Quantum sensitivity of rods in the toad retina.

A dark-adapted toad rod can respond consistently to flashes of light which bleach an average of less than one pigment molecule in its outer segment. These responses are much less variable in amplitude than would be expected if rods were independent quantum detectors. Rods interact with one another by pooling their signals, so that at least 85 to 90 percent of the response recorded from a single rod is generated by pigment molecules bleached in other receptors.