividing input between the hemispheres. When task difficulty was increased a bilateral advantage was observed with respect to both LVF and RVF presentations. Inspection of their figures indicates that the bilateral advantage was greater with

W.D. Norman and Others

634 Fig. 4 - Mean Bilateral Advantage scores (RTs and percent errors) as a function of Number of Dots per pattern for "same" and "different" judgements.

200

_

LVF minus Bilat. (same)

.......... LVF minus Bilat. (diff)

~ ...•... (I)

N

RVF m'o", B".,. (dill)

.......,:::.::

en

o

L -_ _, -_ _ _ _ _ _, -_ _ _ _ _ _, -_ __

4

2

6

Number of Dots

"C

e

20

m

~ 10 ,,;

_

LVF minus Bilat. (same)

_

RVF minus Bilat. (same)

iii

.- ..

"0

« ~

'0

-"'-0----

0

~--.

LVF minus Bilat. (diff) RVF minus Bilat. (diff)

(I)

N

en -10 2

4 Number of Dots

6

respect to LVF rather than RVF presentations. This pattern of bilateral advantage emerged as they increased difficulty (adding a backward visual mask) to their 2 letter condition. However, as they increased difficulty in the 4 letter condition, even though a bilateral advantage was present, it did not differ with respect to LVF or RVF presentations. Comparing their results with our accuracy scores, we observed an approximately equal increase in the bilateral advantage with respect to LVF and RVF presentations for "same" judgements. Since Banich and Belger (1990) employed a go/no-go response their results should, perhaps, only be compared with our results for "same" judgements. Even this comparison must be tentative as recent work in our lab indicates a possible response type effect ("same" /"different" versus go/no-go) on the bilateral advantage. Moving from a simple task (physical identity matching of letters) to a more difficult task (name matching of letters), they observed a bilateral advantage in the latter condition. This advantage was larger with respect to RVF than LVF presentations for both RT and percent error scores. For two other tasks which Banich and Belger considered as difficult (ordinal and summation tasks) they obtained similar results. We obtained the same pattern of re-

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635

sults for RT scores but found no difference between LVF and RVF presentations for accuracy scores. Banich and Belger did not discuss their results in terms of whether the bilateral advantage was greater with respect to LVF or RVF presentations. However, it is interesting to note from inspection of their tables that their results and ours are similar even though they employed tasks requiring letter name matching, ordinal comparisons, and summation operations, whilst we used patterns of dots that supposedly required vi suo spatial progressing. Whereas Banich and Belger and Merola and Liederman failed to find a bilateral advantage for their low difficulty tasks, we found such an advantage even when 2 Dot patterns were being compared. They suggest that a bilateral advvantage may only be observed when task difficulty is above a given threshold. It may be that our lowest level of difficulty was still difficult enough to reach such a threshold, an interpretation which seems to make the definition of difficulty post hoc. Or it may be that the threshold hypothesis needs to be modified for visuospatial tasks like one we used.

"Same" versus "Different" Judgements As noted above for "same" judgements, whilst we observed an increase in the size of the bilateral advantage with respect to RVF responding, no such increase was apparent with respect to L VF responding. For "different" judgements there was, in fact, a trend in the opposite direction. To see whether these findings for mean RT scores were representative of individual subjects' responding we examined the percentage of subjects who responded faster to bilateral as opposed to L VF or RVF presentations. These percentages support the picture from the group means. For "same" judgements the percentages were as follows for 2,4, and 6 dot conditions, respectively: bilateral advantage with respect to LVF - 80, 76, and 640/0; with respect to RVF - 76,92, and 92%. For "different" judgements the percentages were: with respect to LVF - 80, 72, and 56%; with respect to RVF72,76, and 56%. Thus, the size ofthe bilateral advantage (in terms of processing time or the percentage of subjects showing an effect) increases as task difficulty increases only when compared with RVF presentations.

Models of Processing "Same"rDifferent" Judgements It is evident that "same" and "different" judgements need to be considered separately when investigating the relationship between the bilateral advantage and task difficulty. These two types of judgements have been shown to be mediated by different processes and subject to different influences (see review articles by Farell, 1985; Krueger, 1978; Proctor, 1981). In perceptual matching tasks "same" responses are typically faster, but less accurate, than "different" responses. Our results fit this pattern. It has been suggested that "same" judgements are mediated by a fast, wholistic processor whilst "different" judgements are processed by a slower, analytic processor or a latter stage of the wholistic processor (see Farell, 1985). Attempts to locate the fast-"same" processor in the RH and the slower-"differ-

636

W.D. Norman and Others

ent" processor in the LH have not been successful. Neither studies by Bagnara, Boles, Simi on and Umilta (1982), Fairweather, Brizzolara, Tabossi and Umilta (1982) nor the present study found faster "same" judgements for LVF presentations and faster "different" judgements for RVF presentations, a requirement for the theory of lateralized "same" /"different" processors. Krueger's noisy-operator theory (Krueger, 1978, 1987; Krueger and Allen, 1987; Krueger and Chignell, 1985) proposes that "different" judgements are slower but more accurate because internal noise is more likely to produce spurious featural mismatches than matches. To reduce these errors rechecking on "different" judgements is required. Eriksen's response-competition interpretation (Eriksen and O'Hara, 1982; Eriksen, O'Hara and Eriksen, 1982) suggests that "different" responses are slowed by a high level of priming in the competing "same" response. When the inputs to be compared are different, but similar, a greater priming of "same" responses will produce more competition and hence a slower "different" response. The present study was not designed so as to determine which, if any, of these models best accounts for the results reported here. It is clear that, overall, similar effects were obtained for both "same" and "different" judgements. Responding to bilateral presentations was faster (but not necessarily more accurate) and increasing the number of dots made the task more difficult for both types of judgements. It is only when one examines how the size of the bilateral advantage varies as a function of task difficulty that these two response types yield differing results.

A Stage Process Model of Hemispheric Interaction Basic Assumptions In order to interpret the present results it may be helpful to think of the demands made on the subjects by our task in terms of successive stages of processing, viz: (1) the initial registration of the perceptual input; (2) the interhemispheric transfer of information if and when required; (3) the comparison of the two dot patterns and judgement as to whether they are the same or different; and (4) the execution of an appropriate key pressing motor response. The overall response time, as measured by the experimenter, includes the cumulative times of these four stages. Increases in task difficulty may produce increases in processing times at any or all of the first three stages, but are assumed to be negligible at the motor output stage. If we assume that the requirements of the present task, namely to compare two patterns of dots visually and to decide whether they are the same or different, is in essence a vi suo spatial task then we may further assume that it is a task for which the right hemisphere (RH) is relatively specialised. Whilst the LH is capable of processing these stimuli such processing will be slower. If we further assume that "same" responses are processed in a fast, wholistic manner, whereas "different" responses are processed in a slower, analytic manner (see Farell, 1985) we can build these two assumptions into our model. Following the argu-

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ment put forward by Bagnara et al. (1982) and Fairweather et al. (1982), we make the assumption that both hemispheres are capable of emitting "same" and "different" responses whilst noting the alternative view held by Patterson and Bradshaw (1975) that the RH is specialised for wholistic processing (and hence "same" responses) and the LH for analytic processing (and hence "different" responses). (Eriksen's response-competition or Krueger's noisy-operator models could be substituted so long as it is assumed that the two hemispheres do not differ with respect to these operations.) Basing their theorizing exclusively on the results of studies using unilateral presentation of simuli, Zaidel (1987) and Zaidel, Clarke and Suyenobu (1990), have proposed two models to explain how the two hemispheres handle such lateralized inputs. In the callosal relay model information will be processed by the hemisphere exclusively specialised for handling it. Thus, callosal transfer will be necessary if the information is initially received by the nonspecialised hemisphere. In the direct access model the hemisphere which initially receives the information processes it though the two hemispheres may use different strategies and exhibit different competencies at that task or with that kind of information. The condition of bilateral presentations, where each hemisphere receives part of the stimulus load and a correct response requires integration or comparison of the initially segregated information, would, presumably in all cases, necessitate callosal transfer and hence some form of a callosal relay model. Zaidel (1987) has stated that tasks involving direct access are more common than those requiring callosal relay. If Zaidel is correct and his assertion applies to the task being used in this study, then we may assume that on unilateral trials the direct access model holds. On bilateral trials some sort of callosal relay model must, of necessity, apply. However, since our task is a visuospatial task we make the further assumption that on bilateral trials, in order to make a "same" / "different" judgement, information will be transferred to the hemisphere generally assumed to be more competent for visuospatial processing (Le., transfer will be from the LH to the RH). Predictions from the Model for "Same" Judgements Given these assumptions we may ask what pattern of results we would expect when "same" responses are required. When the stimuli both appear in the RVF we should expect slower responding since the stimuli are going to a hemisphere which is not specialised for processing visuospatial material. However, because a "same" response is required, the fast-(on our view non-Iateralized) "same" processor will operate. As regards L VF input we should expect fast responding because the input goes to a hemisphere which is relatively specialised for visuospatial tasks and the fast-"same" processor will be in operation. As regards bilateral presentations we note that the initial registration of the percentual input is now shared between two hemispheres as compared with a L VF presentation where one hemisphere has to do all the processing. This advantage may, however, be to some extent offset by the requirement to transfer the information received in the LH to the RH, a process that will involve transfer

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time and probably some degradation of the information in the course of transfer (see Berlucchi, 1972, and Rizzolatti, 1979). Evidence from earlier studies is ambivalent in suggesting that the bilateral condition confers an advantage (see references in Introduction).

Predictions from the Model for "Different" Judgements Turning now to "different" responses the situation is as follows. In general, slower responding is expected for "different" responses possibly because (1) a slower processor is involved, (2) competition from primed "same" responses compete with "different" responses, or (3) rechecking occurs on "different" but not "same" trials. RVF presentations may produce slower responding than LVF input in so far as the former is not going to a hemisphere which is relatively specialised for the visuospatial task. As regards the bilateral condition the same factors noted above apply though presumably since the "same" processor is not operative, overall, the bilateral latencies will be slower now when "different" responding is required.

Predictions from the Model as Task Difficulty Increases As task difficulty is increased by increasing the number of dots per pattern the following is predicted, first for RTs on "same" trials. LVF presentations will be processed by the RH which is relatively specialised for this task. Response latencies will be longer as the numer of dots per pattern increases. Input to the RVF will be processed in the LH which is not specialised for this task and should show an even steeper increase as task difficulty increases. Bilateral trials will result in parallel processing of the input in the two hemispheres followed by transfer of the information in the LH to the RH. In all cases, it is assumed that the increase in task difficulty has its effect during the comparison stage of processing. It takes longer to compare patterns with 6 than with 2 dots. A similar explanation applies on "different" trials for the various presentation conditions with the important exception that response latencies, overall, will be longer for the reasons listed above. In predicting percent error scores we here tentatively assume Krueger's noisy-operator model. As the number of dots per pattern increases there should be an increasing number of false-"different" judgements (Le., "same" trials judged as "different"). According to Krueger and Chignell (1985), more errors are expected on "same" than "different" trials due to the fact that internal noise is more likely to change physical matches into spurious mismatches than vice versa. We make the assumption that patterns with 6 dots will be more susceptible to this internal noise effect than patterns with 2 dots. The model assumes that on "different" trials rechecking occurs until a high degree of certainty is reached. If the LH and RH use the same criterion of certainty then LVF, RVF and Bilateral (information finally processed in RH following transfer) responding will be equated in terms of error scores. Averaged processing time during the comparison stage will be approximately equal and the only factor differentiating the three presentation conditions will be that only on bilateral trials will

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there be parallel processing during the initial registration stage. This will lead to a constant bilateral advantage relative to L VF and RVF presentations across levels of task difficulty. On "same" trials where rechecking does not occur low error rates will be observed in all three presentation conditions at 2 Dots. As task difficulty increases, however, more false-"different" responses will be emitted leading to higher error rates than under comparable conditions for "different"trials.

Tfsts of the Model's Predictions Turning now to the actual data which is summarised in Figures 2-4 we see that indeed for "same" judgements RVF responses are slower than LVF responses at all levels. Faster responding following LVF input presumably results from the RH's superiority in handling visuospatial processing. We further find that bilateral responding is faster than L VF responding which presumably means that the sharing of initial imputs confers a greater benefit than the combined effects of added transfer time and stimulus degradation. As regards the picture for the "different" data we see no difference between LVF and RVF responding. This may be due to the factors noted above as predicted by Krueger's noisy-operator model. LVF responding is not significantly faster than RVF responding at the 2 and 4 dot levels implying that both hemispheres may be approximately equal in their ability to process visuospatial material at low levels of task difficulty. Sergent (1991) has recently presented evidence that the two hemispheres are equally competent at processing categorical and coordinate spatial relations. However, as task difficulty is increased to 6 dots differences in processing strategy may result in differences in reaction times. As regards the bilateral condition responding is faster than either unilateral condition. Again this advantage presumably results from the initial sharing of processing during registration. It is worth emphasizing that the bilateral advantage is indeed present whether the data on which it is based are from "same" or "different" responses. And moreover, in both cases, the response latencies reflect the task difficulty as it is increased with an increasing number of dots in the patterds. At the same time, the overall advantage (in terms of response latency) of the bilateral condition is reduced when making "different" judgements. It is not clear why more errors are not emitted during the bilateral condition at 6 Dots for "same" judgements. Perhaps having the inputs separated during early registration may protect further processing of those inputs from false-"different" errors which would be likely during the comparison stage. It is also worth noting from inspecting the functions in Figure 2 that the functions for the L VF data and the bilateral data are indeed closely similar in shape. This is consistent with the assumption made earlier that on bilateral trials interhemispheric transfer is from the LH to the RH and thus that the comparison of dqt patterns occurs in the RH for both the LVF and the bilateral conditions.

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The experiment reported was undertaken for two reasons. First, to develop a procedure to investigate the relationship between hemispheric interactions and task difficulty using visuospatial stimuli that varied along a quantified dimension of stimulus complexity and within the capability of children and certain patient populations. Second, to contribute to the ongoing debate regarding the advantage of bilaterally rather than unilaterally presented stimuli. Requiring subjects to judge whether pairs of patterns were identical or not was found to produce longer RTs and more errors as the number of dots per pattern increased. The results in general agreed with those reported by Jeeves and Lamb (1988) who used an information theory metric to vary stimulus complexity. The bilateral advantage appears to be a reliable phenomenon when using visuospatial stimuli in a perceptual matching task. Both type of judgement (i.e., "same" vs. "different") and the visual field (i.e., L VF vs. RVF) during unilateral presentations need to be considered when examining the relationship between the size of the bilateral advantage and task difficulty. As difficulty increased this advantage increased only for "same" judgements and only with respect to RVF responding for latency scores. The procedure described here has since been shown to be within the competence of normal six and seven year-old children and an acallosal child. ABSTRACT

Twenty-five normal subjects made "same-different" responses to dot patterns presented in the L VF, R VF or bilaterally. Task difficulty was manipulated in each condition by varying the number of dots in the two patterns presented from two to four to six. The pairs of patterns always had the same number of dots on a given trial. Response latency and accuracy worsened as the number of dots increased for all three presentation conditions and for both "same" and "different" judgements. Overall, responding was faster and the number of errors lower on Bilateral presentations. For response latencies to identical patterns of dots, the size of the bilateral advantage increased relative to RVF responding as task difficulty increased but did not change significantly relative to L VF responding. When the two patterns were not identical the size of the advantage did not change as task difficulty increased. "Same" judgements were faster but less accurate than "different" judgements. A model of hemispheric interactions is proposed to account for the findings.

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Prof. M.A. Jeeves, Psychological Laboratory, University of St. Andrews, SI. Andrews, Fife, KYl6 9JU, Scotland, U.K.