Cortical processing of configurally perceived odor mixtures.
Résumé
Most natural odors are not composed of a single volatile chemical species, but rather are mixtures of two to hundreds of different volatile molecules, the perception of which is dependent on the identity and relative concentrations of the components. Changing either the identity or ratio of components can led to shifts between configural and elemental perception of the mixture. For example, a 30/70 ratio of ethyl isobutyrate (a strawberry scent) and ethyl maltol (a caramel scent) is perceived as pineapple by humans - a configural percept distinct from the components. In contrast, a 68/32 ratio of the same odorants is not perceived configurally but elementally, and is not identified as pineapple scent but as the component odors. Data from a variety of assays, either involving explicit training or not, suggest a similar configural (or at least partially configural) perception at this same 30/70 ratio of EI and EM mixture when tested in newborn rabbits, and adult honey bees, rats and mice. Here, we examined single-unit and single-unit ensemble responses in the anterior piriform cortex (aPCX) and posterior PCX (pPCX) of both rats and mice to the EI and EM mixture. We compared responses to the configural mixture to those evoked by the same chemical mixture presented at a different ratio that evokes an elemental percept, as well as to the individual chemical components. Hierarchical cluster analyses of the results suggest that in both the rat and mouse aPCX, the configural mixture was coded as distinct from both components, while the elemental mixture was coded as very similar to the components. Thus, aPCX ensemble coding matched known odor perceptual characteristics. In contrast, known mixture perception did not predict pPCX ensemble coding in either rats or mice, further demonstrating the distinction between odor coding in the aPCX and pPCX.