Can humans navigate by their noses, as dogs do when following a scent on the ground? In a 2007 study, blindfolded participants followed a chocolate-scented trail through grass on all fours, and 21 of 32 reached the end.
Beyond establishing whether people could do it, the real focus was whether having two nostrils helps with tracking. Combining practice, measurements of inhaled air, and specialized devices worn on the nose, the researchers investigated the spatial functions of human olfaction.
- Following a trail, not just detecting an odor
- A scent trail about ten meters long through grass
- Practice improved speed and reduced deviation from the trail
- Two nearby nostrils draw air from different places
- From blocking one nostril to combining two air inlets into one
- Two nostrils assist tracking
- Summary
Following a trail, not just detecting an odor
Smell helps us identify odors and locate their sources. When animals follow a scent trail on the ground, do they compare information from the two nostrils as well as moving the nose to collect successive samples?
With hearing, differences between sounds reaching the two ears help indicate direction. To investigate whether the nose offers a similar advantage, humans are useful participants because they can understand instructions and cooperate with manipulations such as blocking one nostril.
The researchers therefore first established whether humans could perform the task, then examined the roles of practice and input from both nostrils. Rather than assuming that human smell is weak based on appearances, they measured its abilities under a different way of using it.
A scent trail about ten meters long through grass
A string infused with chocolate odor created a trail about 10 m long through grass, with an approximately 45-degree turn partway along. Participants began about 3 m from the trail, facing its approximate direction.
They wore blindfolds, earmuffs, thick gloves, and knee and elbow pads to reduce visual, auditory, and fine tactile cues from the string. With their noses near the ground, they first located the trail and then followed it. Trials ended upon arrival or after ten minutes, with no feedback about whether they were on course.
Of the 32 first-time participants, 21, approximately two-thirds, succeeded. Their paths showed a side-to-side movement across the trail.
The researchers also tested whether non-olfactory cues could explain success. Eleven participants performed the same task with their noses clipped shut, and none succeeded. This control supports the scent’s role as the main cue for locating the trail.
Practice improved speed and reduced deviation from the trail
Four participants then practiced three times a day on three days over a two-week period. Starting positions and the lengths of trail segments varied so that the task could not be solved simply by memorizing movements.
With practice, deviation from the scented trail decreased and tracking speed increased. Average speed rose from approximately 2.6 cm per second initially to about 5.7 cm per second at the end. This small experiment therefore showed improvement after a short training period.
Sniffing frequency also increased as participants moved faster. The researchers suggested that more frequent sampling may have helped participants avoid missing information about the scent on the ground as their speed increased.
This approach treats smell not merely as a passive sense, but as an active behavior involving head movement, inhalation, and checking the next location. Improvement in scent tracking may depend partly on how these movements are performed.
Two nearby nostrils draw air from different places
The nostrils are close together. To investigate whether they nevertheless collect information from different locations, the researchers filmed the movement of tiny airborne particles.
Measurements in one 23-year-old man showed that air entering the two nostrils originated from spatially distinct regions. The centers of those regions were approximately 3.5 cm apart. This is not the distance between the nostril openings themselves, but a difference between the areas from which inhaled air originates.
When the nose is at the edge of a scent trail, one side could sample scented air while the other samples air outside it. These measurements demonstrated physical conditions that could allow comparisons between the two sides.
From blocking one nostril to combining two air inlets into one
In a comparison involving 14 participants, blocking one nostril reduced tracking success and speed relative to using both. The paper reported a success rate of 36% with one nostril versus 66% with two, and an average speed reduction of 26%.
Blocking one nostril changes not only spatial information but also the amount of odor available and the delivery of input to both sides of the nose. The researchers therefore conducted a more targeted experiment using a specialized device.
A device called a nasal prism combined the external air inlet into a single central opening, with passages supplying both nostrils. Both sides still received air, but the advantage of sampling from two spatially separated external locations was removed.
A control device kept the left and right air inlets separate. After becoming familiar with both devices, the four trained participants followed the trail three times with each.
With separate inlets, participants succeeded in 9 of 12 trials. With a single inlet, success fell to 5 of 12 trials. Average speed also decreased by 24%, and both differences were statistically significant. This provides more direct support for the advantage of simultaneously sampling different locations with the two nostrils.
Two nostrils assist tracking
From these experiments, the researchers concluded that humans can track a scent and benefit from comparing the two nostrils, in addition to collecting sequential information by moving the nose. Some trials succeeded with one nostril or a single inlet, suggesting that tracking combines several sources of information.
However, the training and device experiments involved only four people, and airflow was measured in one person. The findings came from following an artificially created scent trail with the nose close to the ground. They did not directly compare human and canine abilities or determine how useful the mechanism is during everyday walking.
Nevertheless, viewing the two nostrils as a system that collects information from slightly different locations, rather than simply two entrances for the same odor, reveals another aspect of the nose’s function.
Summary
Humans could follow a scent trail through grass even with other sensory cues reduced, and a small training experiment showed improvements in speed and accuracy.
Performance also declined when external air was collected through a single inlet while still being delivered to both nostrils. The study demonstrated a concrete advantage of sampling separate locations with two nostrils when locating an odor’s direction and following its trail.
The research was funded by the US Army Research Office and the US National Institutes of Health (NIH). The authors declared no competing financial interests.
References
Porter J, et al. (2007). Mechanisms of scent-tracking in humans. Nature Neuroscience, 10, 27–29 (published online in 2006). Original paper/Author-shared full text
