Neural correlates of attentive selection for color or luminance in extrastriate area V4.

Rhesus monkeys were trained on a conditional orientation discrimination task in order to assess whether attentive selection for a color or luminance stimulus feature would affect visual processing in extrastriate area V4. The task required monkeys to select a bar stimulus based on its color or luminance and then to discriminate the angular tilt of the selected stimulus. The majority of neurons (74%) were selectively activated when the color or luminance of the stimulus in the receptive field matched the color or luminance of the cue. The activity was attenuated when there was not a match between the stimulus and the cue. The differential activation was based on the presence or absence of the stimulus feature and was independent of spatial location. Across the population of V4 neurons, optimal stimuli that matched the selected color or luminance elicited about twice the activity as stimuli that did not match the selected feature. The feature-selective changes in activity were observed to develop beginning about 200 msec after the stimulus onset and were maintained over the remainder of the behavioral trial. In this task the activity of V4 neurons reflected a selection based on the cued feature and not simply the physical color or luminance of the receptive field stimulus. Under these conditions, the topographic representation of the neural activity in area V4 highlights the potential targets in the visual scene at the expense of background objects. These observations offer a physiological counterpart to psychophysical studies suggesting that stimuli can be preferentially selected in parallel across the visual field on the basis of a unique color or luminance feature.