How Do Brains Process Scents? Study Reveals How People Interpret Odors

For the first time in science, neurobiologists from Harvard Medical School have explored new insights into how the brain perceives scent. A recent study published in the journal Nature on July 1, describes how different odors are encoded in the brain's olfactory complex.

In their study, the researchers used carefully chosen molecular structures and analyzed how they invoked neural activity in mice. They were able to categorize scents by analyzing neuronal representations of the odors in the brain's smell complex.

According to Sandeep Robert Datta, an associate professor of neurobiology in the Blavatnik Institute at HMS, all of us share a natural frame of reference with smells. The senior author of the study added that people might agree that both lime and lemon smell similar and that they smell different from pizza. However, it has been unclear how exactly the brain processes these similarities and differences in scents.

Furthermore, the study's findings suggest a neurobiological structure that could explain why individuals have common but highly personalized associations with a certain smell. Their results open opportunities for a better understanding of how the brain converts information on odor chemistry into the perception of the scent.

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The Chemistry of Odor

To investigate further, the researchers came up with an approach to compare odor chemicals comparable to how differences in wavelength, for instance, can be used to compare colors of light.

They used machine learning to look at thousands of chemical structures known to have odors. They also analyzed thousands of different features for each structure, such as the molecular weight, number of atoms, electrochemical properties, and more. Collectively, the data allowed the researchers to compute how similar or different any odor was compared to another.

From the collection of scents, the researchers were able to design different sets of odors. They then exposed mice to different combinations of odors from the different sets. Moreover, they used multiphoton microscopy to image patterns of neural activity in various parts of the brain.

Encoding Scents in the Brain

The experiments uncovered that similarities in neural activity reflected similarities in odor chemistry. Furthermore, the researchers also found that these neural representations were flexible.

In the experiments, the mice were repeatedly given a mixture of two odors. Over time, the equivalent neural patterns of these odors in the cortex became more strongly associated with each other. This happened even when the two odors had different chemical structures.

According to Datta, the team presented two odors as if they're from the same source. Later on, they observed that the brain could reorganize itself to reflect passive olfactory experiences.

Collectively, the study results show how the brain encodes relationships between odors for the first time. According to the author of the study, identifying how the olfactory cortex maps similar odors now gives new insights to understand and potentially control the sense of smell.

Datta concludes that although they still don't have a full understanding of how chemistries translate to perception, they hope to eventually build a machine that would create a controllable, virtual olfactory world for a person.

However, to do so, he says it is first necessary to understand how the brain encodes information about different smells. He hopes that the findings of their study bring them a step closer to that goal.