Contrast's effect on spatial summation by macaque V1 neurons

Stimulation outside the receptive field of a primary visual cortical (V1) n euron reveals intracortical neural interactions(1-6). However, previous inv estigators implicitly or explicitly considered the extent of cortical spati al summation and, therefore, the size of the classical receptive field to b e fixed and independent of stimulus characteristics or of surrounding conte xt. On the contrary, we found that the extent of spatial summation in macaq ue V1 neurons depended on contrast, and was on average 2.3-fold greater at low contrast. This adaptive increase in spatial summation at low contrast w as seen in cells throughout V1 and was independent of surround inhibition.