Contrast changes the weighting of On and Off pathways.

A. Linear filters computed from the On and Off pathway contributions at high contrast (top) and low contrast (bottom). In this representation, the gain is shown as the amplitude of the filter, so that the filter is equal to the normalized filter as shown in Fig 1B times the average slope of the nonlinearity. The Off pathway contribution was measured as the response in the presence of APB, and the On pathway was estimated by subtracting the APB response from the control response. B. Normalized gain of the On and Off pathways computed as a function of contrast, measured as the amplitude of the filter in A. Each cell’s gain was normalized so that the total gain of the full response averaged over all contrasts was one.