d from one cell to the other or has a common origin. The gamma-r renewal process model, often used in the analysis of visual-neuron interevent intervals, describes certain short-term features of the RGC and LGN data reasonably well but fails to account for the long-duration correlation. We present a new model for visual-system nerve-spike firings: a gamma-r renewal process whose mean is modulated by fractal binomial noise. This fractal, doubly stochastic point process characterizes the statistical behavior of both RGC and LGN data sets remarkably well. 1997 Optical Society of America [S0740-3232(97)00202-0] 1. INTRODUCTION The sequence of action potentials recorded from cat retinal ganglion cells 1--16 (RGC's) and lateral-geniculatenucleus (LGN) cells 17--21 remains irregular even when the retina is thoroughly adapted to a steady stimulus of fixed luminance. The statis

Vol. 2, No. 2/February 1985/J. Opt. Soc. Am. B 275

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Teich et al. Vol. 14, No. 3/March 1997/J. Opt. Soc. Am. A 529 Fractal character of the neural spike train in the visual system of the cat