How does the human brain encode smell? Harvard scientists found out

  • Scientists have been investigating how our brains organise odour information for many years
  • A team of Harvard University researchers has recently found some answers
  • The study also explains why lemon and lime smell so alike

For many years, scientists have been trying to understand how the brain translates odour chemistry into perceptions of smell, but, until now, the answer has remained elusive.

Thanks to neuroscientists from Harvard University, we now have a bit more insight into how odours are encoded in the brain. According to the scientists’ results, which were published in the journal Nature, there may be a specific mechanism that explains why individuals have common, but intensely personal experiences with smell.

Why do lemon and lime smell so similar?

According to senior study author, Sandeep Robert Datta, who is an associate professor of neurobiology in the Blavatnik Institute at Harvard Medical School, this study is the first demonstration of how our olfactory cortex – which processes olfactory information and is involved in the sense of smell – encodes information about odour chemistry.

“All of us share a common frame of reference with smells," Datta said in a news release on the HMS site. “You and I both think lemon and lime smell similar and agree that they smell different from pizza, but until now, we didn’t know how the brain organises that kind of information.”

Computing odour

If you ever wondered how we process odours, the process starts in the sensory neurons in our noses, which detect odour molecules, and then relay signals to the olfactory bulb (a structure in our forebrains). The olfactory bulb then transmits information to the piriform cortex (the main structure of the olfactory cortex), for more comprehensive processing.

Subtle chemical changes, such as a variation in a few carbon or oxygen atoms, can bring about notable differences in smell perception. Along with his co-author Stan Pashkovski, research fellow in neurobiology at HMS, Datta focused on how exactly the brain identifies related but distinct odours.

“The fact that we all think a lemon and lime smell similar means that their chemical makeup must somehow evoke similar or related neural representations in our brains,” Datta said.

Machine learning

The researchers used machine learning to investigate thousands of chemical structures known to have odours, and further analysed thousands of different features of each structure, including the number of atoms and electrochemical properties. This allowed the team to calculate the similarities or differences between odours.

Three sets of odours were designed: a set with high, intermediate and low diversity. The researchers then exposed mice to different combinations of odours from the different sets. They used multiphoton microscopy (an imaging technique) to image patterns of neural activity in the mice’s piriform cortex and olfactory bulb.

What the experiments revealed

"We presented two odours as if they were from the same source and observed that the brain can rearrange itself to reflect passive olfactory experiences," Datta said, and explained that part of the reason why things like lemon and lime smell alike is because animals of the same species have similar genomes (an organism's complete set of DNA, including its genes) and therefore similarities in smell perception. However, each individual has personalised perceptions as well.

Datta’s point is supported by a 2013 study by Duke University researchers who found that no two people experience smells the same way. This is because we have nearly a million variations on 400 smell receptors:

“A difference at the smallest level of DNA – one amino acid on one gene – can determine whether you find a given smell pleasant. A different amino acid on the same gene in your friend's body could mean he finds the same odour offensive,” the researchers explained.

The results of the HMU study demonstrate for the first time how the brain encodes relationships between odours and, according to the researchers, this new information can help to better understand and potentially control the sense of smell. Datta did, however, point out a gap in their research:

"We don't fully understand how chemistries translate to perception yet. There's no computer algorithm or machine that will take a chemical structure and tell us what that chemical will smell like.

"To actually build that machine and to be able to someday create a controllable, virtual olfactory world for a person, we need to understand how the brain encodes information about smells," Datta said. "We hope our findings are a step down that path."

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