Continuous spatial representations in the olfactory bulb may reflect perceptual categories

Benjamin Auffarth

doi:10.3389/fnsys.2011.00082

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.

Highlights

The sense of smell is crucial for survival, including for aspects such as maternal bonding, mating, kinship recognition, territorial defense, and aggressive behavior (Savic et al, 2001; SanchezAndrade and Kendrick, 2009; Tirindelli et al, 2009; Martin et al, 2011; Schleich and Zenuto, 2010; Dehnhard, 2011)
Sensory nerves from olfactory receptor neurons (ORNs) converge and terminate on the surface of the OB in spheroidal neuropil structures, called glomeruli, where they synapse with periglomerular interneurons and OB output neurons (MT cells; Kosaka et al, 1998)
We found that errors of fit between spatial representations were similar to perceptual orderings by human subjects and were smaller than for population codes

Summary

Introduction

The sense of smell is crucial for survival, including for aspects such as maternal bonding, mating, kinship recognition, territorial defense, and aggressive behavior (Savic et al, 2001; SanchezAndrade and Kendrick, 2009; Tirindelli et al, 2009; Martin et al, 2011; Schleich and Zenuto, 2010; Dehnhard, 2011). Each ORN expresses only one of a possible 1,200 OR and axons from ORNs that express the same OR are bundled in approximately two glomeruli at stereotyped positions (typically one on each of both mirror-symmetric sides) of about 1800 in each olfactory bulb (OB, Zhang et al, 2007). In this fashion, each odorant elicits a specific map of glomerular activation (Ressler et al, 1994). Mitral and tufted cells (MT cells), project to primary olfactory cortical areas, such as olfactory cortical areas, such as the anterior olfactory nucleus, piriform cortex, olfactory tubercle and lateral entorhinal cortex, and the amygdala (Shipley et al, 2008)

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Conclusion