Responses of neurons in macaque V4 to object and texture images

Abstract

AbstractHumans and monkeys can effortlessly recognize objects in everyday scenes. This ability relies on neural computations in the ventral stream of visual cortex. The intermediate computations that lead to object selectivity are not well understood, but previous studies implicate V4 as an early site of selectivity for object shape. To explore the mechanisms of this selectivity, we generated a continuum of images between “scrambled” textures and photographic images of both natural and manmade environments, using techniques that preserve the local statistics of the original image while discarding information about scene and shape. We measured the responses of single units in awake macaque V4 to these images. On average, V4 neurons were slightly more active in response to photographic images than to their scrambled counterparts. However, responses in V4 varied widely both across different cells and different sets of images. An important determinant of this variation was the effectiveness of image families at driving strong neural responses. Across the full V4 population, a cell’s average evoked firing rate for a family reliably predicted that family’s preference for photographic over scrambled images. Accordingly, the cells that respond most strongly to each image family showed a much stronger difference between photographic and scrambled images and a graded level of modulation for images scrambled at intermediate levels. This preference for photographic images was not evident until ∼50 ms after the onset of neuronal activity and did not peak in strength until 140 ms after activity onset. Finally, V4 neural responses seemed to categorically separate photographic images from all of their scrambled counterparts, despite the fact that the least scrambled images in our set appear similar to the originals. When these same images were analyzed with DISTS (Deep Image Structure and Texture Similarity), an image-computable similarity metric that predicts human judgements of image degradation, this same pattern emerged. This suggests that V4 responses are highly sensitive to small deviations from photographic image structure.