Psychophysical correlates of photoreceptor activity

Abstract

Psychophysical experiments done on a rod monochromatand a normal resulted in the following: (1) above rod saturation, the rod monochromat cannot see any test field with her eyes open, but can detect the test by means of discriminable weak afterimages caused by bleaching as little as 10 −5 of the rhodopsin; (2) these weak afterimages occur in normals and have the spectral sensitivity of rhodopsin. Analysis of the data implies that the weak afterimage originates in the outer segment but can not be explained by the usual equivalent dark light hypothesis which accounts for strong afterimages produced by intense bleaches. An alternative hypothesis about the production of the photocurrent is introduced; (3) strong afterimages show saturation. Two retinal regions that received 70 and 85% bleaches appear subjectively identical for the first 11 min. Thereafter one can discriminate the two different bleached areas; (4) saturation in dark adaptation is shown in the rod monochromat. After a full bleach, it is impossible to detect any increment on the bleached area until 9 min have elapsed; (5) the last two results are consistent with the hypothesis that the rod photoreceptor signal remains saturated for many minutes after an intense bleach is turned off.