Current Biology
Volume 11, Issue 3, 6 February 2001, Pages 177-181
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The dissociation between perception and action in the Ebbinghaus illusion: Nonillusory effects of pictorial cues on grasp

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Abstract

According to a recently proposed distinction [1] between vision for perception and vision for action, visually guided movements should be largely immune to the perceptually compelling changes in size produced by pictorial illusions. Tests of this prediction that use the Ebbinghaus illusion have revealed only small effects of the illusion on grasp scaling as compared to its effect on perception 2, 3, 4. Nevertheless, some have argued that the small effect on grasp implies that there is a single representation of size for both perception and action [5]. Recent findings, however, suggest that the 2-D pictorial elements, such as those comprising illusory backgrounds, can sometimes be treated as obstacles and thereby influence the programming of grasp [6]. The arrangement of the 2-D elements commonly used in previous studies examining the Ebbinghaus illusion could therefore give rise to an effect on grasp scaling that is independent of its effect on perceptual judgements, even though the two effects are in the same direction. We present evidence demonstrating that when the gap between the target and the illusion-making elements in the Ebbinghaus illusion is equidistant across different perceptual conditions (Figure 1a)

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Figure 1. (a) A schematic representation of the three illusory displays. Note that the inner diameter of the adjusted small-circle annulus was matched to that of the traditional large-circle annulus. In the experiments the central targets were three-dimensional plastic disks, while the surrounding elements were two-dimensional. (b) The mean values for the manual-estimation task (left) and the grasping task (right) with the three illusory displays. Results are averaged across disk sizes since the effect of disk size on manual estimations and on grasp scaling did not interact with illusion condition (p > .05 in both cases). (c) The difference scores resulting from each of the possible within-task comparisons between the three displays. For the manual-estimation task, the long-established effect of the illusory displays was seen; targets surrounded by smaller circles appeared to be larger than targets surrounded by larger circles. Significant differences were seen for comparisons between the traditional small-circle annulus and the traditional large-circle annulus t17 = 8.92, p < .001 and between the adjusted small-circle annulus and the traditional large-circle annulus t17 = 7.40, p < .001. In addition, the traditional small-circle annulus resulted in larger estimates than did the adjusted small-circle annulus t17 = 3.48, p < .01, and this effect is consistent with well-known properties of the illusion. For the grasping task, significant differences in grasp aperture were seen only when displays with different gap distances between the target and surrounding annulus were compared. Grasp scaling was significantly greater for targets placed on the traditional small-circle annulus (where there was almost no gap between the target disk and the surrounding annulus) as compared to grasp scaling for targets placed either on the large-circle annulus t17 = 3.17, p < .01 or the adjusted small-circle annulus t17 = 2.72, p < .05. For the comparison between the adjusted small circle-annulus and the large-circle annulus, in which the distances between the target disks and the surrounding annulus were equated, no difference was seen t17 = 0.58, p > .05. The error bars in (b) and (c) represent standard error. A single asterisk indicates that p < .05; a double asterisk indicates that p < .01; and a triple asterisk indicates that p < .001

, the apparent effect of the illusion on grasp scaling is eliminated.

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