Aachen and Bonn Researchers Uncover Key Neurons in Human Odor Perception

We often only realize how important our sense of smell is when it is no longer there: food hardly tastes good, or we fail to react to dangers such as the smell of burning. Researchers from RWTH Aachen University, the University of Bonn, and University Hospital Bonn (UKB) have investigated the neuronal mechanisms of human odor perception for the first time. The results of this study close a long-standing knowledge gap between animal and human odor research and have now been published in the prestigious scientific journal Nature.

Individual nerve cells in the brain recognize smells and react specifically to the scent, image, and written word of an object, such as a banana. While imaging techniques like functional magnetic resonance imaging (fMRI) have shown which regions of the human brain are involved in odor perception, these methods do not allow investigation at the fundamental level of individual nerve cells. A research team led by Professor Marc Spehr from RWTH’s Chemosensation Laboratory and Professor Florian Mormann from the Department of Epileptology at UKB has now succeeded for the first time in recording the activity of individual nerve cells during olfaction.

“We discovered that individual nerve cells in the human brain react to odors. Based on their activity, we were able to precisely predict which scent is being smelled,” says first author Marcel Kehl, a doctoral student at the University of Bonn. The measurements showed that different brain regions such as the primary olfactory cortex, amygdala, hippocampus, and entorhinal cortex are involved in specific tasks. While nerve cell activity in the olfactory cortex most accurately predicted which scent was smelled, nerve activity in the hippocampus predicted whether scents were correctly identified. Only nerve cells in the amygdala – responsible for emotional processes – reacted differently depending on whether a scent was perceived as pleasant or unpleasant.

In the next step, the researchers investigated the connection between the perception of scents and images. They discovered that nerve cells in the primary olfactory cortex responded not only to scents but also to images. “This suggests that the task of the human olfactory cortex goes far beyond mere scent perception,” explains Professor Marc Spehr.

The researchers found individual nerve cells that specifically responded to a smell, an image, and a written word – for example, a banana. This discovery indicates that semantic information is processed early on in human olfactory processing. The results not only confirm decades of animal studies but also show how different brain regions are involved in specific human odor processing functions. “This is an important contribution towards deciphering the human olfactory code,” says co-corresponding author Professor Florian Mormann from UKB, who is also a member of the ‘Life & Health’ Transdisciplinary Research Area at the University of Bonn. “Further research in this field is crucial to eventually develop olfactory aids that could be as commonplace and user-friendly as eyeglasses or hearing aids.”