Photoreceptor signals at visual threshold

Abstract

Electrical responses of cone photoreceptors in the retina of the freshwater turtle have been characterised for flashes and steps of light in darkness and in the presence of background light^1–4. These intracellular measurements have been combined with the behavioural increment threshold curve to yield an estimate of 5–10 µV for the signal developed in a cone when the turtle can just detect an increment flash⁵. The signal developed when the cones under the stimulus image are dark-adapted is of interest, for its measurement would help to explain how known physiological processes subserve visual detection for a variety of photic conditions. The effective quantal absorption of dark-adapted, red-sensitive cones of Pseudemys scripta elegans for a stimulus that the turtle can just detect is reported here. By combining this result with previous electrical measurements on red-sensitive cones of this species, an estimate of 35–70 µV is obtained for the signal developed in a dark-adapted cone at behavioural threshold. This larger signal required for detection of a flash in darkness is of particular interest in view of the recent observation⁶ that the intrinsic noise of turtle cones in darkness is larger than that of illuminated cones.