Mechanisms underlying the generation of averaged modified trajectories |
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Authors: | Ealan A Henis Tamar Flash |
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Institution: | (1) Information Principles Laboratory, Room 2C-440, AT/@T Bell Laboratories, 600 Mountain Ave., Murray-Hill NJ 07974, USA (Phone: (908)582-3531, e-mail: ealan@research.att.com, Fax: (908)582-7308) , US;(2) Department of Applied Mathematics and Computer Science, The Weizmann Institute of Science, Rehovot 76100, Israel , IL |
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Abstract: | In this work we have studied what mechanisms might possibly underlie
arm trajectory modification when reaching toward visual targets. The
double-step target displacement paradigm was used with inter-stimulus
intervals (ISIs) in the range of 10-300 ms. For short ISIs, a high
percentage of the movements were found to be initially directed in
between the first and second target locations (averaged
trajectories). The initial direction of motion was found to depend on
the target configuration, and on : the time difference between the
presentation of the second stimulus and movement onset. To account
for the kinematic features of the averaged trajectories two
modification schemes were compared: the superposition scheme and the
abort-replan scheme. According to the superposition scheme, the
modified trajectories result from the vectorial addition of two
elemental motions: one for moving between the initial hand position
and an intermediate location, and a second one for moving between that
intermediate location and the final target. According to the
abort-replan scheme, the initial plan for moving toward the
intermediate location is aborted and smoothly replaced by a new plan
for moving from the hand position at the time the trajectory is
modified to the final target location. In both tested schemes we
hypothesized that due to the quick displacement of the stimulus, the
internally specified intermediate goal might be influenced by
both stimuli and may be different from the location of the first
stimulus. It was found that the statistically most successful model
in accounting for the measured data is based on the superposition
scheme. It is suggested that superposition of simple independent
elemental motions might be a general principle for the generation of
modified motions, which allows for efficient, parallel planning.
For increasing values of the
inferred locations of the intermediate targets were found to gradually
shift from the first toward the second target locations along a path
that curved toward the initial hand position. These inferred locations
show a strong resemblance to the intermediate locations of saccades generated
in a similar double-step paradigm. These similarities in the
specification of target locations used in the generation of eye and
hand movements may serve to simplify visuomotor integration.
Received: 22 June 1994 / Accepted in revised form: 15
September 1994 |
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