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Jude F. Mitchell, Kristy A. Sundberg, John H. Reynolds; Attentive tracking of multiple objects modulates neuronal responses in area V4 of the macaque. Journal of Vision 2006;6(6):772. doi: 10.1167/6.6.772.
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Human observers can attentively track multiple stimuli as they move along independent random trajectories among distracter stimuli (Pylyshyn and Storm, 1988; Sears and Pylyshyn, 2000). We developed a multi-object tracking task suitable for monkeys and recorded single units and LFPs in area V4 as tracked or untracked stimuli entered a unit's receptive field. Each trial began with fixation of a central point, after which 4 identical stimuli appeared at equally eccentric peripheral positions. Either one or two of these stimuli were briefly flashed, identifying them as targets for tracking. All stimuli then moved along random independent trajectories for 2–3 seconds while the monkey maintained fixation. The trajectories were constrained such that one of the stimuli entered and left the receptive field of an isolated V4 neuron. All stimuli terminated motion at equally eccentric positions that were unpredictable from their initial positions. Monkeys were rewarded only if they made a saccade to each target and to no distracters. We found that V4 single unit responses evoked by a tracked stimulus were on average ∼20% stronger than the responses evoked by the same stimulus when it was not being tracked. The magnitude of the modulation did not differ significantly depending on whether the monkey was tracking a single target or two targets, suggesting that attending to one target did not reduce attention to the other.
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