Bonn researchers unveil how the brain encodes and interprets smells at the single-neuron level.

October 9 – Our sense of smell is a vital aspect of how we interact with the world, guiding both animal and human behavior. However, the underlying neural processes of olfaction, particularly in humans, have remained largely mysterious. In a recent breakthrough study, Prof. Florian Mormann and his team at the University of Bonn, in collaboration with Prof. Marc Spehr from the University of Aachen, have made significant strides in understanding how the brain processes odors. The researchers recorded single-neuron activity in various regions of the human brain, offering a new perspective on the neural encoding of olfactory information.

The study focused on the piriform cortex—the primary olfactory region in the brain—as well as areas of the medial temporal lobe (MTL), including the amygdala, entorhinal cortex, and hippocampus. By recording from single neurons in awake human subjects during odor identification tasks, the team discovered that specific neurons in each of these brain regions accurately represent different aspects of odor processing.

Crucially, piriform cortex neurons were found to play a key role in encoding the chemical identity of odors, reflecting a direct link to the physical properties of the smells themselves. Meanwhile, amygdala neurons were more involved in coding the emotional or subjective valence of odors—whether a smell was perceived as pleasant or unpleasant. The hippocampus, a region better known for its role in memory, was shown to predict a person’s ability to identify odors, highlighting its role in linking odor perception with cognitive processes.

One fascinating discovery was how repeated exposure to the same odor led to a decrease in neuronal firing rates, a phenomenon known as repetition suppression. This suggests that the brain becomes less responsive to familiar smells, a mechanism that likely helps us to focus on new or important olfactory information in our environment.

Furthermore, the researchers identified that neurons in the piriform cortex and amygdala were also responsive to odour-related images, revealing a striking cross-modal integration in these regions. This suggests that these brain areas are not limited to processing smells alone but also play a role in connecting olfactory information with visual stimuli, supporting a more comprehensive understanding of how we experience and interpret the world around us.

The study also revealed conceptual coding in olfaction, with some neurons responding to both odors and images that were semantically related, such as the smell of an orange and a picture of the fruit. This opens up exciting new avenues for understanding how the brain organizes sensory information based on meaning and association.

The findings bridge a long-standing gap between human studies and animal models of olfaction, advancing our understanding of how the human brain processes smells. This research not only sheds light on the neural mechanisms of olfactory perception but also underscores the intricate roles played by different brain regions in interpreting and integrating sensory information.

For further details, please refer to the full publication https://doi.org/10.1038/s41586-024-08016-5

Publication: Kehl, M.S., Mackay, S., Ohla, K. et al. Single-neuron representations of odours in the human brain. Nature (2024).

Scientific contact:
Prof. Florian Mormann
Department of Epileptology- Cognitive and Clinical Neurophysiology
Bonn University Hospital
E-Mail:  fmormann@yahoo.de

Written by: Dr Michela Barboni, Ph.D