The way we perceive odors may change depending on which of our nostrils catches the biggest whiff, new research has revealed. While observing the brain activity of 10 people as they sniffed a series of different smells, the study authors discovered that each nasal passage responds independently, which means that there could be a subtle difference in the way the two nostrils react to a given scent.

Previous research has demonstrated how rodents navigate toward the source of a smell by picking up on differences in odor concentrations across their two nostrils. However, until now it was unclear if human schnozzes encode smells as a single stimulus or as two separate signals.

To investigate, the study authors recruited 10 epilepsy patients who had been fitted with intracranial depth electrodes in order to locate the neural origins of their seizures. Borrowing these devices for a different purpose, the researchers recorded activity in the participants’ olfactory cortex as they took part in a smell test.

During the experiment, sniffers were presented with banana, coffee, and eucalyptus aromas, delivered via tubes into each nostril. Brain activity was measured as these fragrances were injected into either the left, right, or both nostrils, with a photoionization detector being used to ensure the intensity of the smells remained constant across all trials.

Intriguingly, odors that were sniffed by both nostrils simultaneously triggered two distinct representations in the brain, with the right nostril sending signals to the olfactory cortex in the right hemisphere while the left nostril activated the left hemisphere. These two signals were not entirely identical, suggesting that each nostril triggered a unique experience.

Likewise, when the same odor was presented to each nostril individually, the activity patterns produced were similar but not equal. “We show that odor information from the two nostrils is temporally segregated in the primary olfactory cortex,” write the study authors. 

Breaking down the sequence of events, they describe how a smell presented to a particular nostril elicits a response in the corresponding brain hemisphere, with the opposing hemisphere only becoming activated about half a second later. According to the researchers, this in-built smell trick may help humans to identify the source of a scent in much the same way as rodents.

“These results raise the question of whether the human olfactory system, akin to the auditory system using interaural time differences to localize sounds, can engage such a coding scheme to compare odor inputs across nostrils and aid in rapid odor localization within a single sniff,” they write.

The study is published in the journal Current Biology.