Rapid effects of light and dark adaptation upon the receptive field organization of S- potentials and late receptor potentials

Abstract

Since the late RP (receptor potential) of cones decays rapidly, while that of rods decays slowly, relative amplitudes of the rapid and slow decay phases indicate relative cone and rod contributions to a response from a mixed population of receptors. S-potentials of the cat are shown capable of following closely the form of either the cone or rod late RP. When evoked by a small centered stimulus, the S-potential is pure cone in form and does not change form between light and dark adaptation. In the light adapted state, increase of stimulus size increases the amplitude of the cone form of response. After only 2 min of dark adaptation, the same increase of stimulus size produces a predominantly rod form response and decreases the cone contribution elicited by the small centered stimulus. Thus the S-potential shows a dramatic and rapid change in the organization of its receptive field, during dark adaptation as well as light adaptation. Qualitatively similar results were then obtained by studying the local late RP from the peripheral retina of the cynomolgus monkey. Hence the receptors also have receptive fields, which must result from the late RP being influenced presynaptically by receptor interaction. The organization of the receptive field of the receptors likewise changes markedly and rapidly between light and dark adapted states. Hence there are rapid neural mechanisms of adaptation which influence both the late RP and S-potential, and which are probably fundamental to the rapid neural phases of dark and light adaptation. When using large stimulus spots, our results indicate that the cone late RP is suppressed by rod activity under dark adapted conditions, while the rod late RP is suppressed by cone activity after light adaptation. Thus our findings also give a basis for understanding why visual functions are so completely dominated by rods under dark adapted conditions, and by cones after light adaptation, a problem at the heart of the duplicity theory. The hypothesis is advanced that the S-potentials we have recorded are from horizontal cells, which mediate reciprocal lateral inhibition between cones and rods. This hypothesis is supported by several lines of evidence, and can account for the close functional relation demonstrated between the late RP and S-potential, in spite of crucial differences which show that the two responses are generated by separate cells.