A statistical property of fly odor responses is conserved across odors

Significance Although the brain has long been considered to use combinatorial codes (a defined population of neurons whose pattern of rates is specific to a particular object), in no case has the code been well-characterized. Here I characterize the fly odor code by showing it is maximum entropy; what this means is that the response to all odors is statistically the same. This characterization is a significant first step in cracking the odor code. I have reanalyzed the data presented by Hallem and Carlson [Hallem EA, Carlson JR (2006) Cell 125(1):143–160] and shown that the combinatorial odor code supplied by the fruit fly antenna is a very simple one in which nearly all odors produce, statistically, the same neuronal response; i.e., the probability distribution of sensory neuron firing rates across the population of odorant sensory neurons is an exponential for nearly all odors and odor mixtures, with the mean rate dependent on the odor concentration. Between odors, then, the response differs according to which sensory neurons are firing at what individual rates and with what mean population rate, but not in the probability distribution of firing rates. This conclusion is independent of adjustable parameters, and holds both for monomolecular odors and complex mixtures. Because the circuitry in the antennal lobe constrains the mean firing rate to be the same for all odors and concentrations, the odor code is what is known as maximum entropy.

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