Neural Activity and Odor Perception Mapping

According to findings published in Nature Neuroscience, a new Northwestern Medicine study has showed how changes in neural activity within the brain’s olfactory and orbital cortices cause people to perceive the same odors differently.

The study’s senior author was Thorsten Kahnt, PhD, an adjunct associate professor in the Division of Behavioral Neurology at the Ken and Ruth Davee Department of Neurology.

Odor perception is subjective in the sense that different people perceive the same odor differently. Previous research has demonstrated that odor perception is created by odor molecules triggering patterns of neural activity patterns in the brain’s olfactory cortex, which is responsible for generating a person’s sense of smell. However, it has not been fully established how these neural activity patterns are transferred to odor percepts among various people.

“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, PhD, 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.

The researchers conducted functional MRIs on three subjects who each smelled the identical 160 scents in the current investigation; 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 researchers constructed computational models that matched odor attributes to different neural activity patterns. The scientists also developed a model that links odor perception qualities (such as sweet, sour, spicy, flowery, acidic, and so on) to distinct brain activity patterns.

They discovered that, while the olfactory cortex responded to scents as expected, it also captured an objective and low-dimensional representation of an odor. The orbitofrontal cortex, which was previously assumed to serve only a secondary function in olfactory perception, instead caught high-dimensional and subjective odor perceptions.

The olfactory cortex, for example, may assist participants in describing whether an odor was minty or not, but the orbitofrontal cortex gave a more sophisticated knowledge 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 further stated that he is currently investigating if the manner in which odors are sniffed by a person influences how the odors are perceived and how the brain builds value based on how much it likes or hates the odors based on these perceptual characteristics.

“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.

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