Relative size perception - if two circles of identical size are placed near each other and one is surrounded by larger objects, and the other by smaller objects, the two circles will look smaller and larger, respectively. The two spheres in this picture are the same size.
Complete article was published by Holly Gerhard "One More Look at the Ebbinghaus Illusion on University of California, School of Social Science's site.
The Ebbinghaus illusion has generated a great deal of debate in visuomotor literature, and one of our projects in Dr. Wright's visuomotor behavior lab further evaluates the hypothesized illusion size difference observed between subjective judgments of central disk size and motor interactions with the central disks.
In this diagram, the grasping task is represented as mediated by the dorsal processing stream originating in area V1 and feeding into parietal cortex, and the size judgment task is represented as mediated by the ventral processing stream also originating in area V1 but feeding into the temporal lobe.
You may be wondering how it's possible to compare illusion size in these two completely different tasks, and that is a good question.
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