Two stages in visual texture segregation: a lesion study in the cat.

We have used five cats to investigate the effects of two distinct visual cortex lesions on the segregation of two different texture stimuli. The ablation of areas 17 and 18 (tier I) severely impaired the segregation between textures made of line elements differing in orientation, but spared the segregation between annulus and dot textures. In contrast, the ablation of those areas receiving direct afferents from areas 17 and 18 (tier II) destroyed the segregation for both texture stimuli. Strong deficits remained up to 1 year after the lesion, although limited recovery was observed after tier II lesions. We suggest that tier I areas are involved in the local filtering of the texture elements, and that tier II areas compute texture differences on the basis of the filtered image provided by tier I areas. The crucial contribution to texture segregation of visual areas belonging to a second level in the cortical hierarchy challenges the notion that texture segregation is entirely an early process in vision.