Finding a face in the crowd: parallel and serial neural mechanisms of visual selection.

At any given moment, our visual system is confronted with more information than it can process. Thus, attention is needed to select behaviorally relevant information in a visual scene for further processing. Behavioral studies of attention during visual search have led to the distinction between serial and parallel mechanisms of selection. To find a target object in a crowded scene, for example a "face in a crowd", the visual system might turn on and off the neural representation of each object in a serial fashion, testing each representation against a template of the target object. Alternatively, it might allow the processing of all objects in parallel, but bias activity in favor of those neurons representing critical features of the target, until the target emerges from the background. Recent neurophysiological evidence shows that both serial and parallel selections take place in neurons of the ventral "object-recognition pathway" during visual search tasks in which monkeys freely scan complex displays to find a target object. Furthermore, attentional selection appears to be mediated by changes in the synchrony of responses of neuronal populations in addition to the modulation of the firing rate of individual neurons.