Neural activity in area V4 was examined to assess (1) whether the effects of attentive selection for stimulus features could be based on the memory of the feature, (2) whether dynamically changing the feature selection would cause activity associated with the newly selected stimuli to pop out, and (3) whether intrusion of more than one stimulus into the receptive field would disrupt the feature-selective activity. Rhesus monkeys were trained on several variations of a conditional orientation discrimination task. A differential activation of area V4 neurons was observed in the conditional discrimination task based on the presence of a match or a nonmatch between the conditional cue (a particular color or luminance) and the color or luminance of the receptive field stimulus. The differential activation was unchanged when the cue was removed and the animal had to remember its color (or luminance) to perform the task. When the cued feature was switched from one alternative to another in the middle of a trial the differential activation of neurons reversed over the course of 150–300 msec. If the stimulus in the receptive field contained the newly selected feature, V4 neurons became activated without a concomitant change in the stimulus in classical receptive field. Across the topographic map of V4 the activity associated with the newly selected stimuli popped out, whereas the activity of deselected stimuli faded to the background levels of other background objects. Evidence of a suppressive input from stimuli outside the classical receptive field was clear in only 3 of 24 neurons examined. Intrusion into the classical receptive field by a second stimulus resulted in a diminished difference between matching and nonmatching conditions. These physiological data suggest a major role for attentional control in the parallel processing of simple feature-selective differences.