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Anna E. Ipata, Angela L. Gee, James W. Bisley, Michael E. Goldberg; Top-down inhibition of the response to an irrelevant popout stimulus in monkey parietal cortex. Journal of Vision 2005;5(8):781. doi: 10.1167/5.8.781.
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Salient stimuli usually capture attention and the eyes in a stimulus driven manner. However, subjects can withhold saccades to such stimuli, especially when they are irrelevant to the task at hand. We have shown that activity in monkey LIP correlates with saccadic behavior during free visual search (Gee et al., VSS2005). We now show that after a monkey has learnt that a popout stimulus is irrelevant, the response in LIP is reduced. We recorded from LIP neurons while monkeys performed a visual search task in which they reported the orientation of a target by making a non-spatial manual response. The target was embedded in a radial array of 7 distractors that closely resembled the target, one of which popped out by virtue of color and luminance. The monkeys were free to move their eyes in any direction throughout the trial. We have previously shown that after a block of trials in which the target pops out, the monkeys adapt to a block in which a distractor, but never the target, pops out by greatly reducing the number of saccades to the now irrelevant popout. In about 95% of the current trials, the monkeys directed their first saccade away from the popout distractor and to the target or to one of the non-popout distractors. When the first saccade was made to a non-popout distractor the next saccade rarely if ever went to the popout. We found that the activity in LIP discriminated early between the popout and non-popout distractor in the receptive field, with the popout evoking a smaller response than the non-popout distractor. This discrimination occurred both when the monkey made a saccade to the receptive field and when it made a saccade away from the receptive field. These data suggest that an early top down mechanism suppresses LIP's response to a salient stimulus which the monkey has learned is task-irrelevant, and is therefore less likely to be the target of the next saccade.
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