Abstract
Exploration of a natural visual scene involves active eye movements that foveate salient features and facilitate feature identification. Eye movements are thought to be directed by saliency maps that encode the location of interesting features in the scene.
To identify saliency maps we trained a macaque to perform an attentionally demanding visual search task. The task was designed to mimic natural visual exploration and incorporated both natural stimuli and untrained voluntary eye movements. Search targets and distracters were cut from high quality B&W natural scene photographs. Matches were indicated with a touch bar.
We recorded the activity of 60 neurons in extrastriate area V4 along with eye movements during visual search. We found task related attentional modulation consistent with a perceptual saliency map. Neurons were more selective during search than in a passive viewing condition. To further characterize this effect we computed perisaccadic time histograms, conditioned by search target. In 22/60 (37%) neurons we found target-specific additive and multiplicative modulation. Cells were inhibited or facilitated immediately after the search target was cued and remained modulated for the remainder of the 20–30 s trial. In 30/60 (50%) cells we observed presaccadic facilitation of visual responses. Elevated visual responses predicted saccades towards the receptive field.
These effects suggest that V4 functions as a saliency map during visual search. Search target specification causes a persistent top-down modulation of spatiotemporal selectivity that optimizes V4 neurons for match identification. Putative matches are indicated by bottom-up correlations between the modulated neuronal filters and local features in the visual scene. High-activity regions in V4 represent likely matches and are most likely to be foveated by the next saccade.