Summary: A new study shows that specific human brain cells respond not only to odors but also to images and written words associated with those scents, offering fresh insight into how we perceive smells. Researchers recorded single-neuron activity in regions such as the primary olfactory (piriform) cortex, amygdala, hippocampus and entorhinal cortex, and found that these neurons distinguish different odors and link them with visual and semantic cues.
Using recordings from epilepsy patients who had diagnostic intracranial electrodes implanted, the team bridged a long-standing gap between animal models and human olfactory research. Individual neurons sometimes responded to a scent, the matching photograph and the written name of the same object (for example, a banana), indicating that odor processing in humans integrates visual and semantic information at an early stage. The findings raise the possibility of future “olfactory aids” and underscore the close ties between smell, emotion and memory in the human brain.
Key Facts
- Neurons in the human piriform (olfactory) cortex respond to odors, corresponding images and words.
- The amygdala encodes subjective odor valence, distinguishing pleasant from unpleasant smells.
- Different medial temporal lobe regions play distinct roles: piriform cortex encodes chemical odor identity; hippocampus reflects subjective perception and identification performance.
- The study provides single-neuron evidence that links prior animal findings to human olfactory processing.
Source: University of Bonn
We often only notice how vital our sense of smell is when it fades: food loses flavor, and we may fail to detect dangers like smoke.
Researchers at the University Hospital Bonn (UKB), the University of Bonn and RWTH Aachen University investigated the neuronal basis of human odor perception by recording activity from individual nerve cells during smelling. The recordings reveal that single neurons can identify odors and also respond selectively to images and words related to those odors—for instance, the smell, picture and written name of a banana.
These results address a major knowledge gap: while functional imaging (fMRI) has identified brain regions involved in smell, it cannot resolve activity at the level of single neurons. Until now, detailed cellular-level models of odor coding largely came from animal studies, leaving unanswered how well those models translate to humans.
Nerve cells identify odors
The research group led by Prof. Florian Mormann recorded single-neuron activity while people with implanted electrodes were presented with pleasant and unpleasant odors (for example, fresh fruit or the smell of old fish). The patients participated as part of clinical diagnostics at one of Europe’s largest epilepsy centers, making possible direct intracranial measurements in awake humans.
First author Marcel Kehl reports that individual neurons reacted reliably to specific scents, and that neuronal firing patterns could be used to predict which odor a participant was smelling. Measurements showed that the primary olfactory (piriform) cortex provided the most accurate encoding of odor identity, while medial temporal lobe areas contributed specialized functions: hippocampal activity predicted whether participants correctly identified an odor, and amygdala neurons encoded whether a smell was experienced as pleasant or unpleasant.
Cross-modal and semantic coding: smell, image and word
To probe connections between smell and vision, researchers presented matching images after odor exposure—such as a photograph of a banana following the banana scent. Surprisingly, neurons in the piriform cortex not only responded to odors but also to the associated images. The team discovered single neurons that reacted selectively to the scent, the matching picture and the written word for the same object. This indicates that semantic and visual information are integrated with olfactory processing early in the sensory pathway.
Overall, the data show multimodal coding across regions, with notable cross-modal representations in both the piriform cortex and the amygdala. The findings confirm and extend decades of animal research and reveal functional specializations across human brain regions involved in odor processing.
“This is an important contribution on the way to decoding the human olfactory code,” says Prof. Mormann. Co-corresponding author Prof. Marc Spehr notes that the piriform cortex appears to perform tasks beyond pure odor detection, participating in linking smells to visual and conceptual knowledge.
The authors emphasize that further research could enable practical applications, such as olfactory assistive devices that might one day be as commonplace as glasses or hearing aids.
Funding: The study was supported by the German Research Foundation (DFG), the Federal Ministry of Education and Research (BMBF) and the state of North Rhine-Westphalia (NRW) as part of the iBehave project.
About this olfaction and visual neuroscience research news
Author: Inka Väth ([email protected])
Source: University of Bonn
Contact: Inka Väth – University of Bonn
Image credit: Neuroscience News
Original Research: Open access. “Single-neuron representations of odours in the human brain” by Florian Mormann et al., published in Nature.
Abstract
Single-neuron representations of odours in the human brain
Olfaction is a core sensory modality that guides both animal and human behavior, yet its underlying neural processes in humans have remained poorly understood at the single-neuron level. This study reports recordings of single-neuron activity from the piriform cortex and medial temporal lobe while awake humans performed odor rating and identification tasks.
The researchers identified odor-modulated neurons in the piriform cortex, amygdala, entorhinal cortex and hippocampus. Across these regions, neuronal firing patterns encoded odor identity with high accuracy. Repeated odor presentations led to reduced firing rates, revealing central repetition suppression and habituation effects.
Distinct roles emerged for medial temporal lobe structures: amygdala neurons encoded subjective odor valence, whereas hippocampal neurons predicted behavioral odor identification performance. Piriform neurons preferentially encoded chemical odor identity, while hippocampal activity reflected subjective perception.
Importantly, piriform cortex neurons also reliably encoded odor-related images, supporting a multimodal role for this region. Cross-modal coding—responses to both odors and images—was particularly evident in the amygdala and piriform cortex. The team also identified neurons that responded to semantically coherent odor and image pairs, indicating conceptual coding schemes in human olfaction.
These results bridge the gap between animal models and non-invasive human studies and advance our understanding of odor processing by identifying neuronal odor-coding principles, regional functional differences and cross-modal integration.