Stability of cortical responses and the statistics of natural scenes.

The primary visual cortex (V1) of higher mammals contains maps of stimulus features; how these maps influence vision remains unknown. We have examined the functional significance of an asymmetry in the orientation map in cat V1, i.e., the fact that a larger area of V1 is preferentially activated by vertical and horizontal contours than by contours at oblique orientations. Despite the fact that neurons tuned to cardinal and oblique orientations have indistinguishable tuning characteristics, cardinal neurons remain more stable in their response properties after selective perturbation induced by adaptation. Similarly, human observers report different adaptation-induced changes in orientation tuning between cardinal and oblique axes. We suggest that the larger cortical area devoted to cardinal orientations imposes stability on the processing of cardinal contours during visual perception, by retaining invariant cortical responses along cardinal axes.