Saturation of a retinal cone mechanism

Abstract

IN discussions of the several differences between the rods and the cones of the human retina it is usually stated that rods saturate and cones do not. When measurements are made of the intensity (ΔI) of an incremental flash that can just be detected on a steady background field of intensity I, the cones normally continue to obey Weber's law (ΔI/I = constant) at very high intensities when nearly all their pigment has been bleached away; but when only a very small fraction of rhodopsin, the rod pigment, has been bleached the incremental threshold for rods begins to rise more rapidly than is predicted by Weber's law. This saturation of the rod signal can be observed when, by careful choice of wavelengths and by exploitation of the directional selectivity of the cones, the rod response is followed to high intensities in normal observers¹; or when measurements are made on rod monochromats, whose retinae completely, or almost completely, lack cone receptors^2–4. Alpern, Rushton and Torii⁵ have suggested that it is bleaching that prevents saturation in the cones, for the receptor signal depends on the number of quanta actually absorbed and the latter quantity will tend to a limit as more and more pigment is bleached by steady background fields of increasing intensity. Cone mechanisms do saturate when intense, brief flashes are delivered to the unbleached retina^5–6. We show here that the blue-sensitive receptors, which are normally thought to be cones, can be saturated by steady fields. In this respect they resemble the rods rather than the red- or green-sensitive cones.