A Chemical Odor Map in Cortex

Abstract

Odorous molecules trigger specific percepts. Appropriate assignment of odorants to corresponding percepts relies on the brain’s ability to both discriminate distinct odorants, as well as generalize odorants that share chemical features. Although odorants evoke correlated activity across receptor expressing neurons in the olfactory epithelium and glomeruli residing in the olfactory bulb, it is unclear how the chemical attributes of odors are encoded in cortex to support both discrimination and generalization. To address this question, we have developed a preparation that allows us to monitor odor representations in awake mice across cortical layers in the main region of the brain devoted to olfactory processing, the Piriform Cortex (PCx). Using sets of odorants rationally designed to tile different regions of chemical odor space, we demonstrate that responses of neural ensembles in both Layer II (LII) and Layer III (LIII) of PCx preserve information about the physicochemical relationships between odor molecules in a manner conserved across individual mice. LII and LIII differ in terms of their correlation structure, reliability, odor sensitivity, and odor preferences, suggesting parallel representational strategies optimized for odor discrimination and generalization. These findings suggest that PCx harbors an invariant relational map of odorant space. The differential expression of this map across distinct cortical layers further suggests complementary contributions to perception and odor-guided behavior.