The Quarterly Journal of Experimental Psychology Section A
ISSN: 0272-4987 (Print) 1464-0740 (Online) Journal homepage: http://www.tandfonline.com/loi/pqja20
Attentional Modulation of Size Contrast Gordon L. Shulman To cite this article: Gordon L. Shulman (1992) Attentional Modulation of Size Contrast, The Quarterly Journal of Experimental Psychology Section A, 45:4, 529-546, DOI: 10.1080/14640749208401332 To link to this article: http://dx.doi.org/10.1080/14640749208401332
Published online: 29 May 2007.
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THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 1992,45A (4) 529-546
Attentional Modulation of Size Contrast Gordon L. Shulman
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Washington University School of Medicine, St. Louis, Missouri, U.S.A. A test circle surrounded by smaller context circles appears larger if presented in isolation, whereas a test circle surrounded by large context circles is seen as smaller than in isolation. Two experiments are reported indicating that this phenomenon, the Ebbinghaus illusion, depends on whether subjects are attending to the context circles. Subjects first saw a reference circle and then a briefly presented (150msec) test circle. Their task was to determine whether the test circle was larger or smaller than the reference. The test circle was surrounded by smaller context circles of one colour arrayed along a horizontal axis centred on the test, and larger context circles of a different colour arrayed along a vertical axis centred on the test. Subjects judged both the size of the test and the colours of either the small or large context circles. Perceived test size changed systematically, depending on which context circles were task-relevant.
The perception of a stimulus can be greatly affected by a neighbouring stimulus. Assimilation and contrast are common examples of stimulusstimulus interactions that have important implications for theories of shape and colour. Assimilation refers to situations in which differences between a test and context stimulus are lessened, and contrast refers to situations in which test-context differences are exaggerated. Coren and Girgus (1978) have noted that many well known geometrical illusions, such as the Mueller-Lyer, Delbouef, Ponzo, and Ebbinghaus, can be interpreted as reflecting in part assimilation and/or contrast. A number of studies furthermore suggest that the effect of the context elements in some of these illusions depends on whether they are attended (Coren & Girgus, 1972; Coren & Porac, 1983; Pressey, 1974a; Restle, 1971; Tsal, 1984). If true, this would indicate that some of the perceptual
Requests for reprints should be sent to Gordon L. Shulman, Department of Neurology and Neurological Surgery, Box 8111, Washington University School of Medicine, 660 S. Euclid, St. Louis, MO 63110, U S A . This work was supported by the Office of Naval Research, Contract N-00014-89-J-1426, and the McDonnell Center for Higher Brain Function.
0 1992 The Experimental Psychology Society
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mechanisms that produce assimilation or contrast in these displays can be modulated by attention. Pressey (1974a), for example, has shown that illusions produced by the Ponzo configuration and two modified Mueller-Lyer illusions differ in magnitude depending on the placement of the comparison line used for indicating the length of the test object. Pressey has argued in a number of papers (Pressey, 1971; Pressey, 1974a, 1974b) that context elements only affect test perception if they fall within an “attentive field”, which is centred at the mid-point of a line joining the two most extreme elements of the stimulus configuration that must be explicitly judged. The location of the comparison line in Pressey’s (1974a) study determined the placement of the observer’s attentive field and therefore changed the relative effectiveness of different parts of the contextual stimulus in generating an illusion. Coren and Girgus (1972) showed that the magnitude of the MuellerLyer illusion was reduced when subjects were instructed to ignore the contextual arrows. More recently, both Coren and Porac (1983) and Tsal (1984) have shown that the direction of the Mueller-Lyer illusion (overor under-estimation of the test line) could be changed by directing the subject’s attention to the outward- or inward-pointing fins of a composite or ambiguous Mueller-Lyer figure. Although these studies suggest attentional influences on some geometric illusions, they have methodological problems. (1) The judgements were made under conditions of free viewing. The stimuli were present for long durations, and subjects were free to move their eyes as they wished. Subjects presumably made eye movements to different parts of the figure, and these eye movements might well differ according to the attention instructions. This aspect of the methodology makes it difficult to attribute effects in these experiments unambiguously to attention. (2) There was often no objective manipulation for controlling the direction of attention. In some studies (Coren & Girgus, 1972; Coren & Porac, 1983; Tsal, 1984), for example, subjects were simply instructed to concentrate on one or another part of the stimulus configuration. The present study was conducted to correct these problems. Although several studies have investigated the Mueller-Lyer figure, which is thought to involve assimilative processes (Coren & Girgus, 1978), the current study examined the Ebbinghaus illusion, which is typically thought to reflect contrast (Figure 1). The stimulus configuration of the Ebbinghaus illusion is well suited to attentional manipulations as the context elements are physically separated from the test figure. This separation makes it easier to control attention to the context elements, independently of attention to the test figure.
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FIG. 1. An illustration of size contrast (the Ebbinghaus illusion). A circle surrounded by small context circles appears larger than a circle surrounded by large context circles.
EXPERIMENT 1 Method Subjects. Eight naive observers participated in Experiment 1. Apparatus and Stimuli. Stimuli were displayed on a colour monitor controlled by an Amiga 10oO microcomputer. On each trial subjects saw a reference circle and then a test and context circles.' The basic task was to determine whether the test circle was smaller or larger than the reference circle. The reference and test circles were white with a luminance of 43.9 cd/m2. The diameter of the reference circle was 23.5 mm (1.5");the diameter of the test circle depended on condition (see below). There were two pairs of context circles: one pair, with diameters of 7.8 mm (OS"), flanked the test circle on the horizontal axis, the other pair, with diameters of 39.1 mm (2.5"),flanked the test circle on the vertical axis. One pair of context circles was red, the other green, with the assignment of colours to sizes counterbalanced across observers. The separation between the outer edge of the test circle and the outer edge of each context circle was 5.9 mm (0.4'). 'Because of limitations in screen resolution (640 x 200 pixels) in the vertical dimension, the circles showed slight irregularities at the vertical poles.
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Two slightly different red hues were used. One of the red hues was produced by stimulating only the red phosphor of the monitor, the other by adding some stimulation from the blue phosphor (the amount from the blue phosphor was adjusted for each subject, as described below). On half the trials, both of the red circles were the same red hue (either “red” or “red + blue”); on the other half, the two circles were of different hues. Two green hues were also used, one produced by stimulating only the green phosphor of the monitor, the other by adding some stimulation from the blue phosphor. Again, on half the trials, both of the green circles were the same hue (either “green” or “green + blue”), and on the other half the two circles were different hues. According to the manufacturer, the chromaticity coordinates of the red phosphor are x = 0.625, y = 0.340, the coordinates of the green phosphor are x = 0.310, y = 0.592, and the coordinates of the blue phosphor are x = 0.150, y = 0.063. The luminance of the red circles was 35.2 d m ’, the luminance of the green circles was 37.5 cd/m2. The addition of the blue phosphor increased the luminance of the red and green circles by 1.0 to 1.8 cd/m*. Depending on condition, subjects had to determine whether one of the other context pair was the same or different hue. To perform this task, a knowledge of the hue of only one circle provides no information concerning the proper response, as the other circle might be the same or different hue with equal probability. The hue difference yielding a performance of roughly 70-80% correct was determined in an initial practice session, described in the Procedure section. There were four types of blocks: baseline, single-alone, dual-alone, and dual-opponent (Figure 2). In all blocks, subjects saw the reference circle followed by the test circle, and then they made a size judgement. The blocks differed depending on the number of context circles presented and whether subjects had to make a d o u r judgement in addition to the size judgement. Baseline Blocks: On these blocks subjects saw the test circle without any context circles. There were four test circle diameters. Let zero designate the perceived size of the reference circle (the test diameter at which the subject would report the test as smaller on 50% of the trials). The four test sizes were -3, - 1 , 1 , 3 , where each unit refers to a change in diameter of 0.39 mm from the “zero” diameter, and negative units refer to test diameters smaller than the “zero” diameter. The “zero” diameter was estimated separately for each subject in an initial practice session. Single-Alone Blocks: On these blocks subjects saw the test circle with only one of the context pairs (either the large or small pair). These blocks therefore correspond to the normal procedure for measuring size and contrast effects. There were two conditions, involving presentation of the large or small context circles. When the large context circles were pre-
ATTENTIONAL MODULATION OF SIZE CONTRAST Baseline
Reference Frame
0
Single Alone
Dual Alone
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0
533 Dual
QQonent
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0 Response
Size?
Size?
Size?
cdor?
Sire? CdOR
FIG. 2. The four conditions of Experiment 1. The single-alone and dual-alone conditions are shown for trials involving the small context circles.
sented, the four test sizes were -1, 1, 3, and 5; when the small context circles were presented, the test sizes were -5, -3, -1, and 1. The different test series were needed to prevent floor and ceiling effects because of the large changes in the perception of test size produced by the context circles. Although subjects did not make a colour judgement in this block, the context circles were presented in the same colours, with matching or mismatching hues on half the trials, as in the dual-alone and opponent blocks described as follows. Dual-Alone Blocks: The stimulus display on these blocks was identical to the single-alone blocks. Subjects saw a test circle with only one of the context pairs. However, following the size judgement, subjects also made the colour judgement. The test sizes were the same as for single-alone blocks ( - 1 , 1 , 3 , and 5 for the large context circles, and -5, -3, -1, and 1 for the small context circles). There were two dual-alone blocks, corresponding to the two context circle sizes. Dual-Opponent Blocks: Subjects were presented with the test circle and both pairs of context circles. On some blocks, subjects made the test size judgement and then the colour judgement on the small context circles; on other blocks, they made the test size judgement and then the colour judgement on the large circles. The test sizes for both opponent conditions were the same as for the baseline blocks (-3, -1, 1, and 3). The hues of both the red and green context circles matched on 50% of the trials in a block and mismatched on the other 50%, irrespective of whether the colour
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judgement in that block was performed on the red or green circles. For example, the red context circles matched on half the trials and mismatched on the other half, irrespective of whether the colour judgement was performed on the red context circles or the green context circles. The physical displays in the two opponent conditions were therefore identical. The conditions only differed in the response that was required to the context circles. Procedure. A fixation cross was presented for 500 msec. Following a 1-sec interstimulus interval (IS), the reference circle was presented for 1 sec. After a 500-msec ISI, the test and context circles were presented for 150 msec. Subjects first made the size judgement, pressing one key if the test circle was smaller than the reference, a second key if larger. They then made the colour judgement, pressing one key if the context circles were the same colour, a second key if they were a different colour. The next trial was presented following a 1500-msec intertrial interval. Initial Session: The test sizes and context hues were established in an initial practice session. Subjects first received a baseline block with 5 test sizes and then a second block with 4 test sizes to determine an appropriate range of test sizes. Successive test sizes in a series involved the minimum changes in test diameter possible with the present display. From these initial baseline blocks, two test sizes were found that straddled the 50% point: i.e. one size yielded “smaller” test judgements on fewer than 50% of the trials; the next smaller test size yielded “smaller” judgements on more than 50% of the trials. The test diameter midway between these two sizes was defined as the “zero” point. Subjects next received two practice blocks on the colour task in which the first reference frame was eliminated and subjects only made the colour judgement on the test frame. Only the relevant set of context circles was presented. Subjects then received a dual-alone block in which they made both the colour and size judgement, followed by two dual-opponent blocks. On the basis of the data from each block, slight adjustments were made to the blue component of the hue in the succeeding block (increasing it for percent correct values below 70% and decreasing it for percent correct values above 80%). The size of the blue component needed to produce roughly 70-80% correct judgements was similar across subjects, and it was generally straightforward to arrive at a suitable pair of hues. Sometimes the data from these blocks also indicated that the range of test sizes estimated on the basis of the original baseline blocks needed to be adjusted.
Design. There were 7 conditions (1 baseline, 2 single-alone, 2 dualalone, 2 dual-opponent). As the dual-opponent conditions provided the key data for testing the effects of attention on size contrast, they were
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repeated twice. The baseline was also repeated, producing a total of 10 conditions (2 baseline, 2 single-alone , 2 dual-alone , 4 dual-opponent). The order of these 10 conditions was randomized, and subjects went through two orders for a total of 20 blocks. Each block consisted of 32 trials, eight at each test size.
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Results Figure 3 shows for each condition (averaged across subjects) the percentage of trials on which the test circle was reported as smaller than the reference circle. The figure indicates that for both the single-alone and dual-alone conditions large contrast effects were found. When the context circles were small, subjects gave fewer “smaller” responses than when the context circles were large. Contrast effects were also seen in the two dual-opponent conditions, and the sign of the effect was appropriate to the size of the task-relevant context pair. When subjects detected colour changes in the large context pair, they judged the test circle as being smaller than when they detected colour changes in the small context pair. These assertions are supported by the following statistical analyses.
Single-Alone, Dual-Alone. An analysis of variance was conducted with task (single-alone, dual-alone), context circle size (small, large), and test size as factors. The analysis yielded no significant effects other than
loOl + Single small -m-
Dualsmall
+ Opponent small _._.. Baseline
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--*--. Dual large - - -0-- - Single large 0 ‘
. -1.95
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FIG.3. Percentage of trials on which the test circle was judged smaller than the reference circle in Experiment 1.
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test circle size, F(3, 21) = 193, p < 0.001. The absence of any effect of task indicates that adding the colour judgement did not significantly affect the contrast phenomenon. Note that the contrast effect will not be demonstrated in this analysis, as the four test sizes in the small context condition are different from those in the large context condition. The same analysis on the two test circle sizes common to both context conditions yielded significant effects of test size, F(1, 7) = 28.8, p = 0.001, and context circle size, F(1, 7) = 510.4, p < 0.001, reflecting a significant contrast effect. There was again no effect of task.
Dual-Opponent. An analysis of variance with attended context size (small, large) and test circle size as factors yielded significant effects of attended context size, F(1, 7) = 14.33, p < 0.01, and test circle size, F(1, 7) = 166.6, p < 0.001, with a context x test size interaction, F(1, 7) = 3.99, p < 0.05. The main effect of attended context size shows that the perceived size of the test circle depended on which context circle was attended. The baseline condition, however, did not significantly differ from either the small [F(l, 7) = 4.49, p = 0.071 or large [F(1, 7) = 2.16, p > 0.11 opponent conditions. The functions in Figure 3 were converted to z-scores and regression lines were fitted to the resulting functions.* For each function, the test size at which the z-score equalled zero yielded the point of subjective equality (PSE) with the reference circle. These PSEs were subtracted from the baseline PSE to show the overall magnitude of the size shifts in the different contrast conditions (Table 1). The opponent conditions yielded contrast effects roughly 30% of those found in the dual-alone conditions. The slopes of the regression lines correspond to the discriminability of the different test sizes in each condition. The change in test size that produces a change in response from 50% “shorter” to 75% “shorter” was calculated and is shown in Table 2. One might expect that size discriminability would be higher in the single task conditions, and this seems to hold in the means. However, the interaction of test size by condition for the single-alone and dual-alone conditions was not significant (i.e. the singlealone and dual-alone functions in Figure 3 are statistically parallel), indicating no reliable difference in discriminability.
’The statistical analyses were conducted on the probability scores. Another approach would be to fit regression lines and derive PSEs for each condition for each subject and conduct analyses on these PSE scores. Unfortunately, because of the limitations on screen resolution noted earlier, some subjects in some conditions showed either floor (OYOsmaller responses) or ceiling (loOD/, smaller responses) effects. Rather than assign z-scores to these values by convention, it was decided to take a more conservative approach and conduct the analyses on the probability scores.
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TABLE 1
Shifts in the Perceived Diameter of the Test Circle Relative to the Baseline Condition ~~
Separation = 3.0"
Separation = 0.4" Small Context
Large Context
Contrast Effect
1 Single Dual-Alone Opponent
1.o 1.18 0.34
-0.59 -0.75 -0.27
1.59 1.93 0.61
2 Dual-Alone Opponent
1.03 0.06
- 1.04 -0.57
2.07 0.63
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Experiment
Small Context
Large Context
Contrast Effect
0.32 -0.75
-1.66 -1.12
1.98 0.37
Diameters of test circles in mm. Note: Negative quantities refer to shifts to sizes smaller than the baseline.
TABLE 2 Changes in Diameter Needed to Shift Test Responses from 50% Smaller to 75% Smaller
Separation = 3.0"
Separation = 0.4" Small Context
Large Context
1 Baseline Single Dual-Alone Opponent
0.52 0.65 0.59
0.53 0.65 0.65
2 Baseline Dual-Alone Opponent
0.52 0.64
0.74 0.60
Experiment
Small Context
Large Context
0.54 0.66
0.48 0.61
0.48
0.49
Changes in diameter in mm.
Discussion The results indicate that size contrast can be modulated by attention. The two opponent conditions were physically identical. Depending on whether subjects detected colour changes in the large or small context circles, however, the test circle was seen as smaller or larger. Because of the brief duration of the test display, these attentional effects cannot be mediated by eye movements.
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The opponent functions did not significantly differ, however, from the baseline. In an opponent design, the most sensitive measure of attentional effects comes from directly comparing the two opponent conditions rather than comparing each to a baseline. The effect produced by one context stimulus is weakened by the presence of the other, making it more difficult to see departures from a baseline (see Shulman, 1991,1992,for a discussion of this issue). Direct comparisons of two opponent conditions provide unambiguous evidence of attentional modulation, as the stimuli and even the overall tasks in the two conditions are identical. However, this comparison does not allow one to determine separately whether attention modulated the effect of the small or large context circles. The mean contrast effect was greater in the dual-alone conditions than in the single-alone conditions (Table 1), but this difference was not significant. (Although the probability functions for the two conditions in Figure 3 look very similar, they differ in the extreme values. The z-score transform used to compute the contrast effects in Table 1 weight these extreme values.) As subjects were required to process the context circles in the dual-alone condition explicitly but not the single-alone condition, one might expect larger context effects in the former condition. This comparison, however, is a weaker test of the attentional dependence of the illusion than the comparison of the two dual-opponent conditions. The comparison of dual-alone and single-alone conditions is similar to that of a distractor design in which the effect of a context element is compared when attention is directed to that element or to a neutral element (Shulman, 1991). Opponent designs in studies of perceptual after-effects have been shown to have twice the efficiency of distractor designs (Shulman, 1991). The difference in the contrast effect between the dual-alone and single-alone conditions was 0.34, slightly more than half the contrast effect in the dual-opponent conditions (0.61). The size of the contrast effect in the opponent conditions was roughly 30% of those in the dual-alone conditions, suggesting that the irrelevant context circles still had a substantial effect on test perception. This effect may indicate that part of the contrast effect is automatic or that subjects partly attended to the irrelevant context. The context circles were close to the test circle, and in order to attend to the relevant circles without attending to the irrelevant ones subjects would need to spread their attention in a precise fashion along either a horizontal or vertical axis. This may not be possible. It is also possible that the relatively small magnitude of the attentional modulation resulted from the experimental design. Suppose that subjects tend to distribute their responses so that the frequency of “smaller” judgements matches the frequency of “larger” judgements. As the test sizes in the opponent conditions were identical, any tendency to distribute
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responses equally would pull the opponent functions of Figure 3 together, reducing the contrast effect. This effect would not occur in the single-alone or dual-alone conditions as different test sizes were used for the small and large context conditions. A tendency to distribute responses would actually produce a contrast effect by default in these conditions. It was for this reason that identical test sizes were used for both opponent conditions. Experiment 2 was primarily a replication of Experiment 1, as well as an attempt to increase the attentional effects on contrast by making it easier to attend to one context pair vs. the other. In Experiment 2 the separation between the context and test circles was manipulated. If spreading attention along, say, the horizontal axis does not necessarily increase the "width" of the selected area, then it might be easier to attend to a particular context pair at a large context-test separation. Girgus, Coren, and Agdern (1972) found that the contrast effect was unchanged by increasing the distance between the test and context circles, but at large separations the test circle was seen as smaller in all context conditions. In Experiment 1, the small context circles were always along the horizontal axis, the large context circles along the vertical axis. Although it seems unlikely that this was responsible for the observed effects, in Experiment 2 the axes were reversed.
EXPERIMENT 2 Method Eight subjects participated in Experiment 2. Subjects were given baseline, dual-alone and dual-opponent conditions. For the latter two conditions, the test-context separation could either be 5.9 mm (0.4"), as in Experiment 1, or 46.9 mm (3.0"). There were 4 dual-alone conditions (the factorial combination of the two context sizes and two test-context separations), 4 dual-opponent conditions (the factorial combination of the two attended context sizes and two test-context separations), and 1 baseline condition, for a total of 9 conditions. Subjects received three random orderings of these 9 conditions, for a total of 27 blocks. The test sizes for the small separation conditions were the same as in Experiment 1: -3, -1, 1, and 3 for the baseline and opponent conditions, -1, 1, 3, and 5 for the large-context dual-alone condition, and -5, -3, -1, and 1 for the small-context dual-alone condition. The test sizes for the large separation conditions were all increased by two units to compensate for the overall decrease in perceived test size at the larger context-test separation (Girgus et al., 1972). For example, the test sizes for both opponent conditions at the large separation were -1, 1, 3, and 5.
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Initiul Session: The procedure was generally the same as in Experiment 1. Following baseline blocks, the colour task was initially performed without the size task for two blocks. Because of the larger number of conditions, practice on combining the colour and size task was confined to the opponent conditions. One opponent block was given for each of the 4 conditions defined by the factorial combination of relevant context circle (small, large) and context-test separation (small, large).
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Results The percentage of trials on which the test circle was reported as smaller than the reference is shown in Figure 4 €orthe small context-test separation and in Figure 5 for the large separation. The PSEs for the different conditions are shown in Table 1. The dual-alone conditions replicate the results of Girgus et al. (1972). The contrast effect was the same at both circle-test separations, but the test circle was uniformly seen as smaller with the larger separation than for the corresponding condition with the smaller separation. The opponent conditions at both separations show an attentional effect, but this effect is, if anything, reduced at the larger separation. Although the magnitudes of the overall contrast effects and attentional modulation are very similar to those in Experiment 1, the opponent condiTest context separation = .4 deg 1001
Dual small Opponent small
Baseline Opponent large Dual large
-1.95
-1.17
-.39
39
1.17
1.95
Test circle size(mm) FIG. 4. Percentage of trials on which the test circle was judged smaller than the reference circle-mnall test-context separation conditions of Experiment 2.
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+ Dual small --t
c
In 0
c
Opponent small Opponent large
---0---
"
Dual large
. -1.17
-.39
.39
1.17
1.95
2.73
Test circle size(mfn) FIG. 5. Percentage of trials on which the test circle was judged smaller than the reference circle-large test-context separation conditions of Experiment 2. Note that the values on the x-axis have been shifted to larger test sizes.
tions do not shift symmetrically with respect to the baseline. The smallcircle opponent condition is similar to baseline, whereas the large-circle condition shifts in the expected direction. Duaf-Alone: One of the test sizes was common to all four of the dualalone conditions. An ANOVA on the percentage of "smaller" test responses at this size with the context circle size (0.5" or 2.5") and contexttest separation (0.4" or 3") as factors yielded significant main effects of separation, F(1, 7) = 2 0 . 7 5 , ~< 0.005, and context size, F(1, 7) = 98.42, p < 0.001, with no interaction. The absence of an interaction confirms Girgus et al.'s (1972) report that contrast effects are independent of context-test separation, and the main effect of separation also confirms their finding that the test circle is perceived as smaller at larger test-context separations. Opponent: Three of the four test sizes were common to all four opponent conditions. An ANOVA with test size, attended context circle size, and context-test separation as factors yielded significant main effects of separation, F(1, 7) = 9.82, p < 0.05, test size, F(1, 7) = 114.7, p < 0.001, and attended size, F(1, 7) = 19.91, p < 0.005. The latter main effect indicates that perceived test size depended on which of the context
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circles was attended. This effect of attention did not interact with contexttest separation, indicating that the attentional modulation of the contrast effect was the same at both separations. The baseline condition differed from the large, F(1, 7) = 5.7, p = 0.05, but not the small opponent conditions at the small separation.
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Discussion The perceived size of the test circle systematically changed as subjects attended to the small or large context circle. This effect held for both test-context separations. Although it was felt that larger separations might permit subjects to attend more selectively to one or the other context pair, the data did not support this hypothesis. The results from Experiment 2, however, do strongly replicate the basic attentional effect on contrast observed in Experiment 1. The opponent conditions did not shift the perceived test size symmetrically from baseline, as in Experiment 1. Rather, the test size was perceived as equal in the small opponent and baseline conditions and smaller in the large opponent condition. The reason for this asymmetry is unclear. Perhaps large stimuli grab attention more effectively than small stimuli. Given that the baseline did not differ significantly from either opponent condition in Experiment 1, the asymmetry observed in Experiment 2 may not be reliable. The attentional effect did not statistically differ at the two context-test separations. As one reviewer of this paper noted, this result, as well as the additivity of the overall contrast effect with test-context separation, may indicate that selection of the context circles was mediated by non-spatial features such as colour or size. Additionally, the colour task, while explicitly requiring judgements of only colour, apparently led to the preferential coding of the size of the relevant context circles, consistent with object-based accounts of selection (Duncan, 1984). Selection of a colour or size feature may therefore have enabled access to a representation that contains information about many features of the object. The effect of context-test separation replicated the results reported by Girgus et al. (1972). The contrast effect was independent of separation, but the test circle was uniformly perceived as smaller in conditions with the large context-test separation relative to the same conditions with the small separation. This latter result suggested to Girgus et al. (1972) that the Ebbinghaus illusion is actually a variant of the Delbouef illusion. If two concentric circles are presented, the perceived size of the inner circle depends systematically on the separation between the two. At small separations, the inner circle is perceived as larger-an example of assimilationbut at large diameters of the outer circle the inner circle is perceived as
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smaller, an example of contrast. Girgus et al. suggested that the test-context configurations at large separations in the Ebbinghaus illusion is similar to a Delbouef stimulus in which the outer circle has a large diameter. Girgus and Coren (1982) noted that even at small separations the Ebbinghaus illusion can be interpreted in terms of the Delbouef illusion. When the context circles are small, the test is surrounded by a neighbouring outer “ring” that produces assimilation; when the context circles are large, the ring is further away and produces contrast. Studies of illusion decrement by Girgus and Coren (1982) support this analysis. Girgus et al. (1972) conclude that the Ebbinghaus illusion may reflect multiple mechanisms. Coren and Porac (1978) have attributed the shift from assimilation to contrast in a variety of illusions to attentional factors. They propose that when a test and context stimulus are attended together in a single “glance”, assimilation occurs, but when test and context are attended in separate “glances”, contrast occurs. They noted a number of situations in which assimilation occurs for small spatial separations between a test and context element, whereas contrast occurs at large separations (such as the Delbouef illusion). Jordan and Schiano (1986), for example, have shown in studies of the parallel line illusion that assimilation occurs for small test-context spatial separations and contrast occurs at large separations. Separation in time also produces contrast. Brigell and Uhlarik (1979) and Jordan and Uhlarik (1985) report that simultaneous presentation of the test and context line in the parallel line illusion produces assimilation, but sequential presentation produces contrast. One difficulty with tests of the pool and store model, however, is that attention has not been manipulated separately from stimulus factors. As test-context separation increases, one changes both the likelihood that the two stimuli will be jointly attended and the likelihood or type of sensory interactions that will occur between them. The shift from assimilation to contrast may result from either factor. Some phenomena that depend on the distance between a test and context element, for example-such as lateral contour masking-are attributed to sensory rather than attentional factors (although attentional factors could logically play a role here as well). A clean test of the pool and store model therefore requires that attention is manipulated independently of stimulus variables. Attentional explanations of the effects of spatial variables in the above studies also assume that selection is mediated spatially rather than by other features. Although this may be reasonable in the cases noted above, it should not necessarily be assumed. The results of Experiments 1 and 2 might cast doubt on the pool and store model as contrast effects were found even though the test and context stimuli were presented briefly and subjects were required to process both. One might object, however, that different “glances” of the test and context
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figures were still mediated by rapid attention shifts, particularly as the stimuli were not masked. Additionally, if the context circles were selected by size or colour, perhaps they should not be considered as occurring within the same “glance” as the test stimulus.
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GENERAL DISCUSSION In both experiments, the perception of the size of the test object depended on which pair of context circles was task-relevant. When the small context pair was relevant, the test circle was seen as larger than when the large context pair was relevant. These shifts in perceived size occurred even though the physical display was unchanged and display duration was too brief to permit eye movements. Contrast and assimilation phenomena suggest that perceptual judgements are based on relative quantities. At some point in the evaluation of the size of an object, it is compared to other objects in the field. The present experiments suggest that a selection mechanism weights the degree to which contextual objects are entered in this comparison process. The present results do not detail the properties of this selection mechanism. As noted earlier, however, selection in the current experiments may have been mediated by non-spatial features that allowed access to an objectdefined representation. Coren (1971) has presented results suggesting that the context-test comparison occurs at a fairly late stage of perceptual processing. He presented a test circle surrounded by context circles of identical size. By stereoscopically changing the depth of the context circles, he was able to change their perceived size through size/distance scaling. Correspondingly, the test circle appeared smaller or larger, depending on the perceived size of the context circles. This result suggests that the selection process operates on representations that have undergone size/distance scaling. Recent work by Coren and Enns (1991) also indicates that the illusion varies with the conceptual similarity (i.e. the degree to which the test and context elements are from semantically similar categories) of the test and context elements. In a series of papers, Gogel and colleagues (Gogel & McCracken, 1979; Gogel & Sharkey, 1989; Gogel & Tietz, 1976) have shown that attention influences a comparison process in the perception of motion. His work indicates that attention affects a number of induced motion phenomena in which the perceived direction of motion of a test element depends on the direction of motion of context elements. As with the present work, Gogel finds that attention weights the degree to which contextual elements influence a comparison process. It is possible that selective attention plays an important role generally in processes that compare target and context stimuli to form representations emphasizing relational quantities.
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Effects of attention on size perception have also been reported by Epstein and Broota (1986), who find that size constancy requires attention. Size judgements of attended objects reflected objective size, and judgements of unattended objects reflected projective size. Some theories of size constancy have suggested a role for relative size judgements or size comparisons between the judged object and some contextual stimulus. Although size comparisons may help maintain constancy in some situations, however, theories based solely on relative size have been criticized (Rock, 1983), and size constancy is typically thought to require the integration of retinal size and distance information.
REFERENCES Brigell, M., & Uhlarik, J. (1979). The relational determination of length illusions and length aftereffects. Perception, 8, 187-197. Coren, S. (1971). A size contrast illusion without physical size differences. American Journal of Psychology, 84, 565-566. Coren, S., & Ems, J. (1991). Size contrast as a function of conceptual similarity. Meeting of the Psychonomic Society, November, 1991. Coren, S., & Girgus, J. (1972). Differentiation and decrement in the Mueller-Lyer illusion. Perception & Psychophysics, 12, 4-70. Coren, S . , & Girgus, J. (1978). Seeing is deceiving: The psychology of visual illusions. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc. Coren, S., & Porac, C. (1983). The creation and reversal of the Mueller-Lyer illusion through attentional manipulation. Perception, 12, 49-54. Duncan, J. (1984). Selective attention and the organization of visual information. Journal of Experimental Psychology: General, 113, 501-517. Epstein, W., & Broota, K. (1986). Automatic and attentional components in perception of size-at-a-distance. Perception & Psychophysics, 40, 256-262. Girgus, J., & Coren, S. (1982). Assimilation and contrast illusions: Differences in plasticity. Perception & Psychophysics, 32, 555-561. Girgus, J., Coren, S., & Agdern, M. (1972). The interrelationship between the Ebbinghaus and Delbouef illusions. Journal of Experimental Psychology, 95, 453-455. Gogel, W., & McCracken, P. (1979). Depth adjacency and induced motion. Perceptual and Motor Skills, 48, 343-350. Gogel, W., & Sharkey, T. (1989). Measuring attention using induced motion. Perception, 18, 303-320. Gogel, W., & Tietz, J. (1976). Adjacency and attention as determiners of perceived motion. Vision Research, 16, 839-845. Jordan, K., & Schiano, D. (1986). Serial processing and the parallel-lines illusion: Length contrast through relative spatial separation of contours. Perception & Psychophysics, 40, 384-390. Jordan, K., & Uhlarik, J. (1985). Assimilation and contrast of perceived length depend on temporal factors. Perception & Psychophysics, 37, 447454. Pressey, A. (1971). An extension of assimilation theory to illusions of size, area, and direction. Perception & Psychophysics, 9, 172-176. Pressey, A. (1987a). Evidence for the role of attentive fields in the perception of illusions. Quarterly Journal of Experimental Psychology, 26, 464-471.
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Pressey, A. (1974b). Effect of size of angle on the ambiguous Mueller-Lyer illusion. Acta Psychologica, 38, 401-404. Restle, F. (1971). Instructions and the magnitude of an illusion: Cognitive factors in the frame of reference. Perception & Psychophysics, 9, 31-32. Rock, I. (1983). The logic of perception. Cambridge: MIT Press. Shulman, G. (1991). Attentional modulation of mechanisms that analyze rotation in depth. Journal of Experimental Psychology: Human Percpetion and Performance, 17,126-137. Shulman, G . (1992). Attentional modulation of a figural aftereffect. Perception. Tsal, Y. (1984). A Mueller-Lyer illusion induced by selective attention. Quarterly Journal of Experimental Psychology, 36A, 319-333.
Revised manuscript received 25 May I992
ISSN: 0272-4987 (Print) 1464-0740 (Online) Journal homepage: http://www.tandfonline.com/loi/pqja20
Attentional Modulation of Size Contrast Gordon L. Shulman To cite this article: Gordon L. Shulman (1992) Attentional Modulation of Size Contrast, The Quarterly Journal of Experimental Psychology Section A, 45:4, 529-546, DOI: 10.1080/14640749208401332 To link to this article: http://dx.doi.org/10.1080/14640749208401332
Published online: 29 May 2007.
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THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 1992,45A (4) 529-546
Attentional Modulation of Size Contrast Gordon L. Shulman
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Washington University School of Medicine, St. Louis, Missouri, U.S.A. A test circle surrounded by smaller context circles appears larger if presented in isolation, whereas a test circle surrounded by large context circles is seen as smaller than in isolation. Two experiments are reported indicating that this phenomenon, the Ebbinghaus illusion, depends on whether subjects are attending to the context circles. Subjects first saw a reference circle and then a briefly presented (150msec) test circle. Their task was to determine whether the test circle was larger or smaller than the reference. The test circle was surrounded by smaller context circles of one colour arrayed along a horizontal axis centred on the test, and larger context circles of a different colour arrayed along a vertical axis centred on the test. Subjects judged both the size of the test and the colours of either the small or large context circles. Perceived test size changed systematically, depending on which context circles were task-relevant.
The perception of a stimulus can be greatly affected by a neighbouring stimulus. Assimilation and contrast are common examples of stimulusstimulus interactions that have important implications for theories of shape and colour. Assimilation refers to situations in which differences between a test and context stimulus are lessened, and contrast refers to situations in which test-context differences are exaggerated. Coren and Girgus (1978) have noted that many well known geometrical illusions, such as the Mueller-Lyer, Delbouef, Ponzo, and Ebbinghaus, can be interpreted as reflecting in part assimilation and/or contrast. A number of studies furthermore suggest that the effect of the context elements in some of these illusions depends on whether they are attended (Coren & Girgus, 1972; Coren & Porac, 1983; Pressey, 1974a; Restle, 1971; Tsal, 1984). If true, this would indicate that some of the perceptual
Requests for reprints should be sent to Gordon L. Shulman, Department of Neurology and Neurological Surgery, Box 8111, Washington University School of Medicine, 660 S. Euclid, St. Louis, MO 63110, U S A . This work was supported by the Office of Naval Research, Contract N-00014-89-J-1426, and the McDonnell Center for Higher Brain Function.
0 1992 The Experimental Psychology Society
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mechanisms that produce assimilation or contrast in these displays can be modulated by attention. Pressey (1974a), for example, has shown that illusions produced by the Ponzo configuration and two modified Mueller-Lyer illusions differ in magnitude depending on the placement of the comparison line used for indicating the length of the test object. Pressey has argued in a number of papers (Pressey, 1971; Pressey, 1974a, 1974b) that context elements only affect test perception if they fall within an “attentive field”, which is centred at the mid-point of a line joining the two most extreme elements of the stimulus configuration that must be explicitly judged. The location of the comparison line in Pressey’s (1974a) study determined the placement of the observer’s attentive field and therefore changed the relative effectiveness of different parts of the contextual stimulus in generating an illusion. Coren and Girgus (1972) showed that the magnitude of the MuellerLyer illusion was reduced when subjects were instructed to ignore the contextual arrows. More recently, both Coren and Porac (1983) and Tsal (1984) have shown that the direction of the Mueller-Lyer illusion (overor under-estimation of the test line) could be changed by directing the subject’s attention to the outward- or inward-pointing fins of a composite or ambiguous Mueller-Lyer figure. Although these studies suggest attentional influences on some geometric illusions, they have methodological problems. (1) The judgements were made under conditions of free viewing. The stimuli were present for long durations, and subjects were free to move their eyes as they wished. Subjects presumably made eye movements to different parts of the figure, and these eye movements might well differ according to the attention instructions. This aspect of the methodology makes it difficult to attribute effects in these experiments unambiguously to attention. (2) There was often no objective manipulation for controlling the direction of attention. In some studies (Coren & Girgus, 1972; Coren & Porac, 1983; Tsal, 1984), for example, subjects were simply instructed to concentrate on one or another part of the stimulus configuration. The present study was conducted to correct these problems. Although several studies have investigated the Mueller-Lyer figure, which is thought to involve assimilative processes (Coren & Girgus, 1978), the current study examined the Ebbinghaus illusion, which is typically thought to reflect contrast (Figure 1). The stimulus configuration of the Ebbinghaus illusion is well suited to attentional manipulations as the context elements are physically separated from the test figure. This separation makes it easier to control attention to the context elements, independently of attention to the test figure.
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OO: 0
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FIG. 1. An illustration of size contrast (the Ebbinghaus illusion). A circle surrounded by small context circles appears larger than a circle surrounded by large context circles.
EXPERIMENT 1 Method Subjects. Eight naive observers participated in Experiment 1. Apparatus and Stimuli. Stimuli were displayed on a colour monitor controlled by an Amiga 10oO microcomputer. On each trial subjects saw a reference circle and then a test and context circles.' The basic task was to determine whether the test circle was smaller or larger than the reference circle. The reference and test circles were white with a luminance of 43.9 cd/m2. The diameter of the reference circle was 23.5 mm (1.5");the diameter of the test circle depended on condition (see below). There were two pairs of context circles: one pair, with diameters of 7.8 mm (OS"), flanked the test circle on the horizontal axis, the other pair, with diameters of 39.1 mm (2.5"),flanked the test circle on the vertical axis. One pair of context circles was red, the other green, with the assignment of colours to sizes counterbalanced across observers. The separation between the outer edge of the test circle and the outer edge of each context circle was 5.9 mm (0.4'). 'Because of limitations in screen resolution (640 x 200 pixels) in the vertical dimension, the circles showed slight irregularities at the vertical poles.
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Two slightly different red hues were used. One of the red hues was produced by stimulating only the red phosphor of the monitor, the other by adding some stimulation from the blue phosphor (the amount from the blue phosphor was adjusted for each subject, as described below). On half the trials, both of the red circles were the same red hue (either “red” or “red + blue”); on the other half, the two circles were of different hues. Two green hues were also used, one produced by stimulating only the green phosphor of the monitor, the other by adding some stimulation from the blue phosphor. Again, on half the trials, both of the green circles were the same hue (either “green” or “green + blue”), and on the other half the two circles were different hues. According to the manufacturer, the chromaticity coordinates of the red phosphor are x = 0.625, y = 0.340, the coordinates of the green phosphor are x = 0.310, y = 0.592, and the coordinates of the blue phosphor are x = 0.150, y = 0.063. The luminance of the red circles was 35.2 d m ’, the luminance of the green circles was 37.5 cd/m2. The addition of the blue phosphor increased the luminance of the red and green circles by 1.0 to 1.8 cd/m*. Depending on condition, subjects had to determine whether one of the other context pair was the same or different hue. To perform this task, a knowledge of the hue of only one circle provides no information concerning the proper response, as the other circle might be the same or different hue with equal probability. The hue difference yielding a performance of roughly 70-80% correct was determined in an initial practice session, described in the Procedure section. There were four types of blocks: baseline, single-alone, dual-alone, and dual-opponent (Figure 2). In all blocks, subjects saw the reference circle followed by the test circle, and then they made a size judgement. The blocks differed depending on the number of context circles presented and whether subjects had to make a d o u r judgement in addition to the size judgement. Baseline Blocks: On these blocks subjects saw the test circle without any context circles. There were four test circle diameters. Let zero designate the perceived size of the reference circle (the test diameter at which the subject would report the test as smaller on 50% of the trials). The four test sizes were -3, - 1 , 1 , 3 , where each unit refers to a change in diameter of 0.39 mm from the “zero” diameter, and negative units refer to test diameters smaller than the “zero” diameter. The “zero” diameter was estimated separately for each subject in an initial practice session. Single-Alone Blocks: On these blocks subjects saw the test circle with only one of the context pairs (either the large or small pair). These blocks therefore correspond to the normal procedure for measuring size and contrast effects. There were two conditions, involving presentation of the large or small context circles. When the large context circles were pre-
ATTENTIONAL MODULATION OF SIZE CONTRAST Baseline
Reference Frame
0
Single Alone
Dual Alone
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QQonent
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0 Response
Size?
Size?
Size?
cdor?
Sire? CdOR
FIG. 2. The four conditions of Experiment 1. The single-alone and dual-alone conditions are shown for trials involving the small context circles.
sented, the four test sizes were -1, 1, 3, and 5; when the small context circles were presented, the test sizes were -5, -3, -1, and 1. The different test series were needed to prevent floor and ceiling effects because of the large changes in the perception of test size produced by the context circles. Although subjects did not make a colour judgement in this block, the context circles were presented in the same colours, with matching or mismatching hues on half the trials, as in the dual-alone and opponent blocks described as follows. Dual-Alone Blocks: The stimulus display on these blocks was identical to the single-alone blocks. Subjects saw a test circle with only one of the context pairs. However, following the size judgement, subjects also made the colour judgement. The test sizes were the same as for single-alone blocks ( - 1 , 1 , 3 , and 5 for the large context circles, and -5, -3, -1, and 1 for the small context circles). There were two dual-alone blocks, corresponding to the two context circle sizes. Dual-Opponent Blocks: Subjects were presented with the test circle and both pairs of context circles. On some blocks, subjects made the test size judgement and then the colour judgement on the small context circles; on other blocks, they made the test size judgement and then the colour judgement on the large circles. The test sizes for both opponent conditions were the same as for the baseline blocks (-3, -1, 1, and 3). The hues of both the red and green context circles matched on 50% of the trials in a block and mismatched on the other 50%, irrespective of whether the colour
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judgement in that block was performed on the red or green circles. For example, the red context circles matched on half the trials and mismatched on the other half, irrespective of whether the colour judgement was performed on the red context circles or the green context circles. The physical displays in the two opponent conditions were therefore identical. The conditions only differed in the response that was required to the context circles. Procedure. A fixation cross was presented for 500 msec. Following a 1-sec interstimulus interval (IS), the reference circle was presented for 1 sec. After a 500-msec ISI, the test and context circles were presented for 150 msec. Subjects first made the size judgement, pressing one key if the test circle was smaller than the reference, a second key if larger. They then made the colour judgement, pressing one key if the context circles were the same colour, a second key if they were a different colour. The next trial was presented following a 1500-msec intertrial interval. Initial Session: The test sizes and context hues were established in an initial practice session. Subjects first received a baseline block with 5 test sizes and then a second block with 4 test sizes to determine an appropriate range of test sizes. Successive test sizes in a series involved the minimum changes in test diameter possible with the present display. From these initial baseline blocks, two test sizes were found that straddled the 50% point: i.e. one size yielded “smaller” test judgements on fewer than 50% of the trials; the next smaller test size yielded “smaller” judgements on more than 50% of the trials. The test diameter midway between these two sizes was defined as the “zero” point. Subjects next received two practice blocks on the colour task in which the first reference frame was eliminated and subjects only made the colour judgement on the test frame. Only the relevant set of context circles was presented. Subjects then received a dual-alone block in which they made both the colour and size judgement, followed by two dual-opponent blocks. On the basis of the data from each block, slight adjustments were made to the blue component of the hue in the succeeding block (increasing it for percent correct values below 70% and decreasing it for percent correct values above 80%). The size of the blue component needed to produce roughly 70-80% correct judgements was similar across subjects, and it was generally straightforward to arrive at a suitable pair of hues. Sometimes the data from these blocks also indicated that the range of test sizes estimated on the basis of the original baseline blocks needed to be adjusted.
Design. There were 7 conditions (1 baseline, 2 single-alone, 2 dualalone, 2 dual-opponent). As the dual-opponent conditions provided the key data for testing the effects of attention on size contrast, they were
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repeated twice. The baseline was also repeated, producing a total of 10 conditions (2 baseline, 2 single-alone , 2 dual-alone , 4 dual-opponent). The order of these 10 conditions was randomized, and subjects went through two orders for a total of 20 blocks. Each block consisted of 32 trials, eight at each test size.
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Results Figure 3 shows for each condition (averaged across subjects) the percentage of trials on which the test circle was reported as smaller than the reference circle. The figure indicates that for both the single-alone and dual-alone conditions large contrast effects were found. When the context circles were small, subjects gave fewer “smaller” responses than when the context circles were large. Contrast effects were also seen in the two dual-opponent conditions, and the sign of the effect was appropriate to the size of the task-relevant context pair. When subjects detected colour changes in the large context pair, they judged the test circle as being smaller than when they detected colour changes in the small context pair. These assertions are supported by the following statistical analyses.
Single-Alone, Dual-Alone. An analysis of variance was conducted with task (single-alone, dual-alone), context circle size (small, large), and test size as factors. The analysis yielded no significant effects other than
loOl + Single small -m-
Dualsmall
+ Opponent small _._.. Baseline
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. -1.95
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-.39
39
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FIG.3. Percentage of trials on which the test circle was judged smaller than the reference circle in Experiment 1.
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test circle size, F(3, 21) = 193, p < 0.001. The absence of any effect of task indicates that adding the colour judgement did not significantly affect the contrast phenomenon. Note that the contrast effect will not be demonstrated in this analysis, as the four test sizes in the small context condition are different from those in the large context condition. The same analysis on the two test circle sizes common to both context conditions yielded significant effects of test size, F(1, 7) = 28.8, p = 0.001, and context circle size, F(1, 7) = 510.4, p < 0.001, reflecting a significant contrast effect. There was again no effect of task.
Dual-Opponent. An analysis of variance with attended context size (small, large) and test circle size as factors yielded significant effects of attended context size, F(1, 7) = 14.33, p < 0.01, and test circle size, F(1, 7) = 166.6, p < 0.001, with a context x test size interaction, F(1, 7) = 3.99, p < 0.05. The main effect of attended context size shows that the perceived size of the test circle depended on which context circle was attended. The baseline condition, however, did not significantly differ from either the small [F(l, 7) = 4.49, p = 0.071 or large [F(1, 7) = 2.16, p > 0.11 opponent conditions. The functions in Figure 3 were converted to z-scores and regression lines were fitted to the resulting functions.* For each function, the test size at which the z-score equalled zero yielded the point of subjective equality (PSE) with the reference circle. These PSEs were subtracted from the baseline PSE to show the overall magnitude of the size shifts in the different contrast conditions (Table 1). The opponent conditions yielded contrast effects roughly 30% of those found in the dual-alone conditions. The slopes of the regression lines correspond to the discriminability of the different test sizes in each condition. The change in test size that produces a change in response from 50% “shorter” to 75% “shorter” was calculated and is shown in Table 2. One might expect that size discriminability would be higher in the single task conditions, and this seems to hold in the means. However, the interaction of test size by condition for the single-alone and dual-alone conditions was not significant (i.e. the singlealone and dual-alone functions in Figure 3 are statistically parallel), indicating no reliable difference in discriminability.
’The statistical analyses were conducted on the probability scores. Another approach would be to fit regression lines and derive PSEs for each condition for each subject and conduct analyses on these PSE scores. Unfortunately, because of the limitations on screen resolution noted earlier, some subjects in some conditions showed either floor (OYOsmaller responses) or ceiling (loOD/, smaller responses) effects. Rather than assign z-scores to these values by convention, it was decided to take a more conservative approach and conduct the analyses on the probability scores.
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TABLE 1
Shifts in the Perceived Diameter of the Test Circle Relative to the Baseline Condition ~~
Separation = 3.0"
Separation = 0.4" Small Context
Large Context
Contrast Effect
1 Single Dual-Alone Opponent
1.o 1.18 0.34
-0.59 -0.75 -0.27
1.59 1.93 0.61
2 Dual-Alone Opponent
1.03 0.06
- 1.04 -0.57
2.07 0.63
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Experiment
Small Context
Large Context
Contrast Effect
0.32 -0.75
-1.66 -1.12
1.98 0.37
Diameters of test circles in mm. Note: Negative quantities refer to shifts to sizes smaller than the baseline.
TABLE 2 Changes in Diameter Needed to Shift Test Responses from 50% Smaller to 75% Smaller
Separation = 3.0"
Separation = 0.4" Small Context
Large Context
1 Baseline Single Dual-Alone Opponent
0.52 0.65 0.59
0.53 0.65 0.65
2 Baseline Dual-Alone Opponent
0.52 0.64
0.74 0.60
Experiment
Small Context
Large Context
0.54 0.66
0.48 0.61
0.48
0.49
Changes in diameter in mm.
Discussion The results indicate that size contrast can be modulated by attention. The two opponent conditions were physically identical. Depending on whether subjects detected colour changes in the large or small context circles, however, the test circle was seen as smaller or larger. Because of the brief duration of the test display, these attentional effects cannot be mediated by eye movements.
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The opponent functions did not significantly differ, however, from the baseline. In an opponent design, the most sensitive measure of attentional effects comes from directly comparing the two opponent conditions rather than comparing each to a baseline. The effect produced by one context stimulus is weakened by the presence of the other, making it more difficult to see departures from a baseline (see Shulman, 1991,1992,for a discussion of this issue). Direct comparisons of two opponent conditions provide unambiguous evidence of attentional modulation, as the stimuli and even the overall tasks in the two conditions are identical. However, this comparison does not allow one to determine separately whether attention modulated the effect of the small or large context circles. The mean contrast effect was greater in the dual-alone conditions than in the single-alone conditions (Table 1), but this difference was not significant. (Although the probability functions for the two conditions in Figure 3 look very similar, they differ in the extreme values. The z-score transform used to compute the contrast effects in Table 1 weight these extreme values.) As subjects were required to process the context circles in the dual-alone condition explicitly but not the single-alone condition, one might expect larger context effects in the former condition. This comparison, however, is a weaker test of the attentional dependence of the illusion than the comparison of the two dual-opponent conditions. The comparison of dual-alone and single-alone conditions is similar to that of a distractor design in which the effect of a context element is compared when attention is directed to that element or to a neutral element (Shulman, 1991). Opponent designs in studies of perceptual after-effects have been shown to have twice the efficiency of distractor designs (Shulman, 1991). The difference in the contrast effect between the dual-alone and single-alone conditions was 0.34, slightly more than half the contrast effect in the dual-opponent conditions (0.61). The size of the contrast effect in the opponent conditions was roughly 30% of those in the dual-alone conditions, suggesting that the irrelevant context circles still had a substantial effect on test perception. This effect may indicate that part of the contrast effect is automatic or that subjects partly attended to the irrelevant context. The context circles were close to the test circle, and in order to attend to the relevant circles without attending to the irrelevant ones subjects would need to spread their attention in a precise fashion along either a horizontal or vertical axis. This may not be possible. It is also possible that the relatively small magnitude of the attentional modulation resulted from the experimental design. Suppose that subjects tend to distribute their responses so that the frequency of “smaller” judgements matches the frequency of “larger” judgements. As the test sizes in the opponent conditions were identical, any tendency to distribute
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responses equally would pull the opponent functions of Figure 3 together, reducing the contrast effect. This effect would not occur in the single-alone or dual-alone conditions as different test sizes were used for the small and large context conditions. A tendency to distribute responses would actually produce a contrast effect by default in these conditions. It was for this reason that identical test sizes were used for both opponent conditions. Experiment 2 was primarily a replication of Experiment 1, as well as an attempt to increase the attentional effects on contrast by making it easier to attend to one context pair vs. the other. In Experiment 2 the separation between the context and test circles was manipulated. If spreading attention along, say, the horizontal axis does not necessarily increase the "width" of the selected area, then it might be easier to attend to a particular context pair at a large context-test separation. Girgus, Coren, and Agdern (1972) found that the contrast effect was unchanged by increasing the distance between the test and context circles, but at large separations the test circle was seen as smaller in all context conditions. In Experiment 1, the small context circles were always along the horizontal axis, the large context circles along the vertical axis. Although it seems unlikely that this was responsible for the observed effects, in Experiment 2 the axes were reversed.
EXPERIMENT 2 Method Eight subjects participated in Experiment 2. Subjects were given baseline, dual-alone and dual-opponent conditions. For the latter two conditions, the test-context separation could either be 5.9 mm (0.4"), as in Experiment 1, or 46.9 mm (3.0"). There were 4 dual-alone conditions (the factorial combination of the two context sizes and two test-context separations), 4 dual-opponent conditions (the factorial combination of the two attended context sizes and two test-context separations), and 1 baseline condition, for a total of 9 conditions. Subjects received three random orderings of these 9 conditions, for a total of 27 blocks. The test sizes for the small separation conditions were the same as in Experiment 1: -3, -1, 1, and 3 for the baseline and opponent conditions, -1, 1, 3, and 5 for the large-context dual-alone condition, and -5, -3, -1, and 1 for the small-context dual-alone condition. The test sizes for the large separation conditions were all increased by two units to compensate for the overall decrease in perceived test size at the larger context-test separation (Girgus et al., 1972). For example, the test sizes for both opponent conditions at the large separation were -1, 1, 3, and 5.
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Initiul Session: The procedure was generally the same as in Experiment 1. Following baseline blocks, the colour task was initially performed without the size task for two blocks. Because of the larger number of conditions, practice on combining the colour and size task was confined to the opponent conditions. One opponent block was given for each of the 4 conditions defined by the factorial combination of relevant context circle (small, large) and context-test separation (small, large).
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Results The percentage of trials on which the test circle was reported as smaller than the reference is shown in Figure 4 €orthe small context-test separation and in Figure 5 for the large separation. The PSEs for the different conditions are shown in Table 1. The dual-alone conditions replicate the results of Girgus et al. (1972). The contrast effect was the same at both circle-test separations, but the test circle was uniformly seen as smaller with the larger separation than for the corresponding condition with the smaller separation. The opponent conditions at both separations show an attentional effect, but this effect is, if anything, reduced at the larger separation. Although the magnitudes of the overall contrast effects and attentional modulation are very similar to those in Experiment 1, the opponent condiTest context separation = .4 deg 1001
Dual small Opponent small
Baseline Opponent large Dual large
-1.95
-1.17
-.39
39
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Test circle size(mm) FIG. 4. Percentage of trials on which the test circle was judged smaller than the reference circle-mnall test-context separation conditions of Experiment 2.
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Test context separation = 3.0 deg 1001
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+ Dual small --t
c
In 0
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Opponent small Opponent large
---0---
"
Dual large
. -1.17
-.39
.39
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2.73
Test circle size(mfn) FIG. 5. Percentage of trials on which the test circle was judged smaller than the reference circle-large test-context separation conditions of Experiment 2. Note that the values on the x-axis have been shifted to larger test sizes.
tions do not shift symmetrically with respect to the baseline. The smallcircle opponent condition is similar to baseline, whereas the large-circle condition shifts in the expected direction. Duaf-Alone: One of the test sizes was common to all four of the dualalone conditions. An ANOVA on the percentage of "smaller" test responses at this size with the context circle size (0.5" or 2.5") and contexttest separation (0.4" or 3") as factors yielded significant main effects of separation, F(1, 7) = 2 0 . 7 5 , ~< 0.005, and context size, F(1, 7) = 98.42, p < 0.001, with no interaction. The absence of an interaction confirms Girgus et al.'s (1972) report that contrast effects are independent of context-test separation, and the main effect of separation also confirms their finding that the test circle is perceived as smaller at larger test-context separations. Opponent: Three of the four test sizes were common to all four opponent conditions. An ANOVA with test size, attended context circle size, and context-test separation as factors yielded significant main effects of separation, F(1, 7) = 9.82, p < 0.05, test size, F(1, 7) = 114.7, p < 0.001, and attended size, F(1, 7) = 19.91, p < 0.005. The latter main effect indicates that perceived test size depended on which of the context
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circles was attended. This effect of attention did not interact with contexttest separation, indicating that the attentional modulation of the contrast effect was the same at both separations. The baseline condition differed from the large, F(1, 7) = 5.7, p = 0.05, but not the small opponent conditions at the small separation.
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Discussion The perceived size of the test circle systematically changed as subjects attended to the small or large context circle. This effect held for both test-context separations. Although it was felt that larger separations might permit subjects to attend more selectively to one or the other context pair, the data did not support this hypothesis. The results from Experiment 2, however, do strongly replicate the basic attentional effect on contrast observed in Experiment 1. The opponent conditions did not shift the perceived test size symmetrically from baseline, as in Experiment 1. Rather, the test size was perceived as equal in the small opponent and baseline conditions and smaller in the large opponent condition. The reason for this asymmetry is unclear. Perhaps large stimuli grab attention more effectively than small stimuli. Given that the baseline did not differ significantly from either opponent condition in Experiment 1, the asymmetry observed in Experiment 2 may not be reliable. The attentional effect did not statistically differ at the two context-test separations. As one reviewer of this paper noted, this result, as well as the additivity of the overall contrast effect with test-context separation, may indicate that selection of the context circles was mediated by non-spatial features such as colour or size. Additionally, the colour task, while explicitly requiring judgements of only colour, apparently led to the preferential coding of the size of the relevant context circles, consistent with object-based accounts of selection (Duncan, 1984). Selection of a colour or size feature may therefore have enabled access to a representation that contains information about many features of the object. The effect of context-test separation replicated the results reported by Girgus et al. (1972). The contrast effect was independent of separation, but the test circle was uniformly perceived as smaller in conditions with the large context-test separation relative to the same conditions with the small separation. This latter result suggested to Girgus et al. (1972) that the Ebbinghaus illusion is actually a variant of the Delbouef illusion. If two concentric circles are presented, the perceived size of the inner circle depends systematically on the separation between the two. At small separations, the inner circle is perceived as larger-an example of assimilationbut at large diameters of the outer circle the inner circle is perceived as
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smaller, an example of contrast. Girgus et al. suggested that the test-context configurations at large separations in the Ebbinghaus illusion is similar to a Delbouef stimulus in which the outer circle has a large diameter. Girgus and Coren (1982) noted that even at small separations the Ebbinghaus illusion can be interpreted in terms of the Delbouef illusion. When the context circles are small, the test is surrounded by a neighbouring outer “ring” that produces assimilation; when the context circles are large, the ring is further away and produces contrast. Studies of illusion decrement by Girgus and Coren (1982) support this analysis. Girgus et al. (1972) conclude that the Ebbinghaus illusion may reflect multiple mechanisms. Coren and Porac (1978) have attributed the shift from assimilation to contrast in a variety of illusions to attentional factors. They propose that when a test and context stimulus are attended together in a single “glance”, assimilation occurs, but when test and context are attended in separate “glances”, contrast occurs. They noted a number of situations in which assimilation occurs for small spatial separations between a test and context element, whereas contrast occurs at large separations (such as the Delbouef illusion). Jordan and Schiano (1986), for example, have shown in studies of the parallel line illusion that assimilation occurs for small test-context spatial separations and contrast occurs at large separations. Separation in time also produces contrast. Brigell and Uhlarik (1979) and Jordan and Uhlarik (1985) report that simultaneous presentation of the test and context line in the parallel line illusion produces assimilation, but sequential presentation produces contrast. One difficulty with tests of the pool and store model, however, is that attention has not been manipulated separately from stimulus factors. As test-context separation increases, one changes both the likelihood that the two stimuli will be jointly attended and the likelihood or type of sensory interactions that will occur between them. The shift from assimilation to contrast may result from either factor. Some phenomena that depend on the distance between a test and context element, for example-such as lateral contour masking-are attributed to sensory rather than attentional factors (although attentional factors could logically play a role here as well). A clean test of the pool and store model therefore requires that attention is manipulated independently of stimulus variables. Attentional explanations of the effects of spatial variables in the above studies also assume that selection is mediated spatially rather than by other features. Although this may be reasonable in the cases noted above, it should not necessarily be assumed. The results of Experiments 1 and 2 might cast doubt on the pool and store model as contrast effects were found even though the test and context stimuli were presented briefly and subjects were required to process both. One might object, however, that different “glances” of the test and context
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figures were still mediated by rapid attention shifts, particularly as the stimuli were not masked. Additionally, if the context circles were selected by size or colour, perhaps they should not be considered as occurring within the same “glance” as the test stimulus.
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GENERAL DISCUSSION In both experiments, the perception of the size of the test object depended on which pair of context circles was task-relevant. When the small context pair was relevant, the test circle was seen as larger than when the large context pair was relevant. These shifts in perceived size occurred even though the physical display was unchanged and display duration was too brief to permit eye movements. Contrast and assimilation phenomena suggest that perceptual judgements are based on relative quantities. At some point in the evaluation of the size of an object, it is compared to other objects in the field. The present experiments suggest that a selection mechanism weights the degree to which contextual objects are entered in this comparison process. The present results do not detail the properties of this selection mechanism. As noted earlier, however, selection in the current experiments may have been mediated by non-spatial features that allowed access to an objectdefined representation. Coren (1971) has presented results suggesting that the context-test comparison occurs at a fairly late stage of perceptual processing. He presented a test circle surrounded by context circles of identical size. By stereoscopically changing the depth of the context circles, he was able to change their perceived size through size/distance scaling. Correspondingly, the test circle appeared smaller or larger, depending on the perceived size of the context circles. This result suggests that the selection process operates on representations that have undergone size/distance scaling. Recent work by Coren and Enns (1991) also indicates that the illusion varies with the conceptual similarity (i.e. the degree to which the test and context elements are from semantically similar categories) of the test and context elements. In a series of papers, Gogel and colleagues (Gogel & McCracken, 1979; Gogel & Sharkey, 1989; Gogel & Tietz, 1976) have shown that attention influences a comparison process in the perception of motion. His work indicates that attention affects a number of induced motion phenomena in which the perceived direction of motion of a test element depends on the direction of motion of context elements. As with the present work, Gogel finds that attention weights the degree to which contextual elements influence a comparison process. It is possible that selective attention plays an important role generally in processes that compare target and context stimuli to form representations emphasizing relational quantities.
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Effects of attention on size perception have also been reported by Epstein and Broota (1986), who find that size constancy requires attention. Size judgements of attended objects reflected objective size, and judgements of unattended objects reflected projective size. Some theories of size constancy have suggested a role for relative size judgements or size comparisons between the judged object and some contextual stimulus. Although size comparisons may help maintain constancy in some situations, however, theories based solely on relative size have been criticized (Rock, 1983), and size constancy is typically thought to require the integration of retinal size and distance information.
REFERENCES Brigell, M., & Uhlarik, J. (1979). The relational determination of length illusions and length aftereffects. Perception, 8, 187-197. Coren, S. (1971). A size contrast illusion without physical size differences. American Journal of Psychology, 84, 565-566. Coren, S., & Ems, J. (1991). Size contrast as a function of conceptual similarity. Meeting of the Psychonomic Society, November, 1991. Coren, S., & Girgus, J. (1972). Differentiation and decrement in the Mueller-Lyer illusion. Perception & Psychophysics, 12, 4-70. Coren, S . , & Girgus, J. (1978). Seeing is deceiving: The psychology of visual illusions. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc. Coren, S., & Porac, C. (1983). The creation and reversal of the Mueller-Lyer illusion through attentional manipulation. Perception, 12, 49-54. Duncan, J. (1984). Selective attention and the organization of visual information. Journal of Experimental Psychology: General, 113, 501-517. Epstein, W., & Broota, K. (1986). Automatic and attentional components in perception of size-at-a-distance. Perception & Psychophysics, 40, 256-262. Girgus, J., & Coren, S. (1982). Assimilation and contrast illusions: Differences in plasticity. Perception & Psychophysics, 32, 555-561. Girgus, J., Coren, S., & Agdern, M. (1972). The interrelationship between the Ebbinghaus and Delbouef illusions. Journal of Experimental Psychology, 95, 453-455. Gogel, W., & McCracken, P. (1979). Depth adjacency and induced motion. Perceptual and Motor Skills, 48, 343-350. Gogel, W., & Sharkey, T. (1989). Measuring attention using induced motion. Perception, 18, 303-320. Gogel, W., & Tietz, J. (1976). Adjacency and attention as determiners of perceived motion. Vision Research, 16, 839-845. Jordan, K., & Schiano, D. (1986). Serial processing and the parallel-lines illusion: Length contrast through relative spatial separation of contours. Perception & Psychophysics, 40, 384-390. Jordan, K., & Uhlarik, J. (1985). Assimilation and contrast of perceived length depend on temporal factors. Perception & Psychophysics, 37, 447454. Pressey, A. (1971). An extension of assimilation theory to illusions of size, area, and direction. Perception & Psychophysics, 9, 172-176. Pressey, A. (1987a). Evidence for the role of attentive fields in the perception of illusions. Quarterly Journal of Experimental Psychology, 26, 464-471.
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Pressey, A. (1974b). Effect of size of angle on the ambiguous Mueller-Lyer illusion. Acta Psychologica, 38, 401-404. Restle, F. (1971). Instructions and the magnitude of an illusion: Cognitive factors in the frame of reference. Perception & Psychophysics, 9, 31-32. Rock, I. (1983). The logic of perception. Cambridge: MIT Press. Shulman, G. (1991). Attentional modulation of mechanisms that analyze rotation in depth. Journal of Experimental Psychology: Human Percpetion and Performance, 17,126-137. Shulman, G . (1992). Attentional modulation of a figural aftereffect. Perception. Tsal, Y. (1984). A Mueller-Lyer illusion induced by selective attention. Quarterly Journal of Experimental Psychology, 36A, 319-333.
Revised manuscript received 25 May I992
