THE DYNAMICS OF LIGHT ADAPTATION IN CAT HORIZONTAL CELL RESPONSES

In order to model the dynamic properties of light adaptation processes in cat horizontal (H-) cells, the time course of the gain adjustment following changes in the mean illumination level was studied. H-cell r esponses were recorded intracellularly in the optically intact, in viv o, eye of the cat. The light stimulus consisted of two spots, a large background spot (8.8 deg diameter) and a concentrically arranged small er test spot (3.9 deg). The background was either square wave or sine wave modulated in intensity at a frequency of 0.2-1 Hz. The instantane ous value of the response gain was measured with brief flashes (10 mse c) of the test spot, generated repetitively at a frequency of 5 or 10 Hz. Modulation of the background intensity, at a constrast of 0.6 and in the photopic range, effectively induces a modulation of the gain. T he readjustment of the gain by a stepwise increase or decrease in back ground illumination is completed within about 200 msec. The amplitude of the gain modulation due to a 0.5 Hz background flicker is quantitat ively comparable to that measured between steady illumination levels. Dynamic changes of the gain at low frequency stimuli therefore, have t o be taken into account in modelling H-cell responses. For sinusoidal modulations of the background luminance the time course of gain adjust ment is quantified by the phase shift of the gain modulation relative to background intensity modulation. The results, together with those d escribed in two preceding papers, are used to test and discuss several light adaptation models that have been proposed previously. It was fo und that light adaptation in cat H-cells is described more adequately by a de Vries-Rose type of adaptation model than by a Weber type of li ght adaptation.