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Joo-Hyun Song, Robert McPeek, Naomi Takahashi; Target selection for visually-guided reaching in macaque. Journal of Vision 2007;7(9):296. doi: 10.1167/7.9.296.
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Most real-world visual scenes are complex and crowded: instead of a single isolated object, there are often several different objects competing for attention and directed action. The majority of our knowledge of the neural mechanisms underlying the selection of visual targets for action has come from studies of eye movements. However, vision is also used to guide other actions such as manual reaching movements. To examine the mechanisms involved in target selection for visually-guided reaching, we trained two rhesus monkeys to perform a search task in which they were rewarded for making a reaching movement to an odd-colored target presented with multiple distractors. The color of the target was randomly selected in each trial to be either red or green, and the distractors were of the opposite color. We found that as the number of distractors increased, reaching times decreased and accuracy improved, presumably due to stronger perceptual grouping of the homogenous distractors. Furthermore, reaches were completed more quickly and accurately when the target color remained the same from trial to trial than when it switched, indicating that cumulative color priming across trials facilitates target selection for reaches in monkey. Reaching errors were usually directed to a distractor adjacent to the target, suggesting a spatially coarse-to-fine progression during target selection. These results are very similar to what has been observed for reaching movements in humans (Song and Nakayama 2006), providing a basis to examine the neural underpinnings of reach target selection in the monkey. Similar effects of distractor number and color priming are also seen for saccadic eye movements in both humans and monkeys (Findlay 1997; McPeek et al. 1999; Bichot and Schall 1999, 2002; McPeek and Keller 2001; Arai et al. 2004), suggesting a common neural mechanism of visual selection for both reaching and eye movements.
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