Variability in the maintained discharges of retinal ganglion cells.

The statistics of maintained discharges of goldfish and cat retinal ganglion cells are examined, and simulations are made with integrate-and-fire models. Actual interval distributions are best fitted with a hyperbolic gamma distribution; however, interval distributions derived from simulations demonstrate that this need not imply a skewed variable signal. The high-pass filtering in the maintained discharges of ganglion cells is weaker than would be expected from the transfer function for photic signals; this phenomenon, as well as the observation of two peaks in some of the interval distributions, is modeled by transient changes in the firing threshold after each impulse. Increases in firing rate owing to changes in the level of background illumination are accompanied by declines in the coefficient of variation; simulations predict an inverse-square-root relationship between the rate and the coefficient of variation, an approximation that may be improved by again assuming changes in the firing threshold.