We often only come to appreciate our sense of smell when it’s diminished: food loses its flavor, or we become unresponsive to threats like the odor of smoke. A team of researchers has explored the brain’s neuronal mechanisms that govern human scent perception for the first time. Distinct nerve cells in our brain can identify smells and react specifically to them, as well as to the visual and textual representation of objects, like a banana. This study bridges a significant gap in our understanding of odor research in both animals and humans.
It’s often in the absence of our sense of smell that we understand its significance: food becomes bland, or we fail to notice alarming scents like smoke. Researchers from the University Hospital Bonn (UKB), the University of Bonn, and the University of Aachen have conducted an unprecedented investigation into the neuronal mechanisms that govern human odor detection. They found that individual brain nerve cells can distinguish scents and respond distinctly to the smell, visual representation, and written information of objects, such as a banana. These findings help to fill a long-standing gap in the understanding of smell research across species and have been published in the journal Nature.
Prior studies using imaging techniques like functional magnetic resonance imaging (fMRI) have shown which areas of the human brain are involved in processing smells. However, these techniques do not enable investigation at the level of individual nerve cells. “Thus, our understanding of how scents are processed at a cellular level has largely relied on animal studies, leaving it unclear how these insights apply to humans,” explains Prof. Florian Mormann, a co-corresponding author from the Department of Epileptology at the UKB, who is also part of the Transdisciplinary Research Area (TRA) “Life & Health” at the University of Bonn.
Nerve cells in the brain identify odors
The research team led by Prof. Mormann has managed to record the activity of individual nerve cells in response to smells. This was made possible by collaborating with patients at the Clinic for Epileptology at the UKB, one of Europe’s largest epilepsy centers, who had electrodes placed in their brains for diagnostic reasons. These participants were exposed to a variety of scents, including pleasant ones and smells perceived as unpleasant, like old fish. “We found that individual nerve cells in the human brain are responsive to odors, and we could accurately predict which scent was being detected based on their activity,” states Marcel Kehl, a doctoral candidate from the University of Bonn working in Prof. Mormann’s group at the UKB. The data revealed that different brain regions, including the primary olfactory cortex, also known as the piriform cortex, and specific parts of the medial temporal lobe—namely the amygdala, hippocampus, and entorhinal cortex—play distinct roles in scent processing. While nerve cell activity in the olfactory cortex was best at predicting which scent was being identified, neuron activity in the hippocampus indicated whether the scents were recognized correctly. In contrast, only the nerve cells in the amygdala, associated with emotional responses, showed varied activity based on whether a smell was deemed pleasant or unpleasant.
Nerve cells react to the smell, image, and name of the banana
Next, researchers examined the relationship between scent perception and visual representation. Participants in the Bonn study were shown relevant images that matched each odor, such as the scent followed by a photo of a banana, while monitoring neuronal responses. Interestingly, nerve cells in the primary olfactory cortex responded not only to smells but also to images. “This indicates that the human olfactory cortex is engaged in more than just basic odor detection,” remarks Prof. Marc Spehr, a co-corresponding author from the Institute of Biology II at RWTH Aachen University.
The research unveiled individual nerve cells that specifically responded to the scent, image, and textual representation of, for instance, a banana. This suggests that semantic processing occurs early in human olfactory cognition. The results not only validate decades of research on animals but also illustrate how various brain regions contribute to particular functions of human odor processing. “This represents a vital step in unraveling the human olfactory code,” states Prof. Mormann. “Further exploration in this domain is crucial to potentially developing olfactory aids that we could use as naturally as we use glasses or hearing aids.”
Funding: The research was supported by the German Research Foundation (DFG), the Federal Ministry of Education and Research (BMBF), and the state of North Rhine-Westphalia (NRW) within the scope of the iBehave project.
