Neuroscientists were able to describe for the first time how the relationships between different odors are encoded in the brain and how the brain converts information about the chemistry of odors into the perception of smells. This mechanism helps explain why we have common but very individual experiences with a sense of smell.
A new study carried out by neurobiologists from Harvard Medical School and published in the journal Nature explained the relationships between the different smells encoded in the olfactory cortex, the brain region responsible for processing odors.
Neural representations of odor in the cerebral cortex
The researchers were able to show that the neural representations of odor in the cerebral cortex reflect the chemical similarities between odors so that odors can be categorized by the brain and “renewed” through sensory experience. These findings suggest a neurobiological mechanism that may explain why individuals have common but very personal experiences with the sense of smell. “We all share a common frame of reference with smells,” confirms the lead author of the study, Professor Sandeep Robert Datta of HMS. You and I think that lemon and lime smell similar, and we agree that they smell different from pizza, but until now we did not know how the brain organizes this kind of information”.
The sense of smell enables animals to recognize the chemical nature of the world around them. Sensory neurons in the nose perceive odor molecules and transmit the signals to the olfactory bulb, a structure in the brain where the initial processing of odors takes place. The olfactory bulb transmits the information for further processing to the piriform cortex, the main structure of the olfactory cortex. Unlike light or sound, it is difficult to understand how the brain constructs neural representations of small molecules that transmit odors. Subtle chemical changes can often lead to significant differences in olfactory perception. The results open new avenues for studies to better understand how the brain converts olfactory chemical information into olfactory perception. “This is the first demonstration of how the olfactory cortex encodes information about what is responsible, namely the chemistry of odors, the basic sensory signals of smell,” says Sandeep Robert Datta.
Predicting the identity of an odor
The researchers focused on how the brain identifies related but different odors. To this end, they developed an approach to quantitatively compare chemical odors. They used machine-learning to study thousands of chemical structures that are known to have odors and analyzed thousands of different characteristics for each structure, such as the number of atoms, molecular weight, and electrochemical properties. This data enabled the researchers to systematically calculate the similarity or difference from one odor to another. This helped them to design three groups of odors: one with high diversity, one with medium diversity, where odors are divided into related groups, and one with low diversity, where the structures varied only by gradually increasing the length of the carbon chain.
They then gave odors with carefully selected molecular structures from the different sets to rats and analyzed their neuronal activity. The experiments showed that similarities in olfactory chemistry were reflected by similarities in neuronal activity. The associated odors produced correlated neuronal profiles in the pyriform cortex and in the olfactory bulb, measured by superimposing neuronal activity. Weakly bound perfumes produced weakly bound activity patterns. In the cortex, the associated scents led to more strongly grouped patterns of neuronal activity than the patterns of the olfactory bulb. The cortical representations of the olfactory relations were so well correlated that they could be used to predict the identity of a selected odor in one mouse based on the measurements of another mouse.
The plasticity of the cortex, the key to our differences
Researchers have also found that these neural representations are flexible. The rats repeatedly received a mixture of two fragrances, and over time the corresponding neural patterns of these fragrances correlated more strongly in the cortex. This occurred even when the two scents had different chemical structures. The adaptability of the cortex was partly controlled by neuronal networks that selectively altered the olfactory conditions. When the normal activity of these networks was blocked, the encoded cortex smelled more like the olfactory bulb. “We represented two odors as if they came from the same source and observed that the brain can reorganize itself to reflect passive olfactory experiences,” the researcher noted. Part of the reason why things like the smell of lemon and lime are the same is probably that animals of the same species have a similar genome and therefore similar olfactory perception.
The plasticity of the cortex explains that despite all the similarities, each individual has a different perception of the same smell. “The plasticity of the cortex can help explain why odor is invariant in individuals, but can be individually adapted based on our unique experiences,” confirms Sandeep Robert Datta. Further research is needed to identify these mechanisms more precisely. “We still do not fully understand how chemicals are transformed into perception,” the researcher continues. There is no algorithm or computer that can pick up a chemical structure and tell us what that chemical is similar to. To really build this machine and one day be able to create a virtual, controllable world of scents for a person, we need to understand how the brain encodes information about smells. We hope that our results will be a step in this direction,” he concludes.