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Abstract:
It is an open question whether visual illusions affect motor responses to the same extent
as perceptual responses. In previous studies (e.g.: Franz, Gegenfurtner, Bülthoff, Fahle
2000) we found similar effects of the Ebbinghaus illusion on perception and on grasping.
This finding contradicts a strong version of the action versus perception hypothesis (Milner
Goodale, 1995) which states that the motor system is unaffected by visual illusions.
Here, we tested whether our grasp effects might have been artifacts. This could be
the case if the motor system treated the illusion inducing context elements as obstacles
and tried to avoid them. To test for this possibility, we varied the distance between context
elements and target.
An aluminum disc (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 (10 or 58 mm in
diameter) were drawn at near or far distances (24 or 31 mm midpoint to nearest point on
context circles). Close to the board a monitor was mounted on which a comparison disc
was displayed. In the perceptual task 52 subjects adjusted the size of the comparison
stimulus to match the size of the target. In the grasping task subjects grasped the target.
Subjects wore shutter glasses and could not see their hand during grasping. The grasp trajectory
was recorded and the maximum preshape aperture was calculated.
The motor illusion responded to the variation of distance between context elements and
target in exactly the same way as the perceptual illusion. None of three different obstacle
avoidance hypotheses can explain these results.
Our results suggest that the same signals are responsible for the perceptual and for the
motor illusion. This either indicates that the action versus hypothesis needs modification,
or that the Ebbinghaus illusion is generated before the separation of the perceptual and
the motor streams.
