The influence of temporal frequency and adaptation level on receptive field organization of retinal ganglion cells in cat

Abstract

The discharges of single X and Y ganglion cells (distinguished by a test of linearity of spatial summation) were recorded in the optic tract of anesthetized, paralyzed cats. Fourier techniques were used to analyze the distribution of amplitudes of several component temporal frequencies in the maintained discahrge. X and Y cells were distinguished by their mean rates, but not by the amplitude or variability of other component frequencies. Sensitivites to moving sinusoidal gratings were measured by an automatic procedure in which stimulus contrast was adjusted to give the smallest modulation of discharge that reliably exceeded that of the relevant component frequency in the maintained discharge. Spatial contrast sensitivity curves of X cells and of on center Y cells could be described by a model of the receptive field as 2 concentric Gaussian sensitivity profiles representing the center and the antagonistic surrounding. Changes in temporal frequency altered the shapes of the spatial contrast sensitivity curves of most units. For X cells sensitivity at the optimum spatial frequency was greater at a temporal frequency of 10.4 Hz than at lower or higher temporal frequencies. The relative sensitivity to low spatial frequencies improved as temporal frequency ws raised from 0.16 to 20.8 Hz. The shapes of the contrast sensitivity functions of Y cells were less affected by changes in temporal frequency: at all spatial frequencies sensitivity was greater at 2.6 Hz than at lower or higher frequencies. The effect of temporal frequency upon the shape of the spatial contrast sensitivity curve could be explained by assuming that the center and surrounding changed their sensitivities without changing their characteristic radii. A simple model, using a temporal R-C filter in the surrounding pathway, predicted qualitatively similar changes in the shape of contrast sensitivity curves but failed to provide acceptable fits to the observations. A 2nd model, which assumed that surrounding signals are delayed by a fixed amount before being combined with those from the center, fitted the observations of most, but not all, X cells. Dark adaptation produced changes in the shape of the spatial contrast sensitivity curve consistent with a reduction in the relative sensitivity of the surrounding, but did not bring about systematic changes in the space constants of the best-fitting theoretical curves. The effects of adaptation level upon contrast sensitivity were expressed as plots of increment-threshold against mean illumination. The shallowest of these curves, obtained for the optimum spatial stimulus moving at about 10 Hz, had slopes averaging 0.77. Decreases in spatial or temporal frequency increased the slopes of the curves.