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Abstract

The perception-versus-action hypothesis
states that visual information is processed
in two different streams, one for visual
awareness (or perception) and one for motor
performance. Previous reports that the
Ebbinghaus illusion deceives perception but
not grasping seemed to indicate that this
dichotomy between perception and action was
fundamental enough to be reflected in the
overt behavior of non-neurological, healthy
humans. Contrary to this view we show that
the Ebbinghaus illusion affects grasping to
the same extent as perception. We also show
that the grasp effects cannot be accounted
for by non-perceptual obstacle avoidance
mechanisms as has recently been
suggested. Instead, even subtle variations of
the Ebbinghaus illusion affect grasping in
the same way as they affect perception. Our
results suggest that the same signals are
responsible for the perceptual effects and
for the motor effects of the Ebbinghaus
illusion. This casts doubt on one line of
evidence, which used to strongly favor the
perception-versus-action hypothesis.