Nonlinear signal transfer from mouse rods to bipolar cells and implications for visual sensitivity.

We investigated the impact of rod-bipolar signal transfer on visual sensitivity. Two observations indicate that rod-rod bipolar signal transfer is nonlinear. First, responses of rods increased linearly with flash strength, while those of rod bipolars increased supralinearly. Second, fluctuations in the responses of rod bipolars were larger than expected from linear summation of the rod inputs. Rod-OFF bipolar signal transfer did not share this strong nonlinearity. Surprisingly, nonlinear rod-rod bipolar signal transfer eliminated many of the rod's single-photon responses. The impact on sensitivity, however, was more than compensated for by rejection of noise from rods that did not absorb photons. As a consequence, rod bipolars provide a near-optimal readout of rod signals at light levels near visual threshold.