Expression of ?retinal? contrast gain control by neurons of the cat's lateral geniculate nucleus

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Abstract
This paper describes the temporal tuning of cells in the lateral geniculate nucleus of the cat (27 X cells, 51 Y cells) and how this changes with stimulus contrast. Drifting sinusoidal gratings of optimal spatial frequency were presented at 7 temporal frequencies (0.5, 1, 2, 4, 8,16 and 32 Hz) and 4 contrasts (10, 20, 40, 80%). For some cells response growth at all temporal frequencies was proportional to changes in contrast. Thus, their temporal tuning functions, on log-log axes, were displaced vertically with increasing contrast. This shift also largely characterizes the response to low temporal frequencies of the other neurons studied. For these other cells, however, responses to high temporal frequencies grew disproportionately large with increasing contrast generally causing their tuning functions to change shape. Typically the peaks of these functions shifted to higher frequencies at higher contrasts. Most of the X cells studied displayed behavior of the first type, while Y cells usually followed the second pattern. This qualitative impression was confirmed quantitatively. Cubic spline functions were fit to the temporal tuning functions obtained at different contrast levels and the peaks of the curves were extracted. X and Y cells preferred similar temporal frequencies at low contrast levels (X mean=8.1 Hz; Y mean=8.4 Hz) but Y cell values were significantly higher at higher contrasts (80%) (X mean= 12.0 Hz; Y mean=16.8 Hz). These contrast-dependent changes in the temporal characteristics of geniculate cells resemble those described for retinal ganglion cells by Shapley and Victor (1978 and subsequent). Thus, the gain control behavior of geniculate cells probably reflects the temporal properties of their retinal inputs with relatively little modification.