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Abstract

PURPOSE: If presented with a moving object which suddenly disappears observers usually misjudge the object's last seen position as being further
forward along the path of motion. This effect,
called representational momentum, can also be seen
in objects that change size or shape. It has been
argued that the effect is due to perceptual
anticipation. We tested whether a similar effect
is present in the motor system. METHODS: Using
stereo computer graphics we presented cubes of
different sizes on a CRT monitor. In each trial
three cubes were successively presented for 200
msec with increasing or decreasing size (steps of
1 cm width difference). Ten participants either
compared the last cube to a comparison cube
(perceptual task) or grasped the cube using a
virtual haptic setup (motor task). The setup
consisted of two robot arms (Phantom TM) attached
to index finger and thumb. The robot arms were
controlled to create forces equivalent to the
forces created by real objects. The CRT monitor
was viewed via a mirror such that the visual
position of the cubes matched the position of the
virtual haptic objects. RESULTS: In the motor task
participants opened their fingers by 1.1+/-0.4 mm
wider if they grasped a cube that was preceded by
smaller cubes than if they grasped a cube that was
preceded by larger cubes. This is the well-known
representational momentum effect. In the
perceptual task the effect was reversed
(-2.2+/-0.4 mm). The effects correlated between
observers (r=.71, p=.02). CONCLUSIONS: It seems
that a representational momentum occurs also in
grasping tasks. The correlation between observers
suggests that the motor effect is related to the
perceptual effect. However, our perceptual task
showed a reversed effect. Reasons for this
discrepancy will be discussed.