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Visual detectability gradients: The effect of distractors in contralateral field
BRAIN
AND
COGNITION
12, 128-143 (l!%o)
Visual Detectability Gradients: The Effect of Distracters in Contralateral Field R. EFRON, E. W. YUND, AND D. R. NICHOLS Neurophysiology-Biophysics Research Laboratory, Veterans Administration Medical Center, Martinez, California, and Department of Neurology, School of Medicine, University of California at Davis
A number of studies involving recognition of tachistoscopically presented words have reported that the typical right visual field performance superiority associated with linguistic stimuli is enhanced by bilateral presentations (simultaneous stimuli in both visual half-fields) compared to unilateral presentations (stimuli in only one half-field on a trial). We have reported the same phenomenon, however, using visual spatial patterns in a search paradigm (E. W. Yund, R. Efron, & D. R. Nichols, 1990~. Brain and Cognition, 12, 117-127) and have accounted for it in terms of the operating characteristics of a visual scanning mechanism which serially examines a decaying neural representation of the stimuli. In the present experiment we attempted to exploit these operating characteristics to influence this difference between unilateral and bilateral presentations. The results not only are consistent with the assumptions of the scanning hypothesis but they also provide new information pertinent to the operating characteristics of this mechanism. o IWO Academic Press, Inc.
INTRODUCTION
A large number of studies of the accuracy of visual identification, recall, or detection of briefly presented stimuli in neurologically normal subjects have documented a performance superiority in the right visual field (RVF). These observations coupled with the differences in the cognitive deficits associated with right- and left-sided cerebral lesions, have led many investigators to interpret the right/left asymmetrical performance in normal subjects in terms of hemispheric differences in processing capacity, resources, or styles. A number of studies involving recognition of linguistic stimuli (Hines, 1975, 1976; Kershner & Jeng, 1972; McKeever & Huling, 1971a,b) have shown that the RVF performance superiority is more marked when words are delivered to both Address reprint requests to R. Efron, VA Medical Center, 150 Muir Road, Martinez, CA 94553. This work was supported by Research Service, Veterans Administration. 128 0278-2626190$3.00 Copyright All rights
Q 1990 by Academic Press, Inc. of reproduction in any form reserved.
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visual fields simultaneously (bilateral presentation) than is the case when they are delivered to either right or left field (unilateral presentations). It has been suggested (Hines, 1975; McKeever & Huling, 1971a,b) that this difference between unilateral and bilateral presentations may be related to a “competition” for access to the language center in the left hemisphere: Linguistic information presented to the RVF is believed to reach the center with greater “signal fidelity” than identical information presented to the left visual field (LVF), which reaches the left hemisphere in a degraded form as a result of transfer via the corpus callosum. This would cause a weak RVF superiority in unilateral presentations. In a bilateral presentation, however, the degraded signal from the LVF would have to “compete” for access to the right hemisphere language center with higher fidelity signals from the RVF, and this is believed to account for the much stronger RVF superiority in this presentation mode. Our own interest in the difference in the magnitude of the RVF superiority between unilateral and bilateral presentations derives from the previous report (Yund, Efron, & Nichols, 1990~) in which the same effect was observed but with nonlinguistic stimuli-the pattern stimuli used in this series of reports (Efron et al., 1987, 1990a,b; Yund et al., 1990a,b,c). We accounted for the more marked RVF superiority in bilateral than unilateral presentations in terms of a scanning hypothesis (see Efron et al, 1990a) which neither is based on hemispheric processing differences nor does it require the assumption of loss of “signal fidelity” as a consequence of callosal transfer of information. Briefly, the scanning hypothesis accounts for this result in terms of a tendency, across multiple trials, for the scan to examine stimuli in the RVF earlier than those in the LVF-it tends to go to the RVF first. Since the scan is examining a rapidly degrading neural representation of the stimuli, the more time which elapses before it reaches the location occupied by the target the less the information there to support a correct discrimination between the target and a nontarget pattern and the greater the probability of a detection error. However, since the scan tends to go right first the presence of stimuli in the LVF will have little effect on target detectability in the RVF but the presence of stimuli in the RVF will have a more marked effect on target detection in the LVF than would be the case if there were no stimuli in the RVF (see Yund et al., 1990~ for a more detailed explanation). The object of the present experiments was to answer two questions: First, given that bilateral presentations yield larger RVF superiority than unilateral presentations, will any visual stimulus in the contralateral field produce the same enhancement of the RVF detection superiority, or will some type of stimuli in the contralateral field be more effective than others in enhancing the RVF detection superiority? Second, do different types of stimuli in the contralateral field change the shape of the de-
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tectability gradients in the reference field, or do they merely increase (or decrease) the probability of detection at all locations in the reference field equally? The rationale for the design of the present experiments is most easily described by reference to Figs. 1 (Experiment I) and 2 (Experiment II). In each experiment there are four configurations of stimuli (A-D) which were presented to the subject in random order. It will be observed that every condition contains six different patterns, arranged in an arc equidistant from the fovea. The target (the vertical stripe pattern), if present on a trial, was always in that arc. Its location, on the trials in which it was present, was uncertain (randomized). The subject’s task on each trial was to report whether the target was or was not present in this arc. In two configurations (A and B) there were IZOstimuli in the contralateral field, while in two configurations (C and D) six additional stimuli, which we will refer to generically as “distracters”, were present in the contralateral field. As can be seen by comparing Figs. 1 and 2 the only difference between Experiments I and II is the type of distractor which was used in the field contralateral to the target and nontarget patterns in configurations C and D. The results obtained with configurations A and B are the unilateral presentations in which nothing is present in the contralateral field. The detectability at each of the six locations in each field obtained with configurations A and B can then be compared with the detectability at the same location when distracters were present in the contralateral field as in configurations C and D, i.e., the bilateral presentations. We will refer to the distracters of Experiment I as “FILL” stimuli and those of Experiment II as “DOT” stimuli. The FILL stimuli were used because of their overall luminance was exactly the same as that of the patterns, but when viewed 4.6” from the fovea they appear featureless, i.e., without any discernible internal structure or pattern. The DOT stimuli of Experiment II had half the average luminance of the target and nontarget patterns, and when viewed 4.6” from the fovea, are not featureless. Which of the two types of stimuli (if either) when present in the contralateral field will cause changes in detectability in the reference field? If both do, will there be any differences between the two types of stimuli in the magnitude or pattern of target detectability at the six locations in the reference field? Three plausible predictions can be made: On the one hand, it could be argued that the FILL stimuli, which have twice the luminance of the DOT stimuli, would be harder to ignore and would be expected to exhibit a greater effect on the detectability of the target in the reference field than the DOT stimuli. On the other hand, it could be argued that the DOT stimuli will have a greater effect on detectability in the reference field because they have an internal, pattern-like structure, and this struc-
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ture contains both vertical and horizontal spatial frequency components-the vertical component having the same orientation as the target pattern. Finally, it might be predicted that there will be essentially no difference between Experiment I and Experiment II, because it should be a trivial task with either type of stimulus in configurations C and D to merely detect one FILL or DOT stimulus in a field, and then to examine the other field, where the subject knew the target had to be located if present at all. The outcome of these experiments has not been provided in the Abstract to give the reader an opportunity to choose between these predictions-or to make another. A third experiment (Experiment III) was performed in which the FILL and DOT stimuli were not all located in one visual field but were randomly mixed with the patterns. This experiment on a different subject population will be described later. EXPERIMENTS
I AND II
(A) Methods The visual stimuli were presented on the display screen of a computer (Attache, Otrona) which was used as a tachistoscope. Both experiments were performed under identical conditions of ambient lighting. The screen background was 4. I cd/m”, while the illuminated parts of the screen were 73.7 cd/m*. The centers of all patterns were 4.6” from the point of visual fixation and subtended a visual angle of 1.3” on a side. Head position was maintained with a chin and forehead rest and, to minimize as much as possible the effects of sensitivity difference between nasal and temporal retinal regions, the screen was viewed binocularly. Each trial of both experiments consisted of three sequential parts: (I) a fixation routine, a procedure used to ensure that the subject’s fovea was directed at the center of the screen prior to the presentation of the pattern stimuli (see Efron et al., l990a for details); (2) the main task, a brief presentation of one of the four configurations (illustrated in Fig. 1 or 2; and (3) a response routine used to determine whether the subject detected the target on that trial (see Yund et al., l990b for details). The main task consisted of a random presentation for 50 msec of one of the four configurations in Fig. I or 2. The subject was required to report whether the vertical stripe pattern, the “target,” was present in that presentation. On 25% (randomly) of the 512 trials, the target was not present and was replaced by another pattern. The six nontarget patterns which were used are illustrated in configuration A of Fig. 1. On those trials in which the vertical striped target was present, one of these nontarget patterns (randomly) was replaced by the target pattern. The location of the target on the 384 trials (when it was present) was randomized differently for each subject with the constraint that the target appeared n times at every location before it appeared n + I times at any location, for a total of 16 times at each location in each configuration. The locations of the nontarget patterns were also randomized for each trial for each subject to counterbalance for any unique interactions which might have been present between the target and particular adjacent pattern. After the 50-msec exposure of the patterns, the screen was left blank for 333 msec, and then the words “YES OR NO” were displayed as a prompt to obtain the response. (This long interval between the stimuli and the response prompt was used to avoid possible interference in the processing of the stimuli.) The subject was required to press the up arrow on the keyboard if he saw the target or the down arrow if he did not see the target. Upon entering the response, the screen was erased and the next trial
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CONFIG. A
CONFIG. C
CONFIG. B
CONFIG. D
FIG. 1. Contrast-reversed photo~aph from computer screen of the four configurations (A-D) presented randomly in Experiment I. Ail patterns are 4.6’ from the point of visual fixation and subtended a 1.3” visual angle on a side. began with the fixation routine. As two previous studies (Efron et al., 1!?9Ob;Yund et al., 1990~)have shown that the hand used for the fixation and response routines had no influence on the results, the subjects were permitted to use their preferred hand. Prior to the start of each experiment the subjects were shown the stimuli that would be seen (either Fig. I or Fig. 2). They were info~ed that the four con~gurat~ons woutd be presented randomly but that their task was the same regardless of which configuration was presented, i.e., to report the presence or absence of the target in the arc of “patterns.” They were told that the target pattern, if present on a trial, would be in one of the six locations containing “patterns” and to ignore the distrusting stimuli on those triais when they were present. This verbal description of the experiment was followed by 23 practice trials, starting with a long exposure duration (383 msec),of the main task and decreasing on each practice trial so that the last 11 were performed at a SO-msecduration. Except
VISUAL
DETECTABILITY
CONFIG. B
CONFIG. A
ka m E E E
...... ...... :::::: ...... 111111 ...... ...... ..._. .._.. .. ..._...
:::::: .. .. ..._. ... .. ..._.. ....
...... ...... ...... ...... ...... ......
CONFIG. C
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GRADIENTS
...... ...... ...... ...... ...... ......
......_ .. ....__ .._ _..... . ....._.. ..-... .._ ...._._ .... ......_ .. .-.... ......
.. .. .. .. .. .. .. .. .. .. .. .. ::::::
...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ......
X
r H ;
111111
CONFIG. D
FIG. 2. Contrast-reversed photographs of the four configurations presented in Experiment II. Same visual angles for stimuli as in legend to Fig. I.
randomly
for the feedback which was included only for the practice trials, these last 11 practice trials were the same as those of the main experiment. Naive subjects were recruited by advertisement, were paid for their services, and did not know the goal of the experiments other than the fact that the purpose was to obtain normative data on the accuracy of visual pattern perception. Each experiment was performed on 80 right-handed subjects equally divided by sex. Half the subjects of each sex performed Experiment I before Experiment II, and the other 40 subjects performed the two experiments in the reverse order. The mean age of the subjects for Experiments I and II was 23.2 2 5.9 years. The results of each experiment on a subject were computer scored for the probability of a correct detection at each of the six possible target locations in each configuration and the probability of false alarm, i.e., reporting the target to be present when it wasn’t. For the 32 trials of each configuration in which the target was not present, false alarm prob-
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abilities were low: 0.083 f 0.009 for the eight configurations (four each in Experiment I and Experiment II).
An analysis of variance was performed on the number of correct detections as a function of the five independent variables: (1) Configuration (C), distracters present or absent; (2) Field (F), right or left; (3) Position (P), numbered 1 to 6 from top to bottom in each field; (4) Distractor type (D), FILL or DOT; and (5) Sex (S). All significant main effects and first order interactions are presented in Table 1. Three of the five independent variables (configuration, field, and position) exhibited highly significant main effects and interactions. The distractor type variable (D) produced no significant main effect, but it should be emphasized that what might be considered to be the main effect of D would appear as a D x C interaction in this analysis because the distracters were present on only half the trials in each experiment. There was no significant main effect of sex or any interactions involving gender. The highly significant main effects of configuration, field, and position as well as the interactions among them were anticipated from the results of our previous experiments (Efron et al., 1990a,b; Yund et al., 1990a,b,c) and are of interest only in so far as they confirm our previous findings. They reveal the existence of differences in detectability as a function of target location, i.e., that there is a detectability gradient, that there is a right visual field detection superiority, and that the shape of the detectability gradient differed in the two fields. The primary purpose of the present experiment, however, was to determine how these expected effects and inte~~tions would vary, if at all, with the two types of distracters that were used. The nature of these interactions involving distractor types will be described with the aid of a graphic presentation of the data (Fig. 3). Figures 3A and 3B contain all the results of Experiments I and II, TABLE 1 ANALYSIS
Source of Variance Configuration (C) Field (F) Position (P) Distracters (D) CxF CXP FxP DxP DxC
OF VARIANCE
df
F
P
l/78 l/78 5/390 l/78 l/78 5/390 s/390 51390 l/78
25.27 31.98 22.54 2.81 13.95 6.34 10.30 13.77 14.59
<.ooo5 <.0005
<.OOl <.ooo5
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135
GRADIENTS
.90 8ls a
b .8560 -
IO
Exp I -FILL
NO DISTRACTORS D&TRACTORS
c
95 -
90 -
85 80-
b
75 70 -
Exp II o--o
NO DISTRACTORS
-
DOT DISTRACTORS
IO-
95 -
w-
.85 8075 .701
FULL
1 1 IL
2L
1 1 1 1 3L
4L
5L 6L
CIRCLE
I IR
I
I
OF
PATTERNS
1 I
2R 3R 4R
I
5R 6R
LOCATIONS FIG. 3. Results of Experiment I (A) and Experiment II (B). See Discussion for C. The ordinate represents the probability of correct detection. The abscissa represents target location numbered 1-6 from top to bottom of each field. The letter (R or L) after the number indicates the visual field.
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respectively. Each data point represents the mean probability of correct detection (on the ordinate). The abscissa represents the location number of the target from top to bottom of the arc of patterns, with the letter L or R following that number to indicate the field. The two curves in each field in each panel represent the results when distracters were not present in the contralateral field (open circles) and when distracters were present (solid circles). The three main effects (configuration, field, and position) are evident: (1) Target detectabilities in the distractor absent configuration (unfilled circles) are almost always superior to that of the distracters present configuration; (2) The probability of correct detection is predominately superior in the RVF; and (3) Detectability at locations near the top of each field are superior to those near the bottom. The C x F interaction (p < .OOl): When distracters were present in the RVF, the degradation of detectability in the LVF was more marked than was the degradation of detectability in the RVF when distracters were present in the LVF; the two curves in the left half of Figs. 3A and 3B show greater vertical separation than those on the right. ‘The D x C interaction (p < .OOl): The DOT stimuli produced a larger detectability degradation in the reference field (compared to the no distractor condition) than was the case with FILL stimuli. This effect is evident in the greater vertical separation of the two curves in each field in Fig. 3B compared to Fig. 3A. The D x P interaction (p < .0005): DOT stimuli produced their maximal detectability degradation at positions 1 and 6, while FILL stimuli produced their minimal effects at these positions. Figure 3C, containing data from a previous experiment (Experiment I in Yund et al., 1990a), will not be considered until the Discussion of Experiments I and II. (C) Discussion
(Experiments
Z and ZZ)
There were four major findings in the present experiment: (1) There was a gradient of detectability within each field as a function of the spatial locations of the target. (2) When no distracters were present (the unilateral presentations of configurations A and B in Experiments I and II) overall target detectability was superior in the right arc. (3) The presence of distracters in the opposite field degraded detectability in the reference field with DOT stimuli producing distinctly more degradation than FILL stimuli. (4) The degrading effect on detectability induced by the presence of distracters in the opposite field was more marked when the distracters were in the RVF than was the case when they were in the LVF. This had the net effect of increasing the overall RVF detection superiority in the bilateral presentation conditions (configurations C and D in Experiments I and II). How are these findings to be explained? Except for the differential effect of DOT and FILL stimuli-the main phenomena to which the present experiments are directed-all these
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results represent a confirmation of those found in the previous report (Yund et al., 1990~) and are explained in the same way: The scanning mechanism has a tendency to process stimuli in the RVF earlier than those in the LVF-it goes to the right first. When an arc of patterns is in the left field, the scan might be expected to lose at least some time in the empty RVF before examining the LVF. If the arc of patterns was in the right field, however, then it would not lose this time. Thus, when the overall target detection performance of the two arc conditions is compared, a weak RVF detection superiority would be expected-as was the case in the previous experiment and for the unilateral presentations (configurations A and B) of the present experiments. However, if there are stimuli in both visual fields-the bilateral presentation (configuration C and D) of the present experiments-then, when the scan goes to the RVF first, it does find stimuli there and it must determine whether each of them is the target. Since this consumes more time than would have been the case had there been no stimuli in the RVF, target detection in the LVF is relatively worse in the bilateral than unilateral conditionsas is seen in the results. We now consider the more marked degrading effect on overall target detection in the reference field by the presence of DOT than by FILL stimuli in the opposite field-the only new finding in Experiments I and II. We assume that if a stimulus shares one or more spatial frequency components with the target it will take the scanning mechanism longer to “decide” that it is not the target than would be the case if it did not share this feature. DOT stimuli, which share a vertical spatial frequency with the target, would be expected to consume more time at each decision point than FILL stimuli and thus would be expected to produce more degradation of overall target detectability in the reference field. However, since the scan tends to examine the stimuli in the RVF earlier than those in the LVF, the presence of DOT (and to a lesser degree FILL) stimuli in the RVF will have more of a degrading effect on LVF target detectability than would the presence of these distractor stimuli in the LVF have on the RVF performance-as can be seen in the results. Thus, the tendency of the scan to examine RVF stimuli earlier than LVF ones will also produce a greater degrading effect of the distracters on LVF than RVF target detection in the present experiments. The results of a previous study (Experiment I in Yund et al., 1990a) are of particular interest with respect to this issue. In that experiment, which employed the same methods as in the present case, one of the configurations used contained 1 target and 11 nontarget patterns at the identical retinal locations as in the present experiment. The nontarget patterns in the field contralateral to the one containing the target can be considered to be “distracters” also and to be terminologically consistent they will be called PATTERN stimuli. In the design of that experiment,
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performed on a different subject population, only a bilateral presentation was employed. For this reason, the results of that experiment, illustrated in Fig. 3C, contain only one curve in each field, and we cannot determine the magnitude of the degradation of detectability which was produced by the presence of PATTERN stimuli in the field opposite to the one containing the target. However, if DOT stimuli produce more detectability degradation than FILL stimuli because they share a vertical frequency component with the target, then the shape of the detectability gradient with PATTERN stimuli, some of which had vertical spatial frequency components, should resemble that of the DOT stimulus gradients more than that obtained with FILL stimuli. The detectability gradients in the left field when FILL, DOT, and PATTERN stimuli are present are seen in Figs. 3A, 3B, and 3C, respectively (curves with solid circles). In Fig. 3A the detectability falls between locations 1L and 3L and then is essentially the same at locations 4L, 5L, and 6L. The lowest detectability is at 3L. In Fig. 3B there is a similar decrease from IL to 3L, a “recovery” at 4L, and then a precipitous decrease from 4L to 6L. In Fig 3C the pattern is very similar to that in Fig. 3B except that the decrease in detectability at 3L is much less pronounced. Thus there appears to be some type of progression in the shape of the detectability gradients from FILL to DOT to PATTERN stimuli. In the RVF there is only a hint of a similar progression, a result which would be expected since the presence of distracters in the LVF field had much less degrading effect on RVF detectability than the reverse. Parenthetically, the D x P interaction of the present experiment in which DOT stimuli produced their maximal detectability degradation at positions 1 and 6, while FILL stimuli produced their minimal effect at these positions, is further evidence that the effects of DOT stimuli are similar to PATTERN stimuli: In Yund et al. (1990~) the same effect is seen in the configuration by position interaction when PATTERNS were in the field opposite to the one containing the target. EXPERIMENT
III
In the course of this series of reports on the scanning hypothesis we have used several different methods to measure target detectability at multiple locations within the visual field and to correct the data for any response bias. Given this variety of methods and bias-correction procedures, there is necessarily some uncertainty concerning the extent to which some of our conclusions might be method-dependent. To resolve this uncertainty, Experiment III was designed so that a number of the major effects that we have observed in previous experiments using different methods should be evident in a single experiment. In particular, we wanted to confirm (a) the inverse relationship of overall detectability with the number of nontarget patterns present (see Efron et al., 1990a),
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(b) the differential effects of FILL and DOT stimuli (Experiments I and II of the present report), and (c) our inference that a BLANK stimulusdefined as the absence of a stimulus in a location which contained a target or a nontarget pattern on other trials-is ignored completely (skipped over) by the scan or is treated as a very easy nontarget pattern (Efron et al., 1990a). (A) Method The method used was identical to that in Efron et al. (1990a) in which the response routine, which appeared on the screen 500 msec after the termination of the 16.67-msec exposure to the patterns, consisted of the presentation of a single empty box in a location in which a pattern (target or nontarget) had been presented. The subject was required to indicate whether or not the target pattern had been present at this prompted location. The subjects were informed prior to the experiment that if a target had been present on a trial it would have been at this prompted location, and that a “yes” response was required (see op tit for details). With this method hit and false alarm probabilities can be obtained for each possible location of the target. The patterns were in the same 12 locations as in Experiments I and II and were of the same size. The experiment consisted of 1176 trials in which the target was present in 598. There were seven randomly presented conditions: (1) 12 patterns [12P], (2) 8 patterns with 4 randomly located BLANK stimuli [8P + 4B], (3) 8 patterns with 4 randomly located FILL stimuli [8P + 4F], (4) 8 patterns with 4 randomly located DOT stimuli [8P + 4D], (5) 4 patterns with 8 randomly located BLANK stimuli [4P + 8B], (6) 4 patterns with 8 randomly located FILL stimuli [4P + 8F], and (7) 4 patterns with 8 randomly located DOT stimuli [4P + 8D]. At the conclusion of the experiment the prompt had been at each of the 12 possible locations of the target in each condition 14 times and the target had been present in 7 of these occasions. Forty-four right-handed subjects, equally divided by sex, who had not participated in any previous experiments were recruited by advertisement and paid for their services. Their mean age was 22.7 + 5.3.
(B) Results
The raw data consists of the number of hits and the number of false alarms for each subject at each location under each condition. Since the number of possible hits and false alarms was too small to compute a d’ for each condition and location for each subject, we used the a’ statistic and performed the analysis of variance using the following transformation from McNicol (1972): 2 x arcsin ($?). These transformed a’ values range from 0 to 3.1416 (m) and are roughly comparable to d’ values. In keeping with the purpose of Experiment III, only the principal variables of condition and field were included in the analysis of variance. The main effect of field was highly significant (p < .OOOS) and in the expected direction: Detection was superior in the RVF (mean transformed a’ of 2.59 VS. 2.42 for the LVF). The main effect of condition (p < .OOOS)is illustrated in Fig. 4 where target detection is plotted for each condition. The major effects in this experiment are the same as those reported nreviouslv using different methods and thus nrovide reassurance that
140
EFRON, YUND, AND NICHOLS 2.80
-a
2.70
0 bf
2.60
8
%
2.50
z E
2.40
J
2.30 12P
8P+4D
8P+4F
8P+4B
4P+8D
4P+8F
4Pt8B
CONDITION FIG. 4. Results of Experiment III. The seven conditions of this experiment are listed on the abscissa (See Methods, Experiment III for nomenclature). The ordinate is the a’ measure of target detectability. In Experiment III, the BLANK, FILL, and DOT stimuli were randomly distributed among the patterns.
our conclusions are not method-dependent. More specifically, the inverse relationship between target detectability and the number of patterns present is evident by comparison of conditions 12P, 8P + 4B, and 4P + 8B in Fig. 4: Target detectability progressively increased across these three conditions. The same effect, albeit less marked, is observed with FILL stimuli by comparing conditions 12P, 8P + 4F, and 4P + 8F. For DOT stimuli, as would be expected from the results of Experiments I and II, the increase in detectability from 12P to 8P + 4D to 4P + 8D was less marked than for FILL stimuli. In our report describing the effect of randomly presented BLANK stimuli (Efron et al., 1990a) we concluded that a BLANK (defined as the absence of a stimulus in a location where a stimulus was present on many trials) was either “skipped over” entirely by the scan or was processed as a very easy nontarget pattern. The present results suggest the second possibility is the more likely: If BLANKS were skipped over entirely then we would expect that the performance in the 8P + 4B condition would have been appreciably better than in the 8P + 4D and the 8P + 4F conditions-but the differences are only small ones and there is no obvious discontinuity between the 8P + 4D, 8P + 4F, and 8P + 4B conditions. A similar nondiscontinuous relationship of increasing performance is seen in the 4P + 8D, 4P + 8F, and 4P + 8B conditions. Thus, the results of Experiments I, II, and III indicate that BLANKS are processed faster
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than FILL stimuli which in turn are processed faster than DOT stimuli and that DOT stimuli are processed faster than the (average) pattern. CONCLUSIONS
The primary object of this series of investigations (Efron et al., 1987, 1990a,b; Yund et al., 1990a,b,c) has been to understand how we find a specific object in a visual field containing many similar objects when the exposure to the visual scene is too brief to permit a series of eye movements. The scanning hypothesis, presented in larval form in the first of these reports (Efron et al., 1987) and expanded in the third (Efron et al., 1990a), was developed to account for the striking differences in target detectability as a function of its retinal locus. This hypothesis has two fundamental assumptions: That the neural representation of the visual image is serially processed, i.e., scanned, and that the information in that neural representation decays rapidly over time. Since neither of these assumptions is inherently asymmetrical, the scanning hypothesis is no more concerned with right/left, than upper/lower, or any other arbitrarily selected axis of the two-dimensional perceptual surface-the entire visual field. Within the context of the scanning hypothesis itself, there is no reason to divide this surface into halves, quarters, or eighths! However, in the course of our investigations it became apparent that while target detection was superior in the RVF the within-jield differences in target detectability were often more marked than the overall right/left difference. Further experiments (Yund et al., 1990a) revealed that the within-field detectability differences could not be explained either on the basis of the known differences in retinal spatial resolution or known properties of lateral masking-but they could be explained on the basis of a serial process which examines the’\degrading information at various locations within the field in some probabilistic order. The overall RVF detection superiority could be explain&l similarly-a tendency for the scan to examine stimuli in the RVF somewhat earlier than those in the LVF. We recognized, as had White (1973),,$thatthe existence of such a scanning mechanism would have major im&cations for the field of laterality research since the most fundamental premise of that field is that right/left performance differences arise from hemispheric differences, albeit modified or modulated by other task and stimulus variables. Indeed, this series of papers was submitted to this particular journal to alert those interested in hemispheric differences that the scanning hypothesis not only offers an alternative explanation for performance differences between visual fields but also accounts for the within-field differences at the same time. Since the scanning hypothesis contains no assumptions which predict the probabilistic scanning order in which a subject or a group of subjects will examine the stimuli which may be present in the visual field, a
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number of our experiments have been directed to discovering how the scan is influenced by the number ofstimuli present (Efron et al., 1990a), the spatial con$gurution of these stimuli (Efron et al., 1990b; Yund et al., 199Oc),and, in the present report, the type of stimuli which are used. All the results of these studies are consistent with the scanning hypothesis. Although the assumptions of the scanning hypothesis are equally applicable to an attentional or a nonattentional scan, two of our experiments (Yund et al., 199Ob;Efron et al, 1990b) tested the hypothesis that the scan under investigation consists of a sequence of attentional shiftsthe hypothesis originally proposed by Heron (1957). Despite the fact that the results of those studies were consistent with a nonattentional scan and failed to provide evidence for an attentional process, we do not consider this issue resolved at the present time. That we have observed the same kinds of within- and between-field detection differences with visual-spatial patterns that Heron found for the recognition of letters indicates strongly that the scan is not uniquely related to the particular visual stimuli which are used but is a mechanism which is critically important in enabling us to find an object in the presence of similar objects. REFERENCES Efron, R., Yund, E. W., & Nichols, D. R. 1987. Scanning the visual field without eye movements-A sex difference. Neuropsychologia, 25, 637-644. Efron, R., Yund, E. W., & Nichols, D. R. 1990a. Serial processing of visual spatial patterns in a search paradigm. Brain and Cognition, 12, 17-41. Efron, R., Yund, E. W., & Nichols, D. R. 1990b. Detectability as a function of target location: Effects of spatial configuration. Brain and Cognition, 12, 102-l 16. Heron, W. 1957. Perception as a function of retinal locus and attention. American Journal
of Psychology,
70, 38-48.
Hines, D. 1975. Independent functioning of the two cerebral hemispheres for recognizing bilaterally presented tachistoscopic visual-half-field stimuli. Cortex, 11, 132-143. Hines, D. 1976. Recognition of verbs, abstract nouns and concrete nouns from the left and right visual half-fields. Neuropsychologia, 14, 211-216. Kershner, J. R., & Jeng, A. G. 1972. Dual functional hemispheric asymmetry in visual perception: Effects of ocular dominance and postexposural processes. Neuropsychologia, 10, 437-445. McKeever, W. F., & Huling, M. D. 1971a. Lateral dominance in tachistoscopic word recognition performances obtained with simultaneous bilateral input. Neuropsychologia, 9, 15-20. McKeever, W. F., & Huling, M. D. 1971b. Bilateral tachistoscopic word recognition as a function of hemisphere stimulated and interhemispheric transfer time. Neuropsychologia, 9, 281-288. McNicol, D. 1972. A Primer of Signal Detection Theory. London: George, Allen & Unwin Ltd. White, M. J. 1973. Does cerebral dominance offer a sufficient explanation for laterality differences in tachistoscopic recognition. Perceptual and Motor Skills, 36, 479-485. Yund, E. W., Efron, R., & Nichols, D. R. 1990a. Detectability gradients as a function of target location. Bruin and Cognition, 12, 1-16.
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Yund, E. W., Efron, R., & Nichols, D. R. 1!49Ob.Detectability gradients as a function of spatial location: Effects of selective attention. Brain and Cognition, 12, 42-54. Yund, E. W., Efron, R., & Nichols, D. R. 1990~. Target detection in one visual field in the presence or absence of stimuli in the contralateral field by right- and left-handed subjects. Brain and Cognition, 12, 117-127.
AND
COGNITION
12, 128-143 (l!%o)
Visual Detectability Gradients: The Effect of Distracters in Contralateral Field R. EFRON, E. W. YUND, AND D. R. NICHOLS Neurophysiology-Biophysics Research Laboratory, Veterans Administration Medical Center, Martinez, California, and Department of Neurology, School of Medicine, University of California at Davis
A number of studies involving recognition of tachistoscopically presented words have reported that the typical right visual field performance superiority associated with linguistic stimuli is enhanced by bilateral presentations (simultaneous stimuli in both visual half-fields) compared to unilateral presentations (stimuli in only one half-field on a trial). We have reported the same phenomenon, however, using visual spatial patterns in a search paradigm (E. W. Yund, R. Efron, & D. R. Nichols, 1990~. Brain and Cognition, 12, 117-127) and have accounted for it in terms of the operating characteristics of a visual scanning mechanism which serially examines a decaying neural representation of the stimuli. In the present experiment we attempted to exploit these operating characteristics to influence this difference between unilateral and bilateral presentations. The results not only are consistent with the assumptions of the scanning hypothesis but they also provide new information pertinent to the operating characteristics of this mechanism. o IWO Academic Press, Inc.
INTRODUCTION
A large number of studies of the accuracy of visual identification, recall, or detection of briefly presented stimuli in neurologically normal subjects have documented a performance superiority in the right visual field (RVF). These observations coupled with the differences in the cognitive deficits associated with right- and left-sided cerebral lesions, have led many investigators to interpret the right/left asymmetrical performance in normal subjects in terms of hemispheric differences in processing capacity, resources, or styles. A number of studies involving recognition of linguistic stimuli (Hines, 1975, 1976; Kershner & Jeng, 1972; McKeever & Huling, 1971a,b) have shown that the RVF performance superiority is more marked when words are delivered to both Address reprint requests to R. Efron, VA Medical Center, 150 Muir Road, Martinez, CA 94553. This work was supported by Research Service, Veterans Administration. 128 0278-2626190$3.00 Copyright All rights
Q 1990 by Academic Press, Inc. of reproduction in any form reserved.
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visual fields simultaneously (bilateral presentation) than is the case when they are delivered to either right or left field (unilateral presentations). It has been suggested (Hines, 1975; McKeever & Huling, 1971a,b) that this difference between unilateral and bilateral presentations may be related to a “competition” for access to the language center in the left hemisphere: Linguistic information presented to the RVF is believed to reach the center with greater “signal fidelity” than identical information presented to the left visual field (LVF), which reaches the left hemisphere in a degraded form as a result of transfer via the corpus callosum. This would cause a weak RVF superiority in unilateral presentations. In a bilateral presentation, however, the degraded signal from the LVF would have to “compete” for access to the right hemisphere language center with higher fidelity signals from the RVF, and this is believed to account for the much stronger RVF superiority in this presentation mode. Our own interest in the difference in the magnitude of the RVF superiority between unilateral and bilateral presentations derives from the previous report (Yund, Efron, & Nichols, 1990~) in which the same effect was observed but with nonlinguistic stimuli-the pattern stimuli used in this series of reports (Efron et al., 1987, 1990a,b; Yund et al., 1990a,b,c). We accounted for the more marked RVF superiority in bilateral than unilateral presentations in terms of a scanning hypothesis (see Efron et al, 1990a) which neither is based on hemispheric processing differences nor does it require the assumption of loss of “signal fidelity” as a consequence of callosal transfer of information. Briefly, the scanning hypothesis accounts for this result in terms of a tendency, across multiple trials, for the scan to examine stimuli in the RVF earlier than those in the LVF-it tends to go to the RVF first. Since the scan is examining a rapidly degrading neural representation of the stimuli, the more time which elapses before it reaches the location occupied by the target the less the information there to support a correct discrimination between the target and a nontarget pattern and the greater the probability of a detection error. However, since the scan tends to go right first the presence of stimuli in the LVF will have little effect on target detectability in the RVF but the presence of stimuli in the RVF will have a more marked effect on target detection in the LVF than would be the case if there were no stimuli in the RVF (see Yund et al., 1990~ for a more detailed explanation). The object of the present experiments was to answer two questions: First, given that bilateral presentations yield larger RVF superiority than unilateral presentations, will any visual stimulus in the contralateral field produce the same enhancement of the RVF detection superiority, or will some type of stimuli in the contralateral field be more effective than others in enhancing the RVF detection superiority? Second, do different types of stimuli in the contralateral field change the shape of the de-
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tectability gradients in the reference field, or do they merely increase (or decrease) the probability of detection at all locations in the reference field equally? The rationale for the design of the present experiments is most easily described by reference to Figs. 1 (Experiment I) and 2 (Experiment II). In each experiment there are four configurations of stimuli (A-D) which were presented to the subject in random order. It will be observed that every condition contains six different patterns, arranged in an arc equidistant from the fovea. The target (the vertical stripe pattern), if present on a trial, was always in that arc. Its location, on the trials in which it was present, was uncertain (randomized). The subject’s task on each trial was to report whether the target was or was not present in this arc. In two configurations (A and B) there were IZOstimuli in the contralateral field, while in two configurations (C and D) six additional stimuli, which we will refer to generically as “distracters”, were present in the contralateral field. As can be seen by comparing Figs. 1 and 2 the only difference between Experiments I and II is the type of distractor which was used in the field contralateral to the target and nontarget patterns in configurations C and D. The results obtained with configurations A and B are the unilateral presentations in which nothing is present in the contralateral field. The detectability at each of the six locations in each field obtained with configurations A and B can then be compared with the detectability at the same location when distracters were present in the contralateral field as in configurations C and D, i.e., the bilateral presentations. We will refer to the distracters of Experiment I as “FILL” stimuli and those of Experiment II as “DOT” stimuli. The FILL stimuli were used because of their overall luminance was exactly the same as that of the patterns, but when viewed 4.6” from the fovea they appear featureless, i.e., without any discernible internal structure or pattern. The DOT stimuli of Experiment II had half the average luminance of the target and nontarget patterns, and when viewed 4.6” from the fovea, are not featureless. Which of the two types of stimuli (if either) when present in the contralateral field will cause changes in detectability in the reference field? If both do, will there be any differences between the two types of stimuli in the magnitude or pattern of target detectability at the six locations in the reference field? Three plausible predictions can be made: On the one hand, it could be argued that the FILL stimuli, which have twice the luminance of the DOT stimuli, would be harder to ignore and would be expected to exhibit a greater effect on the detectability of the target in the reference field than the DOT stimuli. On the other hand, it could be argued that the DOT stimuli will have a greater effect on detectability in the reference field because they have an internal, pattern-like structure, and this struc-
VISUAL DETECTABILITY
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ture contains both vertical and horizontal spatial frequency components-the vertical component having the same orientation as the target pattern. Finally, it might be predicted that there will be essentially no difference between Experiment I and Experiment II, because it should be a trivial task with either type of stimulus in configurations C and D to merely detect one FILL or DOT stimulus in a field, and then to examine the other field, where the subject knew the target had to be located if present at all. The outcome of these experiments has not been provided in the Abstract to give the reader an opportunity to choose between these predictions-or to make another. A third experiment (Experiment III) was performed in which the FILL and DOT stimuli were not all located in one visual field but were randomly mixed with the patterns. This experiment on a different subject population will be described later. EXPERIMENTS
I AND II
(A) Methods The visual stimuli were presented on the display screen of a computer (Attache, Otrona) which was used as a tachistoscope. Both experiments were performed under identical conditions of ambient lighting. The screen background was 4. I cd/m”, while the illuminated parts of the screen were 73.7 cd/m*. The centers of all patterns were 4.6” from the point of visual fixation and subtended a visual angle of 1.3” on a side. Head position was maintained with a chin and forehead rest and, to minimize as much as possible the effects of sensitivity difference between nasal and temporal retinal regions, the screen was viewed binocularly. Each trial of both experiments consisted of three sequential parts: (I) a fixation routine, a procedure used to ensure that the subject’s fovea was directed at the center of the screen prior to the presentation of the pattern stimuli (see Efron et al., l990a for details); (2) the main task, a brief presentation of one of the four configurations (illustrated in Fig. 1 or 2; and (3) a response routine used to determine whether the subject detected the target on that trial (see Yund et al., l990b for details). The main task consisted of a random presentation for 50 msec of one of the four configurations in Fig. I or 2. The subject was required to report whether the vertical stripe pattern, the “target,” was present in that presentation. On 25% (randomly) of the 512 trials, the target was not present and was replaced by another pattern. The six nontarget patterns which were used are illustrated in configuration A of Fig. 1. On those trials in which the vertical striped target was present, one of these nontarget patterns (randomly) was replaced by the target pattern. The location of the target on the 384 trials (when it was present) was randomized differently for each subject with the constraint that the target appeared n times at every location before it appeared n + I times at any location, for a total of 16 times at each location in each configuration. The locations of the nontarget patterns were also randomized for each trial for each subject to counterbalance for any unique interactions which might have been present between the target and particular adjacent pattern. After the 50-msec exposure of the patterns, the screen was left blank for 333 msec, and then the words “YES OR NO” were displayed as a prompt to obtain the response. (This long interval between the stimuli and the response prompt was used to avoid possible interference in the processing of the stimuli.) The subject was required to press the up arrow on the keyboard if he saw the target or the down arrow if he did not see the target. Upon entering the response, the screen was erased and the next trial
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CONFIG. A
CONFIG. C
CONFIG. B
CONFIG. D
FIG. 1. Contrast-reversed photo~aph from computer screen of the four configurations (A-D) presented randomly in Experiment I. Ail patterns are 4.6’ from the point of visual fixation and subtended a 1.3” visual angle on a side. began with the fixation routine. As two previous studies (Efron et al., 1!?9Ob;Yund et al., 1990~)have shown that the hand used for the fixation and response routines had no influence on the results, the subjects were permitted to use their preferred hand. Prior to the start of each experiment the subjects were shown the stimuli that would be seen (either Fig. I or Fig. 2). They were info~ed that the four con~gurat~ons woutd be presented randomly but that their task was the same regardless of which configuration was presented, i.e., to report the presence or absence of the target in the arc of “patterns.” They were told that the target pattern, if present on a trial, would be in one of the six locations containing “patterns” and to ignore the distrusting stimuli on those triais when they were present. This verbal description of the experiment was followed by 23 practice trials, starting with a long exposure duration (383 msec),of the main task and decreasing on each practice trial so that the last 11 were performed at a SO-msecduration. Except
VISUAL
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CONFIG. B
CONFIG. A
ka m E E E
...... ...... :::::: ...... 111111 ...... ...... ..._. .._.. .. ..._...
:::::: .. .. ..._. ... .. ..._.. ....
...... ...... ...... ...... ...... ......
CONFIG. C
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...... ...... ...... ...... ...... ......
......_ .. ....__ .._ _..... . ....._.. ..-... .._ ...._._ .... ......_ .. .-.... ......
.. .. .. .. .. .. .. .. .. .. .. .. ::::::
...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ......
X
r H ;
111111
CONFIG. D
FIG. 2. Contrast-reversed photographs of the four configurations presented in Experiment II. Same visual angles for stimuli as in legend to Fig. I.
randomly
for the feedback which was included only for the practice trials, these last 11 practice trials were the same as those of the main experiment. Naive subjects were recruited by advertisement, were paid for their services, and did not know the goal of the experiments other than the fact that the purpose was to obtain normative data on the accuracy of visual pattern perception. Each experiment was performed on 80 right-handed subjects equally divided by sex. Half the subjects of each sex performed Experiment I before Experiment II, and the other 40 subjects performed the two experiments in the reverse order. The mean age of the subjects for Experiments I and II was 23.2 2 5.9 years. The results of each experiment on a subject were computer scored for the probability of a correct detection at each of the six possible target locations in each configuration and the probability of false alarm, i.e., reporting the target to be present when it wasn’t. For the 32 trials of each configuration in which the target was not present, false alarm prob-
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abilities were low: 0.083 f 0.009 for the eight configurations (four each in Experiment I and Experiment II).
An analysis of variance was performed on the number of correct detections as a function of the five independent variables: (1) Configuration (C), distracters present or absent; (2) Field (F), right or left; (3) Position (P), numbered 1 to 6 from top to bottom in each field; (4) Distractor type (D), FILL or DOT; and (5) Sex (S). All significant main effects and first order interactions are presented in Table 1. Three of the five independent variables (configuration, field, and position) exhibited highly significant main effects and interactions. The distractor type variable (D) produced no significant main effect, but it should be emphasized that what might be considered to be the main effect of D would appear as a D x C interaction in this analysis because the distracters were present on only half the trials in each experiment. There was no significant main effect of sex or any interactions involving gender. The highly significant main effects of configuration, field, and position as well as the interactions among them were anticipated from the results of our previous experiments (Efron et al., 1990a,b; Yund et al., 1990a,b,c) and are of interest only in so far as they confirm our previous findings. They reveal the existence of differences in detectability as a function of target location, i.e., that there is a detectability gradient, that there is a right visual field detection superiority, and that the shape of the detectability gradient differed in the two fields. The primary purpose of the present experiment, however, was to determine how these expected effects and inte~~tions would vary, if at all, with the two types of distracters that were used. The nature of these interactions involving distractor types will be described with the aid of a graphic presentation of the data (Fig. 3). Figures 3A and 3B contain all the results of Experiments I and II, TABLE 1 ANALYSIS
Source of Variance Configuration (C) Field (F) Position (P) Distracters (D) CxF CXP FxP DxP DxC
OF VARIANCE
df
F
P
l/78 l/78 5/390 l/78 l/78 5/390 s/390 51390 l/78
25.27 31.98 22.54 2.81 13.95 6.34 10.30 13.77 14.59
<.ooo5 <.0005
<.OOl <.ooo5
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.90 8ls a
b .8560 -
IO
Exp I -FILL
NO DISTRACTORS D&TRACTORS
c
95 -
90 -
85 80-
b
75 70 -
Exp II o--o
NO DISTRACTORS
-
DOT DISTRACTORS
IO-
95 -
w-
.85 8075 .701
FULL
1 1 IL
2L
1 1 1 1 3L
4L
5L 6L
CIRCLE
I IR
I
I
OF
PATTERNS
1 I
2R 3R 4R
I
5R 6R
LOCATIONS FIG. 3. Results of Experiment I (A) and Experiment II (B). See Discussion for C. The ordinate represents the probability of correct detection. The abscissa represents target location numbered 1-6 from top to bottom of each field. The letter (R or L) after the number indicates the visual field.
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respectively. Each data point represents the mean probability of correct detection (on the ordinate). The abscissa represents the location number of the target from top to bottom of the arc of patterns, with the letter L or R following that number to indicate the field. The two curves in each field in each panel represent the results when distracters were not present in the contralateral field (open circles) and when distracters were present (solid circles). The three main effects (configuration, field, and position) are evident: (1) Target detectabilities in the distractor absent configuration (unfilled circles) are almost always superior to that of the distracters present configuration; (2) The probability of correct detection is predominately superior in the RVF; and (3) Detectability at locations near the top of each field are superior to those near the bottom. The C x F interaction (p < .OOl): When distracters were present in the RVF, the degradation of detectability in the LVF was more marked than was the degradation of detectability in the RVF when distracters were present in the LVF; the two curves in the left half of Figs. 3A and 3B show greater vertical separation than those on the right. ‘The D x C interaction (p < .OOl): The DOT stimuli produced a larger detectability degradation in the reference field (compared to the no distractor condition) than was the case with FILL stimuli. This effect is evident in the greater vertical separation of the two curves in each field in Fig. 3B compared to Fig. 3A. The D x P interaction (p < .0005): DOT stimuli produced their maximal detectability degradation at positions 1 and 6, while FILL stimuli produced their minimal effects at these positions. Figure 3C, containing data from a previous experiment (Experiment I in Yund et al., 1990a), will not be considered until the Discussion of Experiments I and II. (C) Discussion
(Experiments
Z and ZZ)
There were four major findings in the present experiment: (1) There was a gradient of detectability within each field as a function of the spatial locations of the target. (2) When no distracters were present (the unilateral presentations of configurations A and B in Experiments I and II) overall target detectability was superior in the right arc. (3) The presence of distracters in the opposite field degraded detectability in the reference field with DOT stimuli producing distinctly more degradation than FILL stimuli. (4) The degrading effect on detectability induced by the presence of distracters in the opposite field was more marked when the distracters were in the RVF than was the case when they were in the LVF. This had the net effect of increasing the overall RVF detection superiority in the bilateral presentation conditions (configurations C and D in Experiments I and II). How are these findings to be explained? Except for the differential effect of DOT and FILL stimuli-the main phenomena to which the present experiments are directed-all these
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results represent a confirmation of those found in the previous report (Yund et al., 1990~) and are explained in the same way: The scanning mechanism has a tendency to process stimuli in the RVF earlier than those in the LVF-it goes to the right first. When an arc of patterns is in the left field, the scan might be expected to lose at least some time in the empty RVF before examining the LVF. If the arc of patterns was in the right field, however, then it would not lose this time. Thus, when the overall target detection performance of the two arc conditions is compared, a weak RVF detection superiority would be expected-as was the case in the previous experiment and for the unilateral presentations (configurations A and B) of the present experiments. However, if there are stimuli in both visual fields-the bilateral presentation (configuration C and D) of the present experiments-then, when the scan goes to the RVF first, it does find stimuli there and it must determine whether each of them is the target. Since this consumes more time than would have been the case had there been no stimuli in the RVF, target detection in the LVF is relatively worse in the bilateral than unilateral conditionsas is seen in the results. We now consider the more marked degrading effect on overall target detection in the reference field by the presence of DOT than by FILL stimuli in the opposite field-the only new finding in Experiments I and II. We assume that if a stimulus shares one or more spatial frequency components with the target it will take the scanning mechanism longer to “decide” that it is not the target than would be the case if it did not share this feature. DOT stimuli, which share a vertical spatial frequency with the target, would be expected to consume more time at each decision point than FILL stimuli and thus would be expected to produce more degradation of overall target detectability in the reference field. However, since the scan tends to examine the stimuli in the RVF earlier than those in the LVF, the presence of DOT (and to a lesser degree FILL) stimuli in the RVF will have more of a degrading effect on LVF target detectability than would the presence of these distractor stimuli in the LVF have on the RVF performance-as can be seen in the results. Thus, the tendency of the scan to examine RVF stimuli earlier than LVF ones will also produce a greater degrading effect of the distracters on LVF than RVF target detection in the present experiments. The results of a previous study (Experiment I in Yund et al., 1990a) are of particular interest with respect to this issue. In that experiment, which employed the same methods as in the present case, one of the configurations used contained 1 target and 11 nontarget patterns at the identical retinal locations as in the present experiment. The nontarget patterns in the field contralateral to the one containing the target can be considered to be “distracters” also and to be terminologically consistent they will be called PATTERN stimuli. In the design of that experiment,
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performed on a different subject population, only a bilateral presentation was employed. For this reason, the results of that experiment, illustrated in Fig. 3C, contain only one curve in each field, and we cannot determine the magnitude of the degradation of detectability which was produced by the presence of PATTERN stimuli in the field opposite to the one containing the target. However, if DOT stimuli produce more detectability degradation than FILL stimuli because they share a vertical frequency component with the target, then the shape of the detectability gradient with PATTERN stimuli, some of which had vertical spatial frequency components, should resemble that of the DOT stimulus gradients more than that obtained with FILL stimuli. The detectability gradients in the left field when FILL, DOT, and PATTERN stimuli are present are seen in Figs. 3A, 3B, and 3C, respectively (curves with solid circles). In Fig. 3A the detectability falls between locations 1L and 3L and then is essentially the same at locations 4L, 5L, and 6L. The lowest detectability is at 3L. In Fig. 3B there is a similar decrease from IL to 3L, a “recovery” at 4L, and then a precipitous decrease from 4L to 6L. In Fig 3C the pattern is very similar to that in Fig. 3B except that the decrease in detectability at 3L is much less pronounced. Thus there appears to be some type of progression in the shape of the detectability gradients from FILL to DOT to PATTERN stimuli. In the RVF there is only a hint of a similar progression, a result which would be expected since the presence of distracters in the LVF field had much less degrading effect on RVF detectability than the reverse. Parenthetically, the D x P interaction of the present experiment in which DOT stimuli produced their maximal detectability degradation at positions 1 and 6, while FILL stimuli produced their minimal effect at these positions, is further evidence that the effects of DOT stimuli are similar to PATTERN stimuli: In Yund et al. (1990~) the same effect is seen in the configuration by position interaction when PATTERNS were in the field opposite to the one containing the target. EXPERIMENT
III
In the course of this series of reports on the scanning hypothesis we have used several different methods to measure target detectability at multiple locations within the visual field and to correct the data for any response bias. Given this variety of methods and bias-correction procedures, there is necessarily some uncertainty concerning the extent to which some of our conclusions might be method-dependent. To resolve this uncertainty, Experiment III was designed so that a number of the major effects that we have observed in previous experiments using different methods should be evident in a single experiment. In particular, we wanted to confirm (a) the inverse relationship of overall detectability with the number of nontarget patterns present (see Efron et al., 1990a),
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(b) the differential effects of FILL and DOT stimuli (Experiments I and II of the present report), and (c) our inference that a BLANK stimulusdefined as the absence of a stimulus in a location which contained a target or a nontarget pattern on other trials-is ignored completely (skipped over) by the scan or is treated as a very easy nontarget pattern (Efron et al., 1990a). (A) Method The method used was identical to that in Efron et al. (1990a) in which the response routine, which appeared on the screen 500 msec after the termination of the 16.67-msec exposure to the patterns, consisted of the presentation of a single empty box in a location in which a pattern (target or nontarget) had been presented. The subject was required to indicate whether or not the target pattern had been present at this prompted location. The subjects were informed prior to the experiment that if a target had been present on a trial it would have been at this prompted location, and that a “yes” response was required (see op tit for details). With this method hit and false alarm probabilities can be obtained for each possible location of the target. The patterns were in the same 12 locations as in Experiments I and II and were of the same size. The experiment consisted of 1176 trials in which the target was present in 598. There were seven randomly presented conditions: (1) 12 patterns [12P], (2) 8 patterns with 4 randomly located BLANK stimuli [8P + 4B], (3) 8 patterns with 4 randomly located FILL stimuli [8P + 4F], (4) 8 patterns with 4 randomly located DOT stimuli [8P + 4D], (5) 4 patterns with 8 randomly located BLANK stimuli [4P + 8B], (6) 4 patterns with 8 randomly located FILL stimuli [4P + 8F], and (7) 4 patterns with 8 randomly located DOT stimuli [4P + 8D]. At the conclusion of the experiment the prompt had been at each of the 12 possible locations of the target in each condition 14 times and the target had been present in 7 of these occasions. Forty-four right-handed subjects, equally divided by sex, who had not participated in any previous experiments were recruited by advertisement and paid for their services. Their mean age was 22.7 + 5.3.
(B) Results
The raw data consists of the number of hits and the number of false alarms for each subject at each location under each condition. Since the number of possible hits and false alarms was too small to compute a d’ for each condition and location for each subject, we used the a’ statistic and performed the analysis of variance using the following transformation from McNicol (1972): 2 x arcsin ($?). These transformed a’ values range from 0 to 3.1416 (m) and are roughly comparable to d’ values. In keeping with the purpose of Experiment III, only the principal variables of condition and field were included in the analysis of variance. The main effect of field was highly significant (p < .OOOS) and in the expected direction: Detection was superior in the RVF (mean transformed a’ of 2.59 VS. 2.42 for the LVF). The main effect of condition (p < .OOOS)is illustrated in Fig. 4 where target detection is plotted for each condition. The major effects in this experiment are the same as those reported nreviouslv using different methods and thus nrovide reassurance that
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-a
2.70
0 bf
2.60
8
%
2.50
z E
2.40
J
2.30 12P
8P+4D
8P+4F
8P+4B
4P+8D
4P+8F
4Pt8B
CONDITION FIG. 4. Results of Experiment III. The seven conditions of this experiment are listed on the abscissa (See Methods, Experiment III for nomenclature). The ordinate is the a’ measure of target detectability. In Experiment III, the BLANK, FILL, and DOT stimuli were randomly distributed among the patterns.
our conclusions are not method-dependent. More specifically, the inverse relationship between target detectability and the number of patterns present is evident by comparison of conditions 12P, 8P + 4B, and 4P + 8B in Fig. 4: Target detectability progressively increased across these three conditions. The same effect, albeit less marked, is observed with FILL stimuli by comparing conditions 12P, 8P + 4F, and 4P + 8F. For DOT stimuli, as would be expected from the results of Experiments I and II, the increase in detectability from 12P to 8P + 4D to 4P + 8D was less marked than for FILL stimuli. In our report describing the effect of randomly presented BLANK stimuli (Efron et al., 1990a) we concluded that a BLANK (defined as the absence of a stimulus in a location where a stimulus was present on many trials) was either “skipped over” entirely by the scan or was processed as a very easy nontarget pattern. The present results suggest the second possibility is the more likely: If BLANKS were skipped over entirely then we would expect that the performance in the 8P + 4B condition would have been appreciably better than in the 8P + 4D and the 8P + 4F conditions-but the differences are only small ones and there is no obvious discontinuity between the 8P + 4D, 8P + 4F, and 8P + 4B conditions. A similar nondiscontinuous relationship of increasing performance is seen in the 4P + 8D, 4P + 8F, and 4P + 8B conditions. Thus, the results of Experiments I, II, and III indicate that BLANKS are processed faster
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than FILL stimuli which in turn are processed faster than DOT stimuli and that DOT stimuli are processed faster than the (average) pattern. CONCLUSIONS
The primary object of this series of investigations (Efron et al., 1987, 1990a,b; Yund et al., 1990a,b,c) has been to understand how we find a specific object in a visual field containing many similar objects when the exposure to the visual scene is too brief to permit a series of eye movements. The scanning hypothesis, presented in larval form in the first of these reports (Efron et al., 1987) and expanded in the third (Efron et al., 1990a), was developed to account for the striking differences in target detectability as a function of its retinal locus. This hypothesis has two fundamental assumptions: That the neural representation of the visual image is serially processed, i.e., scanned, and that the information in that neural representation decays rapidly over time. Since neither of these assumptions is inherently asymmetrical, the scanning hypothesis is no more concerned with right/left, than upper/lower, or any other arbitrarily selected axis of the two-dimensional perceptual surface-the entire visual field. Within the context of the scanning hypothesis itself, there is no reason to divide this surface into halves, quarters, or eighths! However, in the course of our investigations it became apparent that while target detection was superior in the RVF the within-jield differences in target detectability were often more marked than the overall right/left difference. Further experiments (Yund et al., 1990a) revealed that the within-field detectability differences could not be explained either on the basis of the known differences in retinal spatial resolution or known properties of lateral masking-but they could be explained on the basis of a serial process which examines the’\degrading information at various locations within the field in some probabilistic order. The overall RVF detection superiority could be explain&l similarly-a tendency for the scan to examine stimuli in the RVF somewhat earlier than those in the LVF. We recognized, as had White (1973),,$thatthe existence of such a scanning mechanism would have major im&cations for the field of laterality research since the most fundamental premise of that field is that right/left performance differences arise from hemispheric differences, albeit modified or modulated by other task and stimulus variables. Indeed, this series of papers was submitted to this particular journal to alert those interested in hemispheric differences that the scanning hypothesis not only offers an alternative explanation for performance differences between visual fields but also accounts for the within-field differences at the same time. Since the scanning hypothesis contains no assumptions which predict the probabilistic scanning order in which a subject or a group of subjects will examine the stimuli which may be present in the visual field, a
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number of our experiments have been directed to discovering how the scan is influenced by the number ofstimuli present (Efron et al., 1990a), the spatial con$gurution of these stimuli (Efron et al., 1990b; Yund et al., 199Oc),and, in the present report, the type of stimuli which are used. All the results of these studies are consistent with the scanning hypothesis. Although the assumptions of the scanning hypothesis are equally applicable to an attentional or a nonattentional scan, two of our experiments (Yund et al., 199Ob;Efron et al, 1990b) tested the hypothesis that the scan under investigation consists of a sequence of attentional shiftsthe hypothesis originally proposed by Heron (1957). Despite the fact that the results of those studies were consistent with a nonattentional scan and failed to provide evidence for an attentional process, we do not consider this issue resolved at the present time. That we have observed the same kinds of within- and between-field detection differences with visual-spatial patterns that Heron found for the recognition of letters indicates strongly that the scan is not uniquely related to the particular visual stimuli which are used but is a mechanism which is critically important in enabling us to find an object in the presence of similar objects. REFERENCES Efron, R., Yund, E. W., & Nichols, D. R. 1987. Scanning the visual field without eye movements-A sex difference. Neuropsychologia, 25, 637-644. Efron, R., Yund, E. W., & Nichols, D. R. 1990a. Serial processing of visual spatial patterns in a search paradigm. Brain and Cognition, 12, 17-41. Efron, R., Yund, E. W., & Nichols, D. R. 1990b. Detectability as a function of target location: Effects of spatial configuration. Brain and Cognition, 12, 102-l 16. Heron, W. 1957. Perception as a function of retinal locus and attention. American Journal
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Hines, D. 1975. Independent functioning of the two cerebral hemispheres for recognizing bilaterally presented tachistoscopic visual-half-field stimuli. Cortex, 11, 132-143. Hines, D. 1976. Recognition of verbs, abstract nouns and concrete nouns from the left and right visual half-fields. Neuropsychologia, 14, 211-216. Kershner, J. R., & Jeng, A. G. 1972. Dual functional hemispheric asymmetry in visual perception: Effects of ocular dominance and postexposural processes. Neuropsychologia, 10, 437-445. McKeever, W. F., & Huling, M. D. 1971a. Lateral dominance in tachistoscopic word recognition performances obtained with simultaneous bilateral input. Neuropsychologia, 9, 15-20. McKeever, W. F., & Huling, M. D. 1971b. Bilateral tachistoscopic word recognition as a function of hemisphere stimulated and interhemispheric transfer time. Neuropsychologia, 9, 281-288. McNicol, D. 1972. A Primer of Signal Detection Theory. London: George, Allen & Unwin Ltd. White, M. J. 1973. Does cerebral dominance offer a sufficient explanation for laterality differences in tachistoscopic recognition. Perceptual and Motor Skills, 36, 479-485. Yund, E. W., Efron, R., & Nichols, D. R. 1990a. Detectability gradients as a function of target location. Bruin and Cognition, 12, 1-16.
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Yund, E. W., Efron, R., & Nichols, D. R. 1!49Ob.Detectability gradients as a function of spatial location: Effects of selective attention. Brain and Cognition, 12, 42-54. Yund, E. W., Efron, R., & Nichols, D. R. 1990~. Target detection in one visual field in the presence or absence of stimuli in the contralateral field by right- and left-handed subjects. Brain and Cognition, 12, 117-127.