THE STRUCTURE AND PRECISION OF RETINAL SPIKE TRAINS

Assessing the reliability of neuronal spike. trains is fundamental to an understanding of the neural code. We measured the reproducibility o f retinal responses to repeated visual stimuli. In both tiger salamand er and rabbit, the retinal ganglion cells responded to random flicker with discrete, brief periods of firing. For any given cell, these firi ng events covered only a small fraction of the total stimulus time, of ten less than 5%. Firing events were very reproducible from trial to t rial: the timing jitter of individual spikes was as low as 1 msec, and the standard deviation in spike count was often less than 0.5 spikes. Comparing the precision of spike timing to that of the spike count sh owed that the timing of a firing event conveyed several times more vis ual information than its spike count. This sparseness and precision we re general characteristics of ganglion cell responses, maintained over the broad ensemble of stimulus waveforms produced by random flicker, and over a range of contrasts. Thus, the responses of retinal ganglion cells are not properly described by a firing probability that varies continuously with the stimulus. Instead, these neurons elicit discrete firing events that may be the fundamental coding symbols in retinal s pike trains.