Perceptual correlates of neural representations evoked by odorant enantiomers

Spatial activation patterns within the olfactory bulb are believed to contribute to the neural representation of odorants. In this study, we attempted to predict the perceptions of odorants from their evoked patterns of neural activity in the olfactory bulb. We first describe the glomerular activation patterns evoked by pairs of odorant enantiomers based on the uptake of [(14)C]2-deoxyglucose in the olfactory bulb glomerular layer. Using a standardized data matrix enabling the systematic comparison of these spatial odorant representations, we hypothesized that the degree of similarity among these representations would predict their perceptual similarity. The two enantiomers of carvone evoked overlapping but significantly distinct regions of glomerular activity; however, the activity patterns evoked by the enantiomers of limonene and of terpinen-4-ol were not statistically different from one another. Commensurate with these data, rats spontaneously discriminated between the enantiomers of carvone, but not between the enantiomers of limonene or terpinen-4-ol, in an olfactory habituation task designed to probe differences in olfactory perception.

Multidimensional chemotopic responses to n-aliphatic acid odorants in the rat olfactory bulb

In an effort to understand the means by which similar chemical odorants are encoded in the mammalian brain, we exposed rats to a homologous series of n-aliphatic acids and mapped the response of the entire olfactory bulb glomerular layer by using a high-resolution [14C]-2-deoxyglucose uptake technique. We found that these similar odorants evoked spatially clustered but distinct responses in the bulb that changed systematically with carbon chain length. In addition to these chemotopic responses, different odorants within the series evoked systematic differences along two other dimensions: amount of deoxyglucose uptake and extent of the glomerular layer showing high activity. Increases along these two dimensions also were correlated with increasing carbon number. The focal glomerular responses were mirrored by responses in deeper bulb layers. Decreasing the odorant concentration decreased the deoxyglucose uptake within focal regions. The focal regions of activity occurred in pairs involving both medial and lateral representations in the bulb, perhaps reflecting the paired medial and lateral projections of olfactory sensory neurons expressing specific types of odorant feature receptor proteins. The observed spatial pattern of response also may explain both the failure of some bulb lesions to interfere with behavioral olfactory responses and the success of other lesions in blocking olfactory responses. These data support a model of parallel, distributed processing of odorants along multiple dimensions. They also support the notion that analyses of the spatial relationships among odorant responses in the olfactory bulb can demonstrate aspects of the mechanism for odor chemical coding.