Figure 4. Sublinear bump summation.

A, only one QB is produced to simultaneous multi‐photon hits, with the resultant QB being smaller than the sum of those produced independently. B, the relationship between the bump charges for three different cases: linear summation, sublinear summation and no summation (C_x is the x‐photon induced QB charge). C, the contribution of sublinear bump summation to light adaptation depends upon the photon arrival rate. Charges/photon is normalized to the single‐photon‐induced QB charge for the normalized gain. This normalized gain is from linear summation, but it varies nonlinearly with the photon rate for sublinear summation (grey area) and no summation (red line). The black line is an example produced by our Stochastic Bump Model. Sublinear bump summation contributes to light adaptation marginally in a fly photoreceptor (<1%, green square).