Interpreting trans-retinal recordings of spectral sensitivity.

A quantitative model is developed to describe spectral sensitivity functions recorded extracellularly from heterogeneous populations of receptors in different states of adaptation. This treatment identifies the most important influences and clarifies several general features of experimental results. The shapes of retinal spectral sensitivity curves in different states of chromatic adaptation depend in predictable fashion on whether the primary effect of the adapting light on individual receptors is to decrease Vmax (response compression) or to increase the quantum demand for half-saturation. Some response compression is necessary in order for one or more receptors to drop out of the response at modest levels of adaptation. The apparent ease of adaptation also depends on the criterion voltage, particularly in the presence of response compression. The technique of selective adaptation of the ERG is capable of revealing the presence of receptors that comprise only a few percent of the total population. The short wavelength absorption of all visual pigments normally makes it impossible to use uv or violet light to adapt selectively those receptors with maximal sensitivity in the uv or violet region of the spectrum while sparing receptors with maximal sensitivity at longer wavelengths. The presence of cone oil droplets absorbing at short wavelengths, however, can effectively screen visual pigments in some of the receptors from uv or violet adapting lights.