Incremental responses to light recorded from pigment epithelial cells and horizontal cells of the cat retina.

1. Rod-dependent incremental responses were recorded intracellularly in both pigment epithelial cells and horizontal cells of the cat retina. They were elicited by test flashes which were superimposed on background flashes after a delay.2. In pigment epithelial cells smaller test responses were produced as background intensity was raised. The incremental sensitivity function was linear for about 1.4 log units, with a slope of 0.86, and the approach of saturation occurred at about 2.5 log td scotopic.3. The amplitude of pigment epithelial test responses could be estimated from the dark-adapted amplitude-log intensity function obtained with single flashes. Test flashes produced the voltage increment predicted by the slope of this function just above the point on the curve equal to the background intensity. The pigment epithelial response to a test flash, therefore, is the response expected if the background were presented alone and made more intense by the amount of the test flash.4. Rod-dependent incremental sensitivity functions of horizontal cells closely resembled the ones obtained from pigment epithelial cells.5. It was concluded that the adaptive effects observed in pigment epithelial cells originated in individual rods. These effects arose from the compressive nature of the dark-adapted amplitude-intensity function. In horizontal cell responses these effects may be modified by the failure of the background response to maintain its initial voltage.