Spectral sensitivity of single neural units in the bullfrog retina.

Abstract

Nerve impulses from retinal units, presumably ganglion cells, were elicited by calibrated light flashes of various luminances, wavelengths, and durations, delivered to the eye of the intact bullfrog under scotopic and photopic conditions. Although several response measures were utilized, latency to the first impulse was particularly useful. A simple linear function relating reciprocal of latency to log light luminance was associated with the dark-adapted state and more complex latency-luminance functions were associated with the light-adapted state. Spectral sensitivity curves were derived from these latency-luminance functions by determining the relative energy at the various wavelengths required to produce a criterion latency. A wide variety of relative spectral-sensitivity curves were found. The average shift of spectral sensitivity toward the longer wavelengths (Purkinje shift) between scotopic and photopic conditions was in approximate quantitative agreement with a shift from the rhodopsin to the iodopsin absorption spectrum. The Purkinje shift was also seen in the results obtained from some individual neural units. Among the variety of spectral sensitivity curves obtained from the 20 dark-adapted units, a homogeneous group of curves (11 units) was identified which was well fitted by the rhodopsin absorption curve. Of the 16 light-adapted units, one group (7 units) was roughly fitted by the iodopsin curve. Of special interest to color theory are the units not included in the rhodopsin or iodopsin groups. While none of the curves were completely fitted by Granit’s modulators, many of the sensitivity peaks were at wavelengths that correspond to his green and red modulators (frog). Relatively narrow spectral-sensitivity curves with maxima below 450 mμ were obtained from several of the dark-adapted units; these were reliably different from Granit’s blue modulator (frog) and were approximately fitted by Dartnall’s blue-sensitive pigment.