Photoreceptor input and temporal summation in turtle retinal ganglion cells.

The relation between photoreceptor input and temporal summation in ganglion cells was investigated in the optical nerve of fresh-water turtle, Pseudemys. Action spectra derived for constant, high-criterion responses showed most cells to be maximally sensitive to 620 nm light, indicating dominant input from red-sensitive cones. At a lower criterion, sensitivity to 520 nm light greatly increased indicating an increase in rod input. Temporal summation curves for these intensity-dependent cells showed reciprocity between flash intensity and duration up to 128 msec for high-criterion summation curves. Low-criterion summation curves showed reciprocity up to the critical duration of 285 msec. Cells maximally sensitive to 620 nm light at all intensities often showed secondary sensitivity to 560 nm light, and were fitted by action spectra taken from red- and green-sensitive cones. Temporal summation curves for these cells had critical durations near 100 msec. Some cells responded best to light near 520 nm at all intensities and demonstrated little or no input from red-sensitive cones. These cells were fitted well by the action spectrum of rods and showed the longest critical durations of all at 375 msec. Chromatic input and temporal characteristics are intimately related: red-cone-dominated ganglion cells have the shortest critical durations while rod-dominated cells shows much longer ones. These findings are in keeping with psychophysical determinations of critical durations in this same animal.