Modulation of V4 shifts from dependent to independent on feature during target selection

We analyzed neuronal activities of visual area V4 of monkeys performing a delayed visual search task. To examine the temporal profile of factors influencing the neuronal activities, we conducted multiple regression analyses at 5 ms steps. During the period from 110 to 155 ms after the stimulus onset, there were neurons whose activity was suppressed when a target was presented near but beyond the neuron's receptive field (RF) compared to that when a target was within the RF. We referred this suppressive effect as an early period modulation. During the period from 155 to 280 ms after the stimulus onset, V4 activities were suppressed when a target was presented in any location outside of the neuron's RF. We referred this suppressive effect as a late period modulation. The magnitudes of the effect were equivalent across target locations when a target was beyond its RF. At the population level, while the modulation in the early period was correlated with stimulus selectivity, the modulation in the late period did not show such a correlation. These results suggest that V4 neurons have at least two distinct phases of modulations and those modulations contribute to select a target in the visual search task.