Mapping neural activity patterns and odor perception

A new Northwestern Medicine study has demonstrated how differences in neural activity within the brain's olfactory and orbital cortices cause people to perceive the same odors differently, according to findings published in Nature Neuroscience.

Thorsten Kahnt, Ph.D., adjunct associate professor in the Ken and Ruth Davee Department of Neurology's Division of Behavioral Neurology, was senior author of the study.

Odor perception is subjective in that people can perceive the same odor differently. Previous work has shown that odor perception is caused by odor molecules evoking patterns of neural activity patterns in the brain's olfactory cortex, a portion of the cerebral cortex that is responsible for generating a person's sense of smell. However, how these activity patterns are mapped to odor percepts across different people had not been well-established.

"Different people can smell the same odor and perceive it quite differently, so how does the brain carry out this transformation from this purely objective chemical compound into this purely subjective percept," said Vivek Sagar, Ph.D., a postdoctoral fellow in the Department of Neurology, a former student in the Northwestern University Interdepartmental Neuroscience (NUIN) program and lead author of the study.

In the current study, the investigators performed functional MRIs on three participants who each smelled the same 160 odors; each participant was scanned for a total of 18 hours.

"There were over 4,000 trials of odors from each subject, so there is a lot of depth in the data. We call this 'precision imaging' in the field because it allows us to probe the brain in a highly person-specific way," Sagar said.

Using this imaging data, the investigators then created computational models mapping how odor properties mapped to different neural activity patterns. The team also created a model linking perceptual properties of odors (i.e. sweet, sour, spicy, floral, acidic, etc.) to different neural activity patterns.

They found that while the olfactory cortex responded to odors in the expected way, it captured an objective and low-dimensional picture of an odor. Instead, it was the orbitofrontal cortex, which was previously thought to play only a secondary role in olfactory perception, that captured high-dimensional and subjective percepts of an odor.

For example, the olfactory cortex could help participants describe whether an odor was minty or not, but the orbitofrontal cortex provided participants with a nuanced understanding of whether the given minty odor was peppermint or spearmint.

"The orbitofrontal cortex is not merely capturing a large dimensional perceptual space, but those high dimensions also tend to be more different across people. The reason why the odors appear different to us is because our olfactory cortexes are not quite the same, so it has that subjective information of how we're smelling things," Sagar said.

Sagar added that he is now studying if the way odors are sniffed by a person impacts how the odors are perceived and how the brain creates value through how much it likes or dislikes the odors based on these perceptual dimensions.

"It's not always about how much you like a certain odor, it's also the magnitude of your tendency to approach or avoid the odor that is important for our behavior," Sagar said.

More information:
Vivek Sagar et al, High-precision mapping reveals the structure of odor coding in the human brain, Nature Neuroscience (2023). DOI: 10.1038/s41593-023-01414-4

Provided by Northwestern University