Odor representations in the mammalian olfactory bulb

Abstract

A first key step in studying a sensory modality is to define how the brain represents the features of the sensory stimulus. This has proven to be a challenge in olfaction, where even the stimulus features have been a matter of considerable debate. In this review, we focus on olfactory representations in the first stage of the olfactory pathway, the olfactory bulb (OB). We examine the diverging viewpoints on spatially organized versus distributed representations. We then consider how odor sampling through respiration is a key part of the odorant code. Finally, we ask how the bulb handles the challenging task of representing mixtures. We suggest that current evidence points toward a representation that is spatially organized at the inputs but later distributed, with the spatial organization not being used for much computation. Nevertheless, this is a simple representation that effectively represents multiple individual odorants, as well as odor mixtures.

Continuous spatial representations in the olfactory bulb may reflect perceptual categories

“…For interpretation of these maps, please note that patterns are bilaterally symmetric across a tilted axis around the horizontal midline (cf. Khan et al, 2010). For anatomic terms of location, please refer to Figure 1.…”

Competing Mechanisms of Gamma and Beta Oscillations in the Olfactory Bulb Based on Multimodal Inhibition of Mitral Cells Over a Respiratory Cycle

“…We chose to simulate variations in odor intensity by MC spike phase locking, which seems to be one of the critical parameters varying with intensity ( Margrie and Schaefer, 2003 ; Courtiol et al, 2011a ; Fukunaga et al, 2012 ), as it also increases the chance of MCs and GCs to fire together during the respiratory cycle. Other MC response parameters that were shown experimentally to vary with odor intensity, such as the firing rate or the number of activated glomeruli, as has been reported in many studies ( Meister and Bonhoeffer, 2001 ; Khan et al, 2010 ), were not directly studied here. Although the influence of the above parameters could explain the decrease in gamma oscillations, as reported in Figure 2 , they cannot by themselves only explain the experimentally observed increase of the beta oscillations because the phase dispersion of MC firing is the critical parameter in our model for the emergence of beta oscillation.…”

Concentration-Dependent Recruitment of Mammalian Odorant Receptors

“…This cascade of events, could, in turn, contribute to the neural representation of elevated odor intensity. Higher concentration of odors activate additional, lower affinity ORs at the olfactory epithelium, resulting in more glomerular activations in the olfactory bulb (Fried et al, 2002;Khan et al, 2010). These additionally recruited, low-affinity ORs may drive differences in odor quality and perceived intensity by modulating neuronal firing in higher olfactory areas (Stettler and Axel, 2009).…”

“…Odorants are represented by combinatorial codes whereby a given odorant, at a specific concentration activates a specific combination of ORs, which in turn activate a specific combination of glomeruli (Malnic et al, 1999;Rubin and Katz, 1999;Bozza et al, 2002;Oka et al, 2006;Saito et al, 2009). Previous studies found an increase in the recruitment of active ORs (in the OSNs) and glomeruli (in the olfactory bulb) in response to higher odorant concentrations (Rubin and Katz, 1999;Fried et al, 2002;Khan et al, 2010;Jiang et al, 2015;Wilson et al, 2017). There is, however, insufficient research determining the identity of these ORs and their collective responses at different odorant concentrations in vivo.…”