Nonlinearity and oscillations in X-type ganglion cells of the cat retina

Intracellularly recorded light-responses of X-type ganglion cells in the cat retina were separated, with the help of a wavelet method, into "slow" membrane ("G")-potentials and the corresponding spike trains. In response to sinusoidally modulated high intensity light spots with different sizes and frequencies, X-type ganglion cells show both oscillations correlated with the stimulus frequency and other, faster, oscillations that were not always locked to the stimulus. A forced van der Pol oscillator model with stimulus-dependent coefficients proved to describe the empirical findings quite well. A linearity-coefficient of the equations indicates strong nonlinearity at a temporal frequency of 8 Hz with spot sizes on the order of 0.5-0.7 deg and decreasing nonlinearity at lower temporal frequencies or smaller spot sizes, while the faster oscillations become more prominent. We could not determine whether the oscillations are intrinsic to the cell-membrane or generated by (or in interaction with) the preganglionic retinal meshwork. The results show that X-cell spike-trains can contain oscillations that are not phase-locked to the stimulus and that are therefore virtually invisible after stimulus synchronous averaging. It is not likely that these retinal oscillations directly induce the well described oscillations in cat visual cortex, since they usually fall in a different frequency range.