Humans navigate with stereo olfaction

Humans navigate with stereo olfaction. Yuli Wu et al. Proceedings of the National Academy of Sciences, June 22, 2020. https://doi.org/10.1073/pnas.2004642117

Significance: The human brain exploits subtle differences between the inputs to the paired eyes and ears to construct three-dimensional experiences and navigate the environment. Whether and how it does so for olfaction is unclear, although humans also have two separate nasal passages that simultaneously sample from nonoverlapping regions in space. Here, we demonstrate that a moderate internostril difference in odor intensity consistently biases recipients’ perceived direction of self-motion toward the higher-concentration side, despite that they cannot report which nostril smells a stronger odor. The findings indicate that humans have a stereo sense of smell that subconsciously guides navigation.

Abstract: Human navigation relies on inputs to our paired eyes and ears. Although we also have two nasal passages, there has been little empirical indication that internostril differences yield directionality in human olfaction without involving the trigeminal system. By using optic flow that captures the pattern of apparent motion of surface elements in a visual scene, we demonstrate through formal psychophysical testing that a moderate binaral concentration disparity of a nontrigeminal odorant consistently biases recipients’ perceived direction of self-motion toward the higher-concentration side, despite that they cannot verbalize which nostril smells a stronger odor. We further show that the effect depends on the internostril ratio of odor concentrations and not the numeric difference in concentration between the two nostrils. Taken together, our findings provide behavioral evidence that humans smell in stereo and subconsciously utilize stereo olfactory cues in spatial navigation.

Keywords: binaral disparityolfactory navigationheading perceptionoptic flow