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 and between the adjusted small-circle annulus and the traditional large-circle annulus . In addition, the traditional small-circle annulus resulted in larger estimates than did the adjusted small-circle annulus , and this effect is consistent with well-known properties of the illusion. For the grasping task, significant differences in grasp aperture were seen 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 or the adjusted small-circle annulus . 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 . 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