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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.