Summary: New research indicates that humans possess a stereo sense of smell that unconsciously influences how we orient and move through space.
Source: Chinese Academy of Science
“If in doubt, always follow your nose,” Gandalf advised in The Lord of the Rings. Despite this literary counsel, many people consider themselves to be microsmatic—having a relatively poor sense of smell. Traditionally, human navigation has been attributed mainly to vision and hearing, where subtle differences between the paired eyes and ears provide directional cues. These binocular and binaural disparities let the brain construct spatial awareness and guide movement through three-dimensional environments.
Humans also have two nostrils that sample the air from slightly different parts of the surrounding space. Until now, it has been widely assumed that internostril differences in odor concentration do not give directional information unless the odor also stimulates the trigeminal nerve—that is, when the scent produces sensations like heat, cooling, spiciness, tingling, or electric-like feelings. In those cases, it is the trigeminal input that is thought to supply directional cues rather than the olfactory system itself.
Researchers at the Institute of Psychology of the Chinese Academy of Sciences, led by graduate student Yuli Wu with senior author Dr. Zhou Wen, have challenged that view. Their experiments tested whether humans can use purely olfactory differences between nostrils to bias perceived self-motion and heading.
To test stereo olfaction, the team used two odorants—phenylethyl alcohol (rose scent) and vanillin (vanilla scent)—both selected because they are known to activate the olfactory nerve without significant trigeminal stimulation. They integrated controlled binaral concentration disparities into a heading-judgment task driven by optic flow. Optic flow is a visual pattern that simulates motion of surfaces in a scene and can create a compelling illusion of self-motion in otherwise stationary observers. By pairing optic flow with asymmetric odorant concentrations between the nostrils, the researchers could assess whether olfactory cues bias perceived direction of movement.
Across four rigorous psychophysical experiments involving a total of 180 participants, the results were consistent. A moderate disparity in odor concentration between the two nostrils produced a reliable bias: participants’ perceived heading shifted toward the side with the higher concentration. This bias occurred even though participants could not reliably report which nostril received the stronger odor. In other words, the olfactory input influenced spatial orientation at a subconscious level, similar in effect to how binocular vision produces a stereo perception of depth.
Importantly, the researchers found that the bias depended on the ratio of concentrations between the nostrils, not on the simple numeric difference. This implies that the human olfactory system uses relative, proportionate cues when integrating odor information for spatial judgments, a mechanism more akin to how other sensory systems weigh comparative signals.
Dr. Zhou Wen summarized the significance: the behavioral evidence supports the notion that humans can “smell in stereo” and that these stereo-olfactory cues are used subconsciously to aid navigation. The findings broaden our understanding of multisensory heading perception and suggest new possibilities for designing olfactory components of virtual reality systems where realistic spatial sensation might be enhanced by stereo scent delivery.
The study, titled “Humans navigate with stereo olfaction,” was published online in PNAS on June 22.
About this research
Source: Chinese Academy of Science
Media contacts: Press Office – Chinese Academy of Science
Image source: The image used is in the public domain.
Original research: “Humans navigate with stereo olfaction” by Yuli Wu et al., published in PNAS. DOI: 10.1073/pnas.2004642117
Abstract
Humans navigate with stereo olfaction
Human navigation relies heavily on paired sensory inputs, such as those from the eyes and ears. While humans also have two nasal passages, there has been limited empirical evidence that internostril differences in odor concentration provide directional information in the absence of trigeminal activation. Using optic flow to induce a sense of self-motion and carefully controlled non-trigeminal odorants, the authors demonstrate through formal psychophysical testing that a moderate binaral concentration disparity biases perceived direction of self-motion toward the nostril receiving the higher concentration. Participants were unable to consciously identify which nostril smelled stronger, yet their heading judgments were affected. The effect scales with the internostril ratio of concentrations rather than with absolute concentration differences. Together, these findings present behavioral evidence that humans possess a stereo sense of smell and that stereo olfactory cues are subconsciously used during spatial navigation.