Olfaction

Abstract

To detect and organize trillions of possible volatile molecules, the human olfactory system deploys approximately 3% of the genes in the human genome as olfactory receptors. Our understanding of how the olfactory system organizes the resulting information has many gaps, and relative to other sensory systems, our current ability to measure and reproduce smells is poor. Here we cover the anatomy, circuitry, and psychophysics of olfaction, with an emphasis on the gaps that prevent us from measuring an odor, representing it digitally, and reproducing a perceptual target from a set of sensory primaries.

Olfactory Perception in Relation to the Physicochemical Odor Space

“…Although a systematic analysis of the factor loadings is challenging due to the large number of descriptors, some associations and trends were explored, especially to check if previous results can be replicated. Similar to previous studies (e.g., see [ 2 , 3 , 8 ]), the first component showed a clear association with molecular weight (as shown by the shade of blue in Figure 2 ) and complexity (high loadings of graph vertex complexity index and graph distance complexity index ; both descriptors from alvaDesc). Further exploration showed that the second principal component shows high loadings of descriptors related to descriptors with some relation to polarity or negativity, such as the eta average electronegativity measure , mean atomic Sanderson electronegativity or topological surface area (alvaDesc descriptors).…”

“…There are some well-known relations between structure and odor perception. In order to be perceived as odorous at all, a molecule must be volatile enough to evaporate and have specific solubility characteristics to pass through the (hydrophilic) nasal mucosa and bind to the (hydrophobic) olfactory receptors in the olfactory epithelium [ 2 ]. In addition, some functional groups have been associated with characteristics of odor quality.…”

“…This suggests that intensity encoding may function independently of odor discrimination. In this context, it must also be taken into account that the perception of intensity, especially at high concentrations of odors, is influenced by both olfactory and trigeminal processing [ 2 , 6 ]. In addition, odor molecules with a high molecular weight were found to have higher rates of specific anosmia, not being able to smell a specific odor, than lighter molecules [ 6 ].…”

“…However, direct relationships between specific physical or chemical characteristics with odor perception are rare, and structurally similar odor molecules in some cases lead to very different olfactory perceptions [ 1 ]. In addition, the understanding is complicated by many other processes in the pathway of olfaction, for example, influences due to chemical reactions during transport through the nasal mucosa or the interaction of odors in odor mixtures [ 1 , 2 , 14 , 15 ]. Thus, olfactory perception remains a “black box” in many respects.…”

“…As a first step, many studies have attempted to characterize a “physicochemical odor space” [ 2 , 3 , 8 , 16 ]. For this purpose, a large number (typically thousands or tens of thousands) of chemical and physical molecular properties, so-called descriptors, are calculated using special software or online databases.…”