See how smell is processed in the brain


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Summary: Olfactory information in the brain is unrelated to perception in the early stages of processing, but when perception occurred later, unpleasant odors were processed faster than pleasant odors.

Source: University of Tokyo

Does the smell of a warm cup of coffee help you start the day off right? Or can’t you stand the strong, heady stuff?

How quickly your brain processes the smell of your morning drink could be related to whether or not you find that smell pleasant, new research suggests.

A team at the University of Tokyo has developed a special device that can accurately and timely emit 10 different scents. The scents were administered to participants who rated their comfort level while wearing non-invasive scalp-recorded electroencephalogram (EEG) caps, which record signals in the brain.

The team was then able to process the EEG data using machine learning-based computational analysis to see for the first time with high temporal resolution when and where the olfactory palette was being processed in the brain.

“We were surprised that we could detect signals from presented odors from very early EEG responses as early as 100 milliseconds after the onset of the odor, suggesting that brain representation of olfactory information is rapid,” said Graduate School PhD student Mugihiko Kato of Agricultural and Life Sciences at the University of Tokyo.

The brain’s detection of the odor occurred before the odor was consciously perceived by the participant, which happened several hundred milliseconds later.

“Our study showed that different aspects of perception, specifically olfactory fear, unpleasantness, and quality, arose through differential spatial and temporal cortical processing,” Kato said.

“The representation of unpleasantness in the brain arose earlier than pleasantness and perceived quality,” said Project Associate Professor Masako Okamoto, also of the Graduate School of Agricultural and Life Sciences.

When unpleasant odors (such as foul and rancid odors) were administered, the participants’ brains could distinguish them from neutral or pleasant odors as little as 300 milliseconds after onset.

However, representation of pleasant odors (such as floral and fruity odors) in the brain did not occur until 500 milliseconds, around the same time that the quality of the odor was also being represented. Then, from 600 to 850 milliseconds after the onset of smell, significant areas of the brain involved in emotional, semantic (language), and memory processing became most involved.

The earlier perception of unpleasant odors can be an early warning system against possible dangers.

“The way each sensory system recruits the central nervous system differs depending on the sensory modality (smell, light, sound, taste, pressure and temperature). Elucidating when and where in the brain olfactory (smell) perception arises helps us understand how the olfactory system works,” said Okamoto.

“We also believe that our study has broader methodological implications. For example, it was not known that an EEG recorded from the scalp would allow us to assess the representation of odors from periods as short as 100 milliseconds.”

This shows a little girl smelling a flower
How quickly your brain processes the smell of your morning drink could be related to whether or not you find that smell pleasant, new research suggests. The image is in the public domain

This high-resolution temporal representation of how our brain processes odors could be a stepping stone to better understanding the mechanisms of neurodegenerative diseases such as Parkinson’s and Alzheimer’s, in which dysfunction of the sense of smell is an early warning sign. The team is interested in exploring several other research avenues.

“In our daily lives, smells are perceived along with other sensory information, such as sight, and each sense affects the perception of the other,” Kato said.

“Although we only presented olfactory stimuli in the current study, we think it is important to analyze brain activity under more natural conditions, e.g. E.g. when presenting smells in a film.” Perhaps Smell-O-Vision will make a comeback after all?

Financing: This work was supported by the Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research on Innovative Areas to MO (18H04998 and 21H05808) and the JST-Mirai program to KT (JPMJMI17DC and JPMJMI19D1).

About this olfactory processing of research news

Author: Joseph Krisher
Source: University of Tokyo
Contact: Joseph Krisher – University of Tokyo
picture: The image is in the public domain

See also

This shows brain scans from the study

Original research: Closed access.
“Spatiotemporal dynamics of olfactory representations in the human brain revealed by EEG decoding” by Masako Okamoto et al. PNAS


Spatiotemporal dynamics of olfactory representations in the human brain revealed by EEG decoding

How the human brain translates olfactory input into different perceptions, from pleasant floral scents to sickening smells of decomposition, is one of the fundamental questions in the field of olfaction.

To investigate how different aspects of olfactory perception arise in space and time in the human brain, we performed a time-resolved multivariate pattern analysis of scalp-recorded electroencephalogram responses to 10 perceptually distinct odors and linked the resulting decoding accuracies to perceptual and source activities.

The mean decoding accuracy of odors exceeded the probability level 100 ms after odor onset and reached a maximum at 350 ms. The result suggests that the neural representations of individual odors were maximally separated at 350 ms.

Perceptual representations emerged after the decoding peak: unipolar unpleasantness (neutral to unpleasant) from 300 ms and pleasantness (neutral to pleasant) and perceptual quality (applicability to verbal descriptors such as “fruity” or “floral”) from 500 ms after the onset of the odor, with all of these Perceptual displays reach their maximum after 600 ms.

A source estimation showed that the areas representing the olfactory information estimated based on the decoding accuracies were localized in and around the primary and secondary olfactory areas 100 to 350 ms after odor onset.

Olfactory representations then expanded to larger areas associated with emotional, semantic, and memory processing, with the activities of these later areas being significantly associated with cognition.

These results suggest that initial olfactory information collected in the olfactory areas (< 350 ms) kodiert sind, durch Berechnungen in weit verteilten kortikalen Regionen zu ihren Wahrnehmungsrealisierungen (300 bis > 600 ms) develop, with different aspects of perception showing different spatiotemporal dynamics.

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