Scientific discovery sheds light on the fascinating complexity of human smell thanks to the remarkable processing power of the brain.
A recent scientific discovery sheds light on the fascinating complexity of human smell. It has been revealed that our two nostrils operate with a degree of independence, each seemingly possessing its own distinct olfactory capabilities. This previously unknown aspect of the olfactory system is attributed to the remarkable processing power of the brain.
This research builds upon previous studies conducted with both animals and humans, suggesting the brain’s remarkable ability to handle information from each nostril independently, while also creating a unified olfactory experience.
The researchers, in their published paper, acknowledge the significant existing knowledge on olfactory responses, but highlight the gap in understanding regarding how the human brain integrates and differentiates information received through the two nostrils.
To further explore this concept of “smelling in stereo,” the research team, comprised of scientists from the University of Pennsylvania, the Barrow Neurological Institute, and Ohio State University, recruited ten epilepsy patients who already had electrodes implanted in their brains.
The Study
During the experiment, one of three distinct scents or a control of pure air was delivered to either a single nostril or both nostrils simultaneously in each trial. After a brief interval, participants were asked to identify the scent and indicate the nostril used for detection (left, right, or both). Simultaneously, brain activity was monitored via implanted electrodes.
Several intriguing observations were made. When the same scent was presented sequentially to each nostril, the resulting brain activity displayed similarities but also subtle differences, implying a degree of independent processing.
Interestingly, smelling with both nostrils simultaneously generated two distinct bursts of brain activity. While the time difference between them was minimal, it was nonetheless detectable. This finding further suggests that the two nostrils may not always function in complete unison.
The researchers focused on the piriform cortex (PC), the brain region responsible for processing and interpreting smells. This interconnectedness of senses suggests the research may have broader health implications beyond olfaction.
Conclusion
Previous studies have shown “smelling in stereo” capabilities in rats, where both nostrils are used to pinpoint the source of a scent. This new research aims to investigate similar abilities in humans and explore how the brain processes the timing and “odor coding” differences between nostrils.
The researchers, in their published paper, highlight the temporal segregation of odor information from each nostril within the human piriform cortex. They conclude that their findings provide evidence for distinct representations of odor data based on the source nostril, achieved through this temporal separation.
https://doi.org/10.1016/j.cub.2023.10.021
Summary
The human olfactory system has two discrete channels of sensory input, arising from olfactory epithelia housed in the left and right nostrils. Here, we asked whether the primary olfactory cortex (piriform cortex [PC]) encodes odor information arising from the two nostrils as integrated or distinct stimuli. We recorded intracranial electroencephalogram (iEEG) signals directly from PC while human subjects participated in an odor identification task where odors were delivered to the left, right, or both nostrils. We analyzed the time course of odor identity coding using machine-learning approaches and found that uni-nostril odor inputs to the ipsilateral nostril are encoded ∼480-ms faster than odor inputs to the contralateral nostril on average. During naturalistic bi-nostril odor sampling, odor information emerged in two temporally segregated epochs, with the first epoch corresponding to the ipsilateral and the second epoch corresponding to the contralateral odor representations. These findings reveal that PC maintains distinct representations of odor input from each nostril through temporal segregation, highlighting an olfactory coding scheme at the cortical level that can parse odor information across nostrils within the course of a single inhalation.