Acta Psychologica 44 (1980) 235-243

0 North-Holland Publishing Company

DECISION MAKING TION

Bill JONES

AND HEMISPHERIC SPECIALIZA-

Revised version received May 1979

In three experiments it was found that right-handed subjects in a tachistoscopic letter

detection task responded more precisely to changes in Signal to Noise ratios or pay-offs when presentation was to the right rather than to the left visual field. This relationship held

whether or not there was also a right field advantage for detection accuracy. It is argued that

the results reflect left hemisphere specialization in analytical decision making.

There is considerable evidence that under some experimental circum- stances letters tachistoscopically presented to the right or left visual hemifields are more accurately perceived in the right field (see White 1972, for a review). Though the data are not by any means consistent a reasonable summary might be that presentation should be unilateral (bilateral presentation seems to reverse the field advantage, e.g. Heron 1957) and to either field at random (no field advantage has been observ- ed when presentation is consistently to one field for a block of trials, e.g. Heron 1957). In some experiments presentation at approximately 2”to either side of the midline has resulted in a right field advantage (Bryden 1965) though in others even a wider separation between fields has not been related to significant differences in accuracy between the fields (Jones and Santi 1978). However, the right field advantage when it is observed probably does not depend upon particular dependent measures since signal detection estimates of accuracy (Jones and Santi 1978; Robertshaw and Sheldon 1976) and measures which in principle confound accuracy per se with decisional strategies (Bryden 1965; Kim- ura 1966) give approximately the same results.

Two broad classes of explanation appear to give a reasonable fit. The first is based upon the premise that each visual hemifield has direct pathways only to the contralateral cerebral hemisphere. Since we can also assume left hemisphere specialization for symbolic activities, the

236 B. Jones/ Decision making and hemispheric specializafion

right visual field advantage for letter perception might be based upon relative or absolute cerebral lateralization of function (Bryden 1965; Kimura 1966). A second account is simply that subjects’ scanning pat- terns (including ‘cognitive’ scanning as well as eye-movement patterns) are biased toward the right of the display (Ayres 1966; Jones and Santi 1978). Although somewhat ad hoc, a scanning hypothesis takes into account known differences in perceptual accuracy given presentation to the top and bottom of the visual fields (e.g. Worrall and Coles 1976) as well as between right and left. A recent experiment by Jones and Santi (1978) lent some support to the scanning notion since they found a right-field advantage for letter discrimination only when subjects were allowed to make eye movements contingent upon a brief presentation of a letter in either visual field. They argued that eye movement control is directionally biased toward the right reflecting some ‘cognitive’ scan- ning of the visual field. Interestingly, Worrall and Coles (1976), who presented letters at clock-face positions in the visual field found that recognition accuracy was highest at 10 o’clock. Although there was a right field advantage when the two positions along the horizontal meri- dian were compared, these investigators observed a slight left-field advantage overall.

If visual asymmetries in accuracy may reflect factors other than, or in addition to, hemispheric differentiation, it may, nevertheless, be pos- sible to demonstrate psychophysically that hemispheric specialization exists for other aspects of performance. The present research is concern- ed with the ability of right-handed persons to analyze optimally the probability structure of events, in short with the decisional aspects of perception rather than with perceptual accuracy per se. As is well known, the theory of signal detection provides convenient ways of obtaining formally independent estimates of perceptual sensitivity and of decisional parameters (Green and Swets 1966). For example, the experimenter may require the subject to categorize events as ‘Signal’ or ‘Noise’ while changing the ratio of Signal to Noise events over blocks of trials. Thus the subject is required to adjust his decision criterion to match the changed probability of particular events. An analysis of optimal decision making can be approached in at least two ways. One can examine the subject’s ability to match the probability of Signal events for each block of trials in terms of the ratio of the prior probabil- ity of Noise events to the prior probability of Signals. This ratio, p, thus represents an ideal decision criterion. However, precise optimal calcula-

tions for a given a priori probability are somewhat less interesting than an examination of the subject’s ability to adjust his criterion appropriat- ely across changes in prior probabilities. If the subject has some intrin- sic bias to respond signal or noise he will not match precisely the a priori probability of signal events for any given probability. Neverthe- less, the probability of responding ‘Signal’ should increase as the proba- bility of signal events increases. A minimal assumption is that the sub- ject’s decision scale is ordinal and hence observed values of p will be monotonically related to optimal values. Where subjects can adjust their decision criteria with precision the rank order correlation between optimal and observed P will approach unity.

Experiment 1

In this experiment. letters were tachistoscopically presented to right-handed Ss in the right and left visual fields at random. One letter was treated as the ‘Signal’ and others as ‘Noise’. The ratio of Signal to Noise letters was varied between blocks of trials. If the left hemisphere is specialized for decision making in this context optimization should be more precise for the right than for the left visual field.

Mrrhotl

The Ss were nine right-handed undergraduates. Handedness was assessed using a standard procedure (Oldfield 1971) and only Ss who reported never using their left- hand were used.

Apparatus and tnarrria1.s Photographic slides were constructed such that the letters X.Y,W and M could be

presented at 5” to the right or left of the midline. These letters are visually highly all confusable at rapid rates of presentation in the periphery. Slides were presented from a Polymetric B2 two-channel projection tachistoscope. The Ss eye-movements were monitored by placing electrodes at the external canthus of each eye. The electrodes fed through a d.c. amplifier to an oscilloscope screen (Tektronix D83). Any deviation of the Ss eye from centre-field fixation during a trial could be monitored and the trial replaced.

In all blocks of trials one of the four letters chosen at random was presented for 40 msec, 5” to the right or left, again chosen at random, of center-field fixation point. The letter X was treated as ‘Signal’ and Y, W, M. as ‘Noise’. Over blocks of 100 trials (50 in each of the right and left visual fields) the ratio of Signal to Noise trials was varied from 0.20 to 0.80 increments of 0.10. The order of ratio blocks was randomized for each S.

238

Table I

Values of rs. the index of precision of optimkation

for left (LVF) and right (RVF) visual fields for all

subjects in Experiment I. Values greater than 0.71

and 0.89 are significant at the 0.05 and 0.01 levels

respectively.

Subject LVF RVF

0.91 I .oo 0.61 I .oo

0.87 I .oo 0.25 I .oo 0.82 I .oo

0.79 0.96

0.61 I .oo

0.87 I .oo

0.63 0.94

Results

The proportion of correct signal responses (‘Hits’) was plotted against the proportion of incorrect signal responses (‘False alarms’) for each Signal to Noise ratio and each visual field. The area under the resulting receiver operating characteristic (ROC) curve, p(A), was calculated as an index of detection accuracy independent of criterion place- ment. Mean f’(A) for the two visual fields was 0.68 and 0.92 for the left and right fields respectively. All nine Ss were more accurate when presentation was to the right visual field and the difference between the fields was highly significant, I = 7.01, tiff’= 8, p < 0.001.

This only confirms many previous experiments. Of more interest is the data in table I which shows values of rs, the rank-order correlation between observed and ideal 8’s for each S. It is readily apparent that all Ss were able to match changing event probabilities more precisely when presentation was to the right visual field and seven of the nine Ss showed a perfect monotone relationship (rs = I) between observed and optimal crite- rion adjustment. The difference between visual fields in terms of rs was also highly significant t = 4.14, ~/f = 8, p < 0.005.

Experiment 2

We might conclude from the first experiment that right-handed Ss exhibit a superior ability to analyze the probability structure of events when information is presented to the right visual field independent qf’differences in detection accuracy. A possible coun- ter might be that while detection accuracy and optimization are formally independent they are empirically co-variates. Given that the S cannot make accurate detections on the left side he would be in no position to determine apriori probabilities of ‘Signal’ and

239

Table 2

OptimiTation by all subjects in experiment 2 for the

left (LVF) and right (RVF) visual fields.

Subject LVF RVF

I 0.86 0.96

2 0.93 0.96

3 0.77 I .oo

4 1.00 0.94

5 0.93 I .oo

6 0.86 I .oo

7 0.47 0.77

‘Noise’. Such a hypothesis appears not to account for the data in Experiment I. Firstly, Ss show, on the average, an adequate level of detection accuracy for the left field. Secondly, one might expect on the basis of this hypothesis, a strong relationship bet- ween detection accuracy and monotonicity of optimization for the left-field. In fact, the rank order correlation between accuracy and the optimization statistic is an insignifi- cant 0.41.

Nevertheless. this hypothesis raises an interesting question. Assuming for whatever reasons, that detection is more difficult when letters are, presented to the left-field. would processing exhibit the same decisional characteristic if accuracy of detection were equated for the two fields? In this second experiment an attempt was made to hold accuracy constant for the two visual fields by presenting target and distractor letters in one visual field consistently for a block of trials rather than randomizing the side of presentation within any given block. Heron (1957) has shown that this procedure elimi- nates the typical right-field advantage in letter perception. Seven right-handed Ss took part in the second experiment. in which presentation w’as consistently to one field and then to the other for 350 trials. The order of visual field presentation was randomized for each S. Otherwise the procedure, c/c.. of Experiment I was followed.

The procedure successfully eliminated the right-field advantage in detection accuracy with a non-significant mean p(A) difference between the fields of (0.003, f = 0.30. c#‘= 6). However, table 2 shows that optimization was more precise for six of seven Ss given right-field presentation. The difference between mean rank-order correlations for the two fields was again significant, t = 2.53, tlf = 6. /J < 0.05.

Experiment 3

From the point of view of signal detection theory ROC curves can be generated by manipulating criterion placement in a number of ways. For example, monetary pay-offs conditional upon signal and noise responses can be systematically varied over blocks of

Table 3 Conditions payoff matrices (cents) for Experiment 3

Ideal P correct ‘Signal

Incorrect Correct ‘Signal ‘Noise’

Incorrect ‘Noise’

4.00 I -4 4 -1 3.00 I 3 3 -1 2.00 I 2 2 I 1.00 I -1 I ~1 0.50 1 -2 2 -I 0.25 I -4 4 -1 0.20 I -5 5 -1

trials and the S is expected to maximize gain. While this procedure may generate ROC curves equivalent to those obtained by directly varying signal to noise ratios. cognitively the two procedures may be rather different. In Experiment I the S had to decide upon an appropriate criterion during the course of successive judgments. In a pay-off experi- ment the S is informed about pay-offs and hence the required decision criteria in advance of a block of trials. Conceivably the learning requirement in Experiment I was at least partly responsible for the right-field advantage. Consequently, this experiment essen- tially repeats the first except that the Ss’ criterion placement was manipulated through prior information about pay-offs.

Method

The Ss were six right-handed undergraduates. handedness assessed as in Experiment I.

Apparatus and material.s The slides X, Y, W and M at 5” to right and left were presented as in the previous two

experiments.

Procedure Seven pay-off conditions were used (table 3). the order being randomized for each S.

In each conditions the signal letter X was presented at random on 50 of 100 trials. On the remaining trials one of the Noise letters Y, W or M, chosen at random, was presented. Half the trials at random were left-field presentations and half right-field. The pay-off matrix was thus the same for both visual fields in each condition.

As in Experiment I, a significant right-field advantage in detection accuracy was found, I = 3.37, @= 5, p < 0.01. Mean p(A) values for the right and left visual fields were 0.903 and 0.775 respectively.

241

Table 4 OptimiTation by all subjects in Experiment III for

the left (LVF) and right (RVF) visual fields.

Subject LVF RVF

I 0.71 I .oo 2 0.66 0.96 3 0.88 I .oo 4 0.54 I .oo 5 0.82 I .oo 6 0.92 I .oo

The relationship between observed and ideal optimization for all Ss is shown in table 4. For all but one S this relationship was perfectly monotone for the right field and all Ss show a significantly higher correlation for the right than for the left. Overall. the difference between fields was highly significant, / = 4.15, df= 5, p < 0.005. These data suggest, therefore, a relatively greater ability on the part of the Sgiven right-field rather than left-field presentation to make decisions which correspond to the aprioristructure of costs and pay-offs.

Discussion

The following points seem worth emphasizing by way of discussion. Although the typical right-field advantage for letter perception was found in the first experiment, eliminating this advantage as in the second experiment does not eliminate differences in decision making independent of detection accuracy. When the subject makes judgments about information in either visual field he may be better able to analyze and reflect the probability structure of events when information is pre- sented to the right visual field. This effect conceivably turns upon the specialization of the left cerebral hemisphere for decision making. Notice, however, that rank order correlations between optimal and observed changes in 0 are typically significant for the left visual field for all three experiments. It might therefore, be more appropriate to speak of the relative specialization of the two hemispheres rather than of rigid localization of abilities in one hemisphere or the other.

Optimization in the sense of matching the changing probability struc- tures of events can be taken as an example of an analytical task and some workers have regarded the left hemisphere as essentially specia- lized for analytical processing with right hemisphere processing being

242 B. Jones/ Decision making and hemisphrric~ specialization

holistic in nature (Bever and Chiarello 1974; Levy-Agresti and Sperry 1968). Clearly matching changes in signal-to-noise ratios requires more than mere calculation (Sperry’s (1968) work indicates in any case that patients after callosal section are capable of arithmetic when the left hemisphere is tested). The subject must be able to judge precisely the relationship between his responses and the changing probability struc- ture even though his responses are to some extent in error; he must be able to grasp the total change in the probability structure of events from an analysis of particular responses.

All the same the analytic-holistic distinction is rather vague and its application to any particular experimental results is ambiguous. (‘Anal- ysis’ conceptually implies, in any case, a ‘whole’ to be analyzed.) Consi- der the letter X, Y, W, and M used in the present experiment. Extrac- tion of significant features (straightness, obliqueness) would not serve to distinguish one letter from another. One could argue, therefore, that Signal-Noise differentiation must here be based upon holistic pattern recognition. Nevertheless, discrimination was more efficient in Experi- ment 1 given presentation to the right visual field.

Use of a payoff matrix or of variations in the posterior probability of signal and noise provide equivalent means of calculating p either in terms of likelihood-ratio decision rules (Green and Swets 1966) or of probability matching (Thomas and Legge 1970). Whether the payoff structure is known in advance or whether subjects must uncover the probability structure of events during the sequence of trials, they must continuously adjust their decisions over time. Recent work by Vroon et

al. (1977) using a wholly different paradigm, suggests that an important structural difference between the cerebral hemispheres is the degree to which information can be analyzed in time. Conceivably left hemi- sphere superiority in temporal analysis underlies the relative differences in probabilistic decision making observed here.

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