Spontaneous quantal events induced in toad rods by pigment bleaching

Barlow proposed that absolute visual threshold is limited by photon-like noise events in the rod photoreceptors¹, and he later extended the idea to explain the elevation of threshold following bright bleaching lights in terms of increased noise in the photoreceptors². Rushton, on the other hand, has proposed that the threshold elevation during dark adaptation involves changes not within the photoreceptors themselves but rather in the gain of a subsequent 'pool'³. I report here measurements of outer segment current in individual rod photoreceptors which demonstrate that spontaneous fluctuations occur at a greatly increased rate following bleaches of around 1%, and that these fluctuations have the form expected for random occurrences of the single photon events. This is consistent with Barlow's ideas but does not indicate whether a gain change subsequent to the receptors also occurs.