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Abstract:
In order to pick up an object, its visual location must be converted into the appropriate
motor commands. Introducing a discrepancy between the seen and felt location of the
object (e.g., via prism goggles) initially impairs our ability to touch it. The sensory systems
rapidly adapt to the discrepancy, however, returning perception and performance to
near normal. Subsequent removal of the discrepancy leads to a renewed performance
decrement -- a Negative Aftereffect (NAE). It is generally believed that this adaptation
consists primarily of “recalibrating” the transformation between the visual and proprioceptive
perception of spatial location (Bedford, 1999). According to such a purely perceptual
account of adaptation, the exact path used to reach the object should not be
important. If, however, it is the transformation from perception to action that is being
altered, then changing the motion trajectory should reduce or eliminate the NAE.
Starting with both hands on the desktop, the chin resting on a horizontal bar, participants
(N=72) had to touch a cross presented at eye level on a touch screen 30 cm in front of
them. Four trajectories were possible: reaching to the cross from below or (swinging the
arm backwards) from above the bar, using either their left or their right hand. Reaching
Accuracy without feedback was determined for all four trajectories before and after adaptation
to a single trajectory with prism goggles (19° horizontal displacement). The NAE
was 46mm (8.7°) for the adapted trajectory, 26mm negligable for both trajectories of the
other hand. The NAE was larger for unfamiliar (above bar, or usage of non-preferred
hand) than for familiar trajectories.
Visuomotor adaptation is not merely a perceptual recalibration. Not only does the structure
of the motion trajectory play a central role, but the familiarity of the trajectory also
seems to be important. These results have strong implications for all models of visuomotor
adaptation.
