Synergy from Silence in a Combinatorial Neural Code

Abstract

The manner in which groups of neurons represent events in the external world is a central question in neuroscience. Estimation of the informationencodedbysmallgroupsofneuronshasshownthatinmanyneuralsystems,cellscarrymildlyredundantinformation.These measures average over all the activity patterns of a neural population. Here, we analyze the population code of the salamander and guinea pig retinas by quantifying the information conveyed by specific multicell activity patterns. Synchronous spikes, even though they are relatively rare and highly informative, convey less information than the sum of either spike alone, making them redundant coding symbols.Instead,patternsofspikinginonecellandsilenceinothers,whicharerelativelycommonandoftenoverlookedasspecialcoding symbols, were found to be mostly synergistic. Our results reflect that the mild average redundancy between ganglion cells that was previously reported is actually the result of redundant and synergistic multicell patterns, whose contributions partially cancel each other when taking the average over all patterns. We further show that similar coding properties emerge in a generic model of neural responses, suggesting that this form of combinatorial coding, in which specific compound patterns carry synergistic or redundant information, may exist in other neural circuits.