Photostimulation of single cones.

Abstract

As far as we know, no experimental study exists concerning the functional activity of single cones of the vertebrate retina. It has not been directly proved that the cones are photosensitive, although such an assumption has been generally accepted mainly because of morphological reasons. On the basis of psychophysical studies on vision, of light absorption measurements of the retina or extracts of it, and of electrophysiological studies on the activity of the second and third order of retinal neurons, a number of more or less likely conclusions have been drawn concerning the functional properties of the cones. The spectral absorption m e a s u r e m e n t s made by Rushton (1-3), offer evidence for the presence of two photopigments in the human foveal cones. Hanaoka and Fujimoto (4), made measurements of the spectral absorption of the outer segments of isolated single cones (outer segment plus ellipsoid), separated from the fish retina. These authors claim the existence of five different photopigments which would indicate the presence of a corresponding number of different types of cones. The spectral response curves recorded from the second order of retinal neurons of fish retina (Motokawa (5), Svaetichin (6), Motokawa, Oikawa, and Tasaki (7), Tomita et al. (8), Mitarai et al. (9), MacNichol and Svaetichin (10), Watanabe and Tosaka (11)), indicate the existence of at least three or possibly five different types of cones. For any theory of vision, a knowledge of the functional characteristics of the cones is of fundamental importance. All relevant theories of color vision are based on the assumption of the existence of cones having different spectral sensitivities, although this has not yet been directly proved. All functions in photopic conditions are proportional to the logarithm of the illumination intensity. This is a basic law for vision, being observed in measurements relating for instance brightness discrimination, visual acuity, and critical fusion to the intensity of illumination. Furthermore, the frequency of the spikes and the amplitude of the slow potentials recorded from the retina are also proportional to the logarithm of the