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Barbara Heider, Kurt F. Ahrens, Ralph M. Siegel; Neural activity in monkey parietal area 7a during reaching and the effects of prism adaptation. Journal of Vision 2005;5(8):121. doi: 10.1167/5.8.121.
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Visuo-motor transformation was studied in macaques by recording from neurons in parietal cortex and studying behavioral performance during a reaching task. In this task the monkeys had to reach to a target (white square) that appeared in one of 9 different locations (3x3 matrix) presented on a touch screen. The monkeys had to fixate the target, which dimmed by 10% to indicate the launch, and then to reach out to touch the target fast and accurately. After touching the target, the monkeys were required to hold their finger on target for 1.5 s. Once the monkeys achieved a high level of performance, 5° or 10° shifting Fresnel prisms were applied to create a mismatch between visually perceived and actual target location. With the prisms, behavioral adaptation and its time course was analyzed. Monkeys adjusted their reaching movements within a few hundred trials but large inter-individual variations were found too. Across multiple recording sessions, the adjustment periods decreased. After prism removal, monkeys showed a transitory after-effect in the opposite direction of the shift induced by the prism. Electrophysiological recordings were made from neurons in area 7a of posterior parietal cortex (PPC). This area plays a crucial role in visuo-spatial transformation from a retinotopic to a head-centered coordinate system, and is likely to be involved in visually guided reaching movements. Single cell responses (>100 neurons) were recorded from single units in area 7a. Temporal (activity changes during specific stages of the task) and spatial (9 stimulus positions) aspects were analyzed. Increased activity was associated mainly with the launch of the hand and only a few cells showed saccade related changes. In about one third of the cells analyzed, these responses were independent of spatial position; in another third, the responses were modulated with location of the reach position.
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