Scotopic visual efficiency: constraints by optics, receptor properties, and rod pooling.

We studied the influence of optics, photoreceptor properties, and rod pooling on scotopic contrast sensitivity by comparing the performance of an ideal discriminator to that of human observers. Comparisons of human and ideal contrast sensitivities indicated that preretinal factors and summation area were not sufficient to explain the shape of the human CSF. Spatial pooling of rods was explored as a possible explanation of this discrepancy. Our highest efficiency, expressed in terms of a human/ideal contrast sensitivity ratio, was about 1:3 (0.33) for a contrast discrimination task.