The role of area V4 in natural vision

Jack Gallant

Natural vision is governed by two critical factors: natural scenes, which have characteristic statistical structure that is exploited by the visual system to facilitate perception; and natural eye movements, which constrain how the scenes are sampled and processed. My laboratory has simultaneously investigated how these factors influence processing in area V4, an important intermediate stage of visual processing. The task requires search for a natural image patch hidden in a large array of similar patches. Eye movements were permitted during search. Arrays were arranged so that whenever any patch was fixated, the receptive field of a recorded V4 neuron would be centered over a different patch. The overall activity of many V4 cells was modulated by the search target; in some cells modulation was additive, in others it was multiplicative. Fourier-domain tuning profiles were also modulated by the search target, revealing top-down influences on spatial frequency and orientation tuning. Finally, the activity of many neurons predicted the direction of subsequent eye movements: the larger the response of the cell, the more likely it was that the next saccade would draw the fovea toward the receptive field. These observations suggest that area V4 acts as a parallel bank of spatially localized multidimensional filters. Their sensitivity and tuning is modulated by top-down influences to optimize performance. During natural vision, filters respond according to their match with the scene and their output, in combination with other areas, is used to drive subsequent eye movements.