Lack of correlation between odor composition and neuron response in the olfactory cortex of mice

In the mammalian brain, sensory circuits are usually organized in a topographical way, meaning that, for a given brain region, neighboring neurons respond to stimuli close to each other in their sensory space. Olfaction is a notable exception to this rule; projections to the olfactory system are sparse and dispersed, leading to no apparent topography. Here, we assessed the presence of a topographical map in the mouse olfactory cortex, using a previously generated online dataset of neuronal recordings. The dataset consisted of about 1,800 olfactory cortical neurons collected from 10 mice, stimulated with a panel of 15 odorants. If there is no odor topography in the olfactory cortex, there should be no correlation between the chemical composition of odorants and their evoked neuronal response. To test this hypothesis, we first calculated odor similarity between each pair of odorants, using their chemical traits. Then, for each odor pair, we computed the similarity between their evoked neuronal responses. Finally, we assessed the relationship between odor similarity and neuronal response similarity. We found little to no correlation between the two variables (R² averaged across all mice tested: 0.015), which suggests the lack of topography in the murine olfactory cortex and opens new questions into what other variables might play a role in odorant distinction.