Practice makes perfect. In human olfaction, such plasticity is generally assumed to occur at the level of cortical synthetic processing that shares information from both nostrils. This view has recently been challenged by a study published in eLife this January, which argues that one nostril does not always know what the other learns.
Guo Feng and Wen Zhou at the Institute of Psychology, Chinese Academy of Sciences employed odor enantiomers in their study. Enantiomers refer to molecules that are non-superimposable mirror images of each other. Such molecules have almost identical physical and chemical properties yet can smell differently to the human nose. This is because olfactory receptors are proteins that use only one enantiomeric form (the L form) of the amino-acid building blocks. The olfactory ability to differentiate between odor enantiomers has genetic basis but can also be acquired through learning by human adults, which speaks to the plasticity of the adult olfactory system. Given the discrimination of odor enantiomers ultimately draws upon the enantioselectivity of olfactory receptors, the characteristics (specificity/generalization) of chiral discrimination learning thus offers a unique window into where the plasticity underlying olfactory learning firstly occurs.
In two experiments, Feng and Zhou trained human adults unirhinally for the discrimination between odor enantiomers over a course of about 10-11 days and examined the extent to which the learning effect transferred to the untrained nostril or to an untrained enantiomer pair that is structurally distinct from or similar to the enantiomer pair used for training. Results showed that training induced perceptual gain in chiral discrimination was specific to the trained nostril, and partially generalized to untrained odor enantiomers in a chemical structure- rather than olfactory quality- based manner. For instance, carvone and limonene share an isopropenyl group at the chiral center and a methyl group at the para-position (i.e. at the opposite side of the ring structure relative to the chiral carbon atom bearing the isopropenyl group) but smell distinctly different from each other. Nonetheless, individuals trained to discriminate the enantiomers of carvone showed significantly improved discrimination for the enantiomers of limonene presented to the trained as opposed to the untrained nostril, and vice versa. No transfer was observed to the enantiomers of α-pinene, whose structure is distinct from that of carvone or limonene in terms of the substituents attached to the chiral center.
These results demonstrate that one nostril does not readily know what the other learns. Moreover, they indicate that the plasticity underpinning the acquisition of chiral discrimination stems from early olfactory regions that analyze the structural features (i.e. chirality) of uninarial olfactory input.
This work, entitled “Nostril-specific and structure-based olfactory learning of chiral discrimination in human adults”, was supported by the Key Research Program of Frontier Sciences (QYZDB-SSW-SMC055) and the Strategic Priority Research Program (XDBS01010200) of the Chinese Academy of Sciences, the National Natural Science Foundation of China (31830037 and 31422023) and Beijing Municipal Science and Technology Commission.
Contact:
Ms.Chen LIU
Institute of Psychology
Email: liuc@psych.ac.cn
