Poor human olfaction is a 19th-century myth

Abstract

It is commonly believed that humans have a poor sense of smell compared to other mammalian species. However, this idea derives not from empirical studies of human olfaction but from a famous 19th-century anatomist's hypothesis that the evolution of human free will required a reduction in the proportional size of the brain's olfactory bulb. The human olfactory bulb is actually quite large in absolute terms and contains a similar number of neurons to that of other mammals. Moreover, humans have excellent olfactory abilities. We can detect and discriminate an extraordinary range of odors, we are more sensitive than rodents and dogs for some odors, we are capable of tracking odor trails, and our behavioral and affective states are influenced by our sense of smell.

Fig. 1

Gross anatomy of the olfactory bulbs of human and mouse. A) Ventral aspect of human brain, with meninges removed from the cortex. Dotted rectangle indicates area of close-up in panel B. B) View of left and right olfactory bulbs and olfactory tracts from panel A. C) Ventral aspect of mouse brain, with olfactory bulbs visible at the top. Up is anterior in all three panels. Dashed lines denote the approximate border between bulb and tract.

Fig. 2

Comparison of the mouse and human olfactory bulb. View is of the ventral aspect of the left olfactory bulb. Both bulbs are at the same scale.

Fig. 3. Comparison of olfactory bulb neuronal numbers across mammalian species

The number of putative neurons per olfactory bulb for each species, as measured by isotropic fractionation. Numbers are drawn from Ribeiro et al. (2014) and Oliveira-Pinto et al. (2014).

Fig. 4. Comparison of human olfactory thresholds across species and odorants

Comparison of detection thresholds (expressed as vapor-phase dilutions in log parts-per-million) across species, where more negative threshold values indicate lower thresholds and thus greater olfactory sensitivity. Shading indicates odors for which humans outperformed all other species tested. (A) Detection thresholds for human subjects (triangles), spider monkeys (squares), and mice (circles) to each of six different thresholds as measured in the Laska laboratory as part of the same experiment. Five individual mouse and spider monkeys are depicted, while the triangles show the range and mean of thresholds from twelve individual subjects. Note that all twelve humans outperformed all mice and monkeys tested for the odorant 3-mercapto-3-methylbutyl-formate and outperformed all mice for 2-propyl thietane. Adapted from Sarrafchi et al. (2013) and used by permission. (B) Pooled olfactory threshold values across species and laboratories for aliphatic carboxylic acids. Note that humans are more sensitive to n-pentanoic acid and n-octanoic acid than all other species tested. Adapted from Can Güven and Laska (2012) and used by permission.