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Abstract:
Size contrast illusions are assumed to exert a smaller effect on human motor behavior than on perception, indicating different cortical pathways
for perception and action (e.g., Aglioti, DeSouza Goodale, 1995 Current Biology 5 679-685). We
tried to replicate these findings for the
Ebbinghaus Illusion. Special effort was taken to minimize effects of motor learning and on
assessment of the size of the perceptual illusion. An aluminum disc (28, 31, 34 or 37 mm in diameter,
5 mm in height) was positioned as target on a
board. Around the target either small or large
context discs were drawn (10 or 58 mm in
diameter). Close to the board a monitor was
mounted on which a comparison disc was
displayed. In a visual task twelve subjects
adjusted the size of the comparison disc to match
the size of the target. In a grasping task
subjects grasped the target. Subjects wore shutter
glasses and could not see their hand during
grasping (open loop condition). The grasp
trajectory was recorded and the maximum preshape
aperture was calculated. Preshape aperture and
adjusted size showed strong and similar linear
relationships to the size of the target. The mean
perceptual effect of the illusion was 1.4 mm (SE =
0.1 mm) while the effect of the illusion on
preshape aperture was 1.5 mm (SE = 0.4 mm). Thus,
grasping was just as much influenced by the
illusion as perception. Possible reasons for this
discrepancy to previous studies are discussed.
