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 photoreceptors, and he later extended the idea to explain the elevation of threshold following bright bleaching lights in terms of increased noise in the photoreceptors. 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'. 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.