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Constanze Hesse, Volker Franz; Adaptive grasping: Corrective processes after perturbations of object size. Journal of Vision 2008;8(6):300. doi: 10.1167/8.6.300.
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© ARVO (1962-2015); The Authors (2016-present)
When grasping an object the pre-shaping of the hand is a highly stable motor pattern which is largely pre-determined by object-related visual input (Jeannerod, 1984). If the object size changes during movement execution, the initially planned motor program has to be adjusted. How these adjustments are accomplished is still a matter of debate. We investigated the corrective responses using a size perturbation paradigm. Participants grasped real objects of different sizes which were visually presented using a mirror setup. In 25% of the trials the visually presented object changed its size and became 1 cm larger or smaller, matching the size of the real object to be grasped. The perturbation could occur at two moments in time: (1) early: as soon as the hand left the starting position, (2) late: after 2/3 of the movement distance. In Experiment 1 participants could see their hand during grasping whereas in Experiment 2 vision of the hand was prevented. By combining the size perturbation paradigm with the presence or absence of visual information about the hand we were able to determine the relative contribution of feedback and feed-forward processes to on-line corrections of the grip. Results indicate that the availability of visual information about the hand influenced grasping kinematics (e.g., larger MGA if the hand was not seen) but had only little effect on the corrections to the new object size. In both experiments maximum grip aperture was perfectly adapted to the new object size after an early perturbation, whereas this correction was not accomplished after a late perturbation. The adaptation of the grip resulted from smooth changes in the aperture over time. These findings suggest that small changes in object size are corrected by a smooth amendment of the initially planned motor program using feed-forward mechanisms.
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