Unpleasant odour is processed more quickly in the brain than perceived quality

According to the findings of a new study by the University of Tokyo, A specially created odour delivery device, along with machine learning-based analysis of scalp-recorded electroencephalogram. This device has enabled researchers to see when and where odours are processed in the brain.

The study found that odour information in the brain is unrelated to perception during the early stages of being processed, but when perception later occurred, unpleasant odours were processed more quickly than pleasant odours. Problems with odour perception can be an early symptom of neurodegenerative diseases, so uncovering more of the neural bases of odour perception could help toward a better understanding of those diseases in 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 odour is pleasant or not.

A team at the University of Tokyo created a special device that can deliver 10 diverse odours 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 odours was processed in the brain with a high temporal resolution for the first time.

"We were surprised that we could detect signals from presented odours from very early EEG responses, as quickly as 100 milliseconds after odour onset, suggesting that representation of odour 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 odour by the brain occurred before the odour 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 odour 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 odours (such as rotten and rancid smells) were administered, participants' brains could differentiate them from neutral or pleasant odours as early as 300 milliseconds after onset. However, the representation of pleasant odours (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 odour was also represented. From 600-850 milliseconds after odour onset, significant areas of the brain involved in emotional, semantic (language) and memory processing then became most involved.

The earlier perception of unpleasant odours 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 wherein 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 the representation of odours from time periods as early as 100 milliseconds."

This high temporal resolution imaging of how our brains process odours may be a stepping stone towards better understanding the mechanisms of neurodegenerative diseases in future, such as Parkinson's and Alzheimer's diseases, in which 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, odours 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 odours with a movie, is important." Perhaps Smell-O-Vision might yet make a comeback?