Independent and redundant information in V1: different stimulus types

Daniel S. Reich1,2, Ferenc Mechler2 and Jonathan D. Victor2

1Rockefeller University
2Department of Neurology & Neuroscience, Weill Medical College, Cornell University

Recently (Society for Neuroscience 2000), we reported that nearby neurons in primary visual cortex (V1), recorded with tetrodes, convey nearly independent stimulus-related information about rapidly modulated, pseudorandom (m-sequence) checkerboard stimuli. We also reported that the instantaneous rate of information transfer -- particularly about the fine spatiotemporal details of a stimulus -- decreases measurably if responses are pooled, through averaging or summation, across groups of nearby neurons. At the same time, information about another stimulus attribute -- contrast -- is not diminished after pooling. Here, we perform a similar analysis with simpler stimuli -- sinusoidal gratings that either drift or appear and disappear abruptly -- and find comparable results. Of particular interest, we find that information is transmitted about contrast in a stimulus-independent fashion. When stimuli vary in both contrast and spatiotemporal pattern, however, some of the transmitted information cannot be unambiguously related to one of these stimulus attributes but is instead confounded between them. We also find that nearby neurons that transmit information independently in response to one type of stimulus also tend to transmit information independently in response to other types, meaning that independent information transmission is more a property of the neurons being studied than of the stimuli used to drive them. In rare cases, the same neurons that transmit information independently in response to the pseudorandom checkerboard stimulus transmit information synergistically in response to grating stimuli, particularly stationary gratings. (Synergistic information transmission means that information is available in the pooled responses that is not available in the responses of individual neurons.) Conventionally, nearby neurons in sensory cortex, which tend to be tuned to similar stimulus features, have been thought to convey redundant information in order to improve the signal-to-noise in their pooled responses. Our results, which reveal the responses of nearby neurons to be largely independent, lend support to a contrary notion -- that the brain may be able to take advantage of the vastly larger signaling capacity of a distributed code in which different neurons, even within a local population, encode different things.