Changes in temporal filtering with light adaptation

Purpose. Over the past several years, many investigators have attempted to develop quantitative models of light adaptation. Our goal was to collect and summarize how the temporal frequency response of the center mechanism of cat X ganglion cells changes over the full operational range of light levels encountered by this animal. Methods. Tungsten-in-glass microelectrodes recorded the discharge of X cells from the optic tract. Sinusoidal gratings of a high spatial frequency were used to isolate the response of the cell's center mechanism and by drifting the gratings at different speeds the cell's temporal frequency response was measured. Different light levels were obtained by attenuating the grating's mean luminance with neutral density filters. Results and Conclusions. A model based on the one presented by Diller, Troy, & Frishman (1994, IOVS, 35,4: 2124), simplified to contain only five parameters, was found to fit the photopic data well. A slightly different five parameter model fitted the scotopic data. For the scotopic range, retinal illuminances less than 1.3 log cat td, the form of an X cell's temporal frequency response was found to be independent of light level: the changes observed with light level could be accounted for simply by changes in peak responsivity and delay in the response. The dependence of peak responsivity and delay upon light level were quantified. In the photopic range, the form of the temporal frequency response was found to change with light level. While peak responsivity was found to remain roughly constant. The change in shape of the frequency response is best characterized as an increase in bandwidth which can be quantified well by changes in just one of the five parameters of our model.