Variability and Information in a Neural Code of the Cat Lateral Geniculate Nucleus

1 Keck Center for Integrative Neuroscience and Department of Physiology, 2 Sloan-Swartz Center for Theoretical Neurobiology, and 3 Department of Otolaryngology, University of California, San Francisco, California 94143-0444 Liu, Robert C., Svilen Tzonev, Sergei Rebrik, and Kenneth D. Miller. Variability and Information in a Neural Code of the Cat Lateral Geniculate Nucleus. J. Neurophysiol. 86: 2789-2806, 2001. A central theme in neural coding concerns the role of response variability and noise in determining the information transmission of neurons. This issue was investigated in single cells of the lateral geniculate nucleus of barbiturate-anesthetized cats by quantifying the degree of precision in and the information transmission properties of individual spike train responses to full field, binary (bright or dark), flashing stimuli. We found that neuronal responses could be highly reproducible in their spike timing (~1-2 ms standard deviation) and spike count (~0.3 ratio of variance/mean, compared with 1.0 expected for a Poisson process). This degree of precision only became apparent when an adequate length of the stimulus sequence was specified to determine the neural response, emphasizing that the variables relevant to a cell's response must be controlled to observe the cell's intrinsic response precision. Responses could carry as much as 3.5 bits/spike of information about the stimulus, a rate that was within a factor of two of the limit the spike train could transmit. Moreover, there appeared to be little sign of redundancy in coding: on average, longer response sequences carried at least as much information about the stimulus as would be obtained by adding together the information carried by shorter response sequences considered independently. There also was no direct evidence found for synergy between response sequences. These results could largely, but not entirely, be explained by a simple model of the response in which one filters the stimulus by the cell's impulse response kernel, thresholds the result at a fairly high level, and incorporates a postspike refractory period.