Continuous Spatial Representations in the Olfactory Bulb may Reflect Perceptual Categories

Abstract

In sensory processing of odors, the olfactory bulb is an important relay station, where odor representations are noise-filtered, sharpened, and possibly re-organized. An organization by perceptual qualities has been found previously in the piriform cortex, however several recent studies indicate that the olfactory bulb code reflects behaviorally relevant dimensions spatially as well as at the population level. We apply a statistical analysis on 2-deoxyglucose images, taken over the entire bulb of glomerular layer of the rat, in order to see how the recognition of odors in the nose is translated into a map of odor quality in the brain. We first confirm previous studies that the first principal component could be related to pleasantness, however the next higher principal components are not directly clear. We then find mostly continuous spatial representations for perceptual categories. We compare the space spanned by spatial and population codes to human reports of perceptual similarity between odors and our results suggest that perceptual categories could be already embedded in glomerular activations and that spatial representations give a better match than population codes. This suggests that human and rat perceptual dimensions of odorant coding are related and indicates that perceptual qualities could be represented as continuous spatial codes of the olfactory bulb glomerulus population.

Keywords: glomeruli; memory organization; odor quality; olfaction; olfactory bulb; perception; population coding; spatial coding.

Figure 1

Ventral-centered chart of the glomerular layer with anatomical locations.

Figure 2

Representations of perceptual categories. Representations for odors (indicated in yellow) corresponding to floral (A), cleaner (B), foul (C), woody (D), medicinal (E), nutty/spicy (F), balsamic (G), fruity (H), herbacious (I), and musky (J).

Figure 3

Absolute error of fit between coding spaces (D_P, D_S) and perceptual spaces (D_Chrea, D_{BH small}, D_{BH big}). Perceptual spaces constructed from pair-wise distances reduced to two-dimensions by multi-dimensional scaling. Please note for comparison that what matters is the relation between categories, i.e., the relative distances. (A) Shows the perceptual space in Chrea (2004), as per Zarzo (2008). (B) Shows the space as given by distances between spatial codes. (C) Shows the space as given by distances between activations of the entire glomerular layer, independent of spatial arrangement of glomeruli. The panel in the middle compares the perceptual space for the BH small data set (D), the distances between the categories in the spatial codes (E), and the population codes (F). The lower panel shows the perceptual ordering for the BH big data set (G), these distances between spatial codes (H), and population codes (I). In (I), fruity and green are indistinguishable.