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Nature of the maintained discharge of Q, X, and Y retinal ganglion cells of the cat.

Cat retinal ganglion cells with center-surround receptive fields have an irregular discharge whose rate is altered by visual stimulation. In assessing the detectability of stimulus-induced changes in the discharge, a consideration of the power spectral density of the discharge is helpful. The power spectral density of Q, X, and Y cells is flat at low frequencies, rises to a peak at the mean frequency of firing, and then decays away at higher frequencies in an oscillatory manner to an asymptotic level equal to the mean rate of discharge. Measured spectra correspond closely with spectra predicted by a renewal-point process with gamma-distributed intervals. When the rate of the discharge is altered by visual stimulation, the spectral density at low frequencies remains roughly constant. Assuming that it is the noise power at these frequencies that is effective in limiting the detectability of visual stimuli, it appears that at the retinal level the irregularity of the discharge can be treated as an additive noise.