Abstract
We present a mathematical model for signal transformation by the receptor-horizontal-bipolar cell triad, which gives a good prediction of the spatial-contrast modulation transfer of the visual system at different levels of retinal illuminance. We assume that rod-driven and cone-driven bipolar output signals, sr and sc, depend on the mean rates of photoisomerisations in rods and cones, yr and yc, and their derivatives, yr' and yc', as expressed by the equations sr= cr yr'/( dr yr+ dc yc), and sc= cc yc'/( dr yr+ dc yc), where cr, cc, dr, dc are constant coefficients. The total output signal is S= sr+ sc. During scotopic vision sr >> sc and spatial frequency characteristics are determined by the rod system. When illumination increases and rods saturate, contrast sensitivity is determined by the cone system. A formula for calculation of the spatial-contrast modulation transfer function is derived with mean illumination as a parameter. The predictions of the model are in good agreement with experimental data measured at various levels of retinal illumination.
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