THE EBBINGHAUS ILLUSION MODULATES

The experiments reported in this chapter investigated whether objects’ apparent size, unlike their retinal size, is coded in pre-attentive vision. Observers searched for a target test circle that was either larger or smaller than the distractor test circles, with both types of test circle surrounded by context circles modulating their apparent size (Ebbinghaus ill usion). The size ratio between the test and context circles was manipulated such that, for example, the test circles were surrounded by smaller context circles (making, e.g., the larger target appear even larger) or by larger context circles (making, e.g., the smaller distractors appear even smaller). These experimental conditions were contrasted with control conditions without context circles, in which the retinal size of the test circles corresponded to the apparent size of the test circles in the experimental conditions. The search times were independent of the number of Ebbinghaus configurations in the display, and the Ebbinghaus apparent-size ill usion modulated the search times in a manner similar to the retinal size manipulation in the control conditions. Taken together with earlier findings (Busch & Müller, 2000), this pattern of results is consistent with pre-attentive, efficient processing of objects’ apparent size. In visual search, a target object that differs from the distractor objects in retinal size can be discerned eff iciently, that is, the search reaction times (RTs) are independent of the number of items in the search display (e.g., Müller, Heller, & Ziegler, 1995). This finding supports the conclusion of spatially parallel, pre-attentive processing of retinal object size. In a series of search experiments, we (Busch & Müller, 2000; 2001) investigated whether display objects’ apparent size, too, is derived and represented pre-attentively (similar to their retinal size). The experiments varied the objects’ apparent size by introducing the geometric-optical ‘Ebbinghaus’ ill usion. This figure, described by Ebbinghaus (1913), consists of an inner test circle surrounded by several context circles. Smaller context circles make the test circle appear larger than an identical comparison circle, whereas larger context circle make the test circle appear smaller (Figure 1). The magnitude of the misjudgement increases with increasing size difference between test and context circles (Massaro & Anderson, 1971). Previous search experiments showed that, under optimal conditions, with the (to-be-detected) target test circle being larger than the distractor test circles, RTs were independent of the number of Ebbinghaus figures in the display and, importantly, that the manipulation of apparent size by smaller context circles can facilit ate the detection of the target in comparison with a control condition without context circles (Figure 2). In contrast, RTs were slowed when test circles were surrounded by larger context circles, perhaps because the size of the target test circle was ‘ in-between’ the size of the distractor test circles and that of the context circles and therefore harder to detect (Wolfe, 1998). Figure 1. Ebbinghaus ill usion. The expedited detection RTs for target test circles surrounded by smaller context circles can be explained by assuming that the manipulation of the test circles’ apparent size reinforces the (retinal) size difference between target and distractor test circles, thereby increasing the saliency of the target. The two experiments reported below were designed to test this assumption. In Experiment 1, a psychophysical experiment, we determined the apparent sizes (the magnitude of misjudgement) of the test circles in the search conditions used by Busch and Müller (2000). These estimated parameters were then introduced as direct control conditions in Experiment 2, a visual search experiment. If the expedited detection of a target test circle surrounded by smaller context circles can in fact be attributed to a pre-attentive representation of the test circles’ apparent size, the eff iciency of target detection under control conditions (without context circles) should directly correspond with that under experimental conditions (with context circles, i.e., with the influence of the size ill usion). Busch & M üller (2000) 400 425 450 475 500 525 550 3 5 7 Display Size R T ( m s) 0 mm 3 mm 6 mm 10 mm 16 mm Figure 2. Results of Busch & Müller (2000). Experiment 1: Method Twenty four observers participated in Experiment 1. Their task was to decide, on each trial, which of two fill ed black circles – the test circle surrounded by six outlined context circles or a comparison circle without context circles – was the larger circle (Figure 3). In line with the parameters in the search experiments of Busch and Müller (2000; 2001), test circles were either 6 or 10 mm and context circles either 3, 6, 10, or 16 mm in diameter. The corresponding comparison circles were varied in steps of 0.5 mm between 3 and 9 mm or between 9 and 13 mm, for 6-mm and 10-mm test circles, respectively. Figure 3. Example display in the psychophysical Experiment 1. Experiment 1: Results and Discussion Figure 4 presents the frequencies of the responses ‘ test circle larger than comparison circle’ as a function of the size of the comparison circle for the four context-circle sizes, separately for two test-circle conditions. In a particular experimental condition, a test circle’s apparent size was estimated as the size of the comparison circle for which the response frequency was 50% (i.e., the ‘point of subjective equality’) . For instance, a 6-mm test circle appeared 6.30 mm in diameter when surrounded by 3-mm context circles, but 5.63 mm in diameter when surrounded by 16-mm context circles. A 10-mm test circle appeared 10.25 mm in diameter when surrounded by 3-mm context circles, but 9.13 mm in diameter when surrounded by 16mm context circles. Thus, the apparent size of the test circles varies according to the size of their surrounding context circles. A test circle surrounded by smaller context circles appears larger, whereas a test circle surrounded by larger context circles appears smaller (Ebbinghaus ill usion). Table 1 summarizes the parameters of the test circles introduced in the control conditions of the subsequent visual search experiment (Experiment 2). Table 1. Apparent sizes of test circles as judged in Experiment 1, that served as parameters for Experiment 2. Test Circles (mm) Context Circles (mm) 6 10 0 6.12 10.00 3 6.30 10.25 6 5.80 10.10 10 5.75 9.70 16 5.63 9.13