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Jacqueline Gottlieb; The monkey's lateral intraparietal area: parallel representations and competitive mechanisms. Journal of Vision 2003;3(9):39. doi: 10.1167/3.9.39.
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© ARVO (1962-2015); The Authors (2016-present)
Neurons in area LIP respond selectively for objects that are physically salient or task-relevant. The pattern of responses in LIP suggests that this area encodes the locus of attention independently of motor orienting. Here I ask just how selective this salience representation is: can LIP represent more than one salient object at a time? Monkeys viewed a circular array of 8 distinct shapes that remained continuously lit for > 100 trials. Each trial began when the monkey achieved central fixation, thereby bringing the receptive field (RF) of the neuron under study upon 1–3 stable objects. A cue that physically matched a randomly-selected stable object was then flashed briefly at a fixed location. Monkeys were rewarded for making a saccade, after a variable delay, to the stable object matching the cue. Neurons responded selectively both to the cue and to the saccade target but not to stable non-target objects. If only the cue (but not the target) was in the RF, neurons had strong responses that peaked within 50–100 ms of cue onset and declined gradually during the delay period, persisting at a low level until the saccade. If only the target (but not the cue) was in the RF, neural activity gradually increased during the delay period, remaining elevated from ∼150–200 ms after cue onset until the saccade. Thus, responses to the cue and to the target were sustained in parallel, throughout the delay period, in populations of LIP neurons with non-overlapping receptive fields. However, if both cue and target were in the RF, neurons responded serially, first to the cue and only later to the target. Directional selectivity for the target evolved 200–300 ms later if the cue was in the RF than if it was not, indicating that the target response was temporarily suppressed if neurons had been recently activated by the cue. This suggests that competitive interactions within a RF curtail the capacity for parallel representation at spatial separations on the order of RF size.
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