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Abstract:
PURPOSE: Visually guided motor behavior is assumed
to be rather unreceptive to size illusions,
indicating two different cortical processing
streams for the purposes of perception and action
(e.g., Aglioti, DeSouza Goodale, 1995; Current
Biology 5, 679-685). Having shown - in contrary -
that grasping and perception are equally
influenced by the Ebbinghaus / Titchener Illusion
(ECVP 98), we tested whether this is also true for
the Mueller-Lyer Illusion. METHODS: Plastic bars
(40, 43, 46 and 49 mm long, 5 mm wide) were
positioned on top of a horizontally oriented
monitor. Fins were presented on the monitor being
directed either outwards or inwards, such that the
fins and the bars resulted in the Mueller-Lyer
Illusion. In the grasping task, twelve subjects
grasped the bars and the maximal aperture between
thumb and index finger was measured using an
Optotrak (TM) system. In the visual perception
task, the subjects adjusted the length of a
comparison bar on the screen to match the length
of the plastic bars. RESULTS: There were strong
effects of the Mueller-Lyer Illusion on grasping
as well as on visual perception. The effect on
grasping (3.5+-0.5 mm) was even larger than on
perception (2.1+-0.3 mm). CONCLUSIONS: Our
results show that grasping is influenced by visual
illusions, indicating that the motor system is
receptive to visual illusions.
