Abstract

Vertebrate vision is mediated by two different types of photoreceptors. Although the range for optimal responsivity of rods and cones differs considerably, both respond over a retinal illuminance range of approximately 1–1000 trolands (T), the visual environment typically provided by modern civilization. Over this range, subjective experience as well as psychophysically measured visual capacity of normal humans relates almost exclusively to cones (e.g. Hecht, 1938). But rods greatly outnumber cones. As a consequence, the corneal electroretinogram (ERG), a field potential reflecting the sum of all transretinal components of stimulus-induced currents, is dominated by the much more numerous rods and other retinal cells involved in the scotopic pathway (Rodieck, 1973; Armington, 1974). This is particularly true of clinical ERGs which are typically elicited by brief duration, high intensity, ganzfeld stimuli and recorded by amplifiers with bandpass set between about 1–1000 Hz. Unless flash intensity is great and/or flashes are presented against a prevailing background level which saturates rods, such ERGs overwhelmingly reflect rod activity. Even with rod–saturating retinal illuminances exceeding 1000 scotopic trolands (T_s), the small amplitude of the cone response makes computer averaging essential. However, the responses so obtained have little obvious relationship to the visual capacity measured by "less objective" psychophysical procedures, since they are usually obtained with uncomfortably bright stimuli rarely encountered in everyday life.

© 1991 Optical Society of America