  To evaluate the nature of the neural code in the cerebral cortex, we have
used a combination of theory and experiment to assess how information is
represented in a realistic cortical population response. We have shown how a
sensory stimulus could be estimated on a biologically-realistic time scale,
given brief individual responses from a population of neurons with similar
response properties. For neurons in extrastriate motion area MT, a
combinatorial code, one that keeps track of the cell identity of action
potentials and silences in individual neurons across the population, carries
twice as much information about visual motion as does spike count averaged over
the same group of cells. The combinatorial code is more informative because of
the diverse firing rate dynamics of MT neurons in response to constant motion
stimuli, and is robust to neuron-neuron correlations. We provide a theoretical
motivation for these observations that challenges commonly held ideas about the
nature of cortical coding at the level of single neurons and neural
populations.
