ON THE COMPLEX DYNAMICS OF INTRACELLULAR GANGLION-CELL LIGHT RESPONSES IN THE CAT RETINA

We recorded intracellular responses from cat retinal ganglion cells to sinusoidal flickering lights, and compared the response dynamics with a theoretical model based on coupled nonlinear oscillators, Flicker r esponses for several different spat sizes were separated in a ''smooth '' generator (G) potential and corresponding spike trains. We have pre viously shown that the G-potential reveals complex, stimulus-dependent , oscillatory behavior in response to sinusoidally flickering lights. Such behavior could be simulated by a modified van der Pol oscillator. In this paper, we extend the model to account for spike generation as well, by including extended Hodgkin-Huxley equations describing local membrane properties. We quantified spike responses by several paramet ers describing the mean and standard deviation of spike burst duration , timing (phase shift) of bursts, and the number of spikes in a burst. The dependence of these response parameters on stimulus frequency and spot size could be reproduced in great detail by coupling the van der Pol oscillator and Hodgkin-Huxley equations. The model mimics many ex perimentally observed response patterns, including non-phase-locked ir regular oscillations. Our findings suggest that the information in the ganglion cell spike train reflects both intraretinal processing, simu lated by the van der Pol oscillator, and local membrane properties des cribed by Hodgkin-Huxley equations. The interplay between these comple x processes can be simulated by changing the coupling coefficients bet ween the two oscillators. Our simulations therefore show that irregula rities in spike trains, which normally are considered to be noise, may be interpreted as complex oscillations that might carry information.