A specially created odor delivery device, along with machine learning-based analysis of scalp-recorded electroencephalogram, has enabled researchers at the University of Tokyo to see when and where odors are processed in the brain. The study found that odor information in the brain is unrelated to perception during the early stages of being processed, but when perception later occurred, unpleasant odors were processed more quickly than pleasant odors. Problems with odor perception can be an early symptom of neurodegenerative diseases, so uncovering more of the neural bases of odor perception could help towards better understanding of those diseases in the future.
Does the smell of a warm cup of coffee
help you start your day the right way? Or can you not stand the strong, heady
stuff? According to new research, how quickly your brain processes the smell of
your morning beverage might depend on whether you think that odor is pleasant
or not.
A team at the University of Tokyo
created a special device that can deliver 10 diverse odors in a way that is
accurate and timely. The odors were administered to participants who rated
their pleasantness while wearing noninvasive scalp-recorded
electroencephalogram (EEG) caps, which record signals inside the brain. The team
was then able to process the EEG data using machine learning-based computer
analysis, to see when and where the range of odors was processed in the brain
with high temporal resolution for the first time.
“We were surprised that we could detect
signals from presented odors from very early EEG responses, as quickly as 100
milliseconds after odor onset, suggesting that representation of odor
information in the brain occurs rapidly,” said doctoral student Mugihiko Kato
from the Graduate School of Agricultural and Life Sciences at the University of
Tokyo.
Detection of odor by the brain occurred
before the odor was consciously perceived by the participant, which didn’t
happen until several hundred milliseconds later. “Our study showed that
different aspects of perception, in particular odor pleasantness,
unpleasantness and quality, emerged through different spatial and temporal
cortical processing,” said Kato.
“The representation of unpleasantness in
the brain emerged earlier than pleasantness and perceived quality,” said
Project Associate Professor Masako Okamoto, also from the Graduate School of
Agricultural and Life Sciences. When unpleasant odors (such as rotten and
rancid smells) were administered, participants’ brains could differentiate them
from neutral or pleasant odors as early as 300 milliseconds after onset.
However, representation of pleasant odors (such as floral and fruity smells) in
the brain didn’t occur until 500 milliseconds onwards, around the same time as
when the quality of the odor was also represented. From 600-850 milliseconds
after odor onset, significant areas of the brain involved in emotional,
semantic (language) and memory processing then became most involved.
Images of smells being processed in the brain. EEG
readings enabled the research team to record which parts of the brain processed
the odors. ©Mugihiko Kato
The earlier perception of unpleasant
odors may be an early warning system against potential dangers. “The way each
sensory system recruits the central nervous system differs across the sensory
modalities (smell, light, sound, taste, pressure and temperature). Elucidating
when and where in the brain olfactory (smell) perception emerges helps us to
understand how the olfactory system works,” said Okamoto. “We also feel that
our study has broader methodological implications. For example, it was not
known that scalp-recorded EEG would allow us to assess representation of odors
from time periods as early as 100 milliseconds.”
This high temporal resolution imaging of
how our brains process odors may be a stepping stone towards better
understanding the mechanisms of neurodegenerative diseases in the future, such
as Parkinson’s and Alzheimer’s diseases, in which a dysfunction in the sense of
smell is an early warning sign. The team is interested in exploring several
further research avenues. “In our daily life, odors are perceived along with
other sensory information like vision, and each sense influences the perception
of the other,” said Kato. “Although we presented olfactory stimuli alone in the
current study, we think that analyzing brain activity under more natural
conditions, such as presenting odors with a movie, is important.” Perhaps
Smell-O-Vision might yet make a comeback?
Source: The University of Tokyo
Journal article: https://www.pnas.org/doi/10.1073/pnas.2114966119
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