Effects of luminance gradient reversal on complex cells in cat striate cortex.

We have examined the responsiveness of various classes of complex cells in the cat's striate cortex to stimuli comprising bars aligned with segments of opposite luminance contrast. In all cases, a short segment of opposite polarity depressed, without abolishing, the bar response; within the length summation zone, the response to the bar in the presence of longer segments matched the length summation characteristics of each cell, depressed by a constant amount. The results could not be accounted for purely on the basis of convergent input from simple cells, whose behaviour to comparable stimuli has previously been reported (Hammond and MacKay 1981a).