Visual transduction in retinal rods of the monkey Macaca fascicularis

The small size and fragility of primate photoreceptors have impeded study of their neural signals by single cell recording. Although physiological information is meagre, psychophysical experiments indicate remarkable performance, including single photon detection by human rods1. What electrical change results from absorption of a quantum? How do the photoreceptors themselves contribute to the sensitivity and temporal resolution of the primate visual system? We report here a study of transduction in single rods of the cynomolgus monkey. From results of colour matching tests2 and microspectrophotometry3 the light receptors of the cynomolgus monkey are thought to be similar to those of man. We have determined the size and shape of the quantal response, and related parameters of the transduction mechanism. These measurements provide a physiological basis for the single photon detection, saturation, and limited temporal resolution characteristic of rod behaviour in psycho-physical experiments. Spectral sensitivity measurements over the entire visible region extend previous determinations of the spectral absorption of rhodopsin, and allow assessment of the factors that determine human scotopic sensitivity.