Effects of Adaptation on the Lateral Geniculate Response to Light Increment and Decrement*

Response characteristics of two types of cells in the lateral geniculate nucleus of the squirrel monkey have been examined in experiments in which the luminance of a stimulus is shifted in stepped increments and decrements from an adaptation luminance. These two types of cells are found to show opposite changes in discharge frequency in response to shifts in stimulus luminance: some respond with an increase in firing rate to increases in luminance and show a decrease in response rate when luminance is decreased; others behave in the opposite fashion. The magnitude of change of the response is graded according to the amount of change of the stimulus. For any cell, the direction of change of the response for a given change of stimulus is dependent on the adaptation luminance. The total range of change of luminance over which any cell shows a good differentiation is usually not more than ±1 log unit around the adaptation luminance. An analysis of the discriminatory ability of these units as a function of adaptation luminance and the amount of change of the stimulus is presented.