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Joo-Hyun Song, Robert M. McPeek; Target selection for visually-guided reaching in the dorsal premotor area during a visual search task. Journal of Vision 2008;8(6):547. doi: 10.1167/8.6.547.
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Most visual scenes are complex and crowded, with several different objects competing for attention and directed action. Thus, an understanding of the production of goal-directed actions must incorporate the higher-level processes involved in the selection of a target stimulus from distractors. To examine the neural substrates of target selection for visually-guided reaches, we recorded the activity of isolated neurons in the dorsal premotor area (PMd) of rhesus monkeys. The role of the PMd when a target must be selected from distractors is not yet fully understood. However, recent studies have demonstrated that when two potential targets are presented for selective reaching, the PMd can simultaneously encode the two competing movement goals during a delay period before the cue to move (Cisek & Kalaska, 2002). Here, we investigated the role of the PMd in a reaction-time visual search task. We trained monkeys to reach to an odd-colored target presented with three distractors. We traced the time course of target/distractor discrimination and found that PMd neurons typically discriminated the target before movement onset, about 150∼200ms after the appearance of the search array. Discrimination in a subset of neurons occurred at a consistent time after search array onset regardless of the latency of the reaching movement, suggesting that these neurons are involved in target selection, as distinct from movement production. In other neurons discrimination time depended on reach latency, suggesting that this latter group of neurons are more involved in movement execution. These results suggest that different groups of PMd neurons are involved in target selection and movement initiation.
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