Researchers decode how the brain identifies and responds to odors

Neurons unlocked: Study reveals how the brain decodes and reacts to smells, linking odors to emotion and memory.

Study: Single-neuron representations of odours in the human brain. Image Credit: Studio Romantic/Shutterstock.com

In a recent study published in Nature, researchers investigated the response of individual neurons in specific regions of the brain to olfactory cues.

Through neural activity recordings during odor identification tasks, they uncovered how neurons in brain regions such as the piriform cortex, hippocampus, and amygdala encode subjective perception and odor identity.

Background

Although olfactory senses are vital for humans and other animals, the mechanisms through which the brain processes odors are not completely understood. The odor molecules are thought to activate sensory neurons in the nose, which then send signals to the olfactory bulb.

The signals are thought to travel from here to regions of the brain such as the amygdala, piriform cortex, and entorhinal cortex, which are the primary olfactory cortex regions.

Studies using animal models have revealed that specific neurons in the piriform cortex, areas in the medial temporal lobe, and amygdala are involved in identifying odors. Imaging studies in humans have also suggested the involvement of these brain regions in the processing of odors.

However, while animal model studies have shown how these regions respond to olfactory cues, no recordings of the activity of individual neurons to odors exist, and how individual neurons process smells remains unclear.

About the study

The present study aimed to understand the response of individual neurons to olfactory stimuli by recording the neuronal activity in the areas in the medial temporal lobe and the piriform cortex while participants identified and rated odors.

The study enrolled participants who were drug-resistant epilepsy patients undergoing treatment and involved the recording of single-neuron activity in specific regions of the brains. In contrast, the participants underwent specific tasks involving odor presentation.

The neuronal responses were measured through depth electrodes that had already been implanted in the patients as part of their seizure monitoring and ongoing treatment.

The participants had Behnke-Fried depth electrodes implanted in them, and these electrodes were specifically designed to measure neuronal activity. These electrodes have cylindrical macroelectrodes containing microwire bundles made of platinum-iridium that are highly sensitive to electrical signals from neurons.

The technical limitations of the electrodes prevented their implantation in larger brain areas such as the hippocampus and the amygdala. Therefore, the neuronal readings were obtained from the medial temporal lobe and piriform cortex.

The participants were presented with 15 different types of olfactory stimuli, delivered using pen-like devices known as “Sniffin’ Sticks,” that contained the various odors. The odors were presented one at a time, and the participants were required to inhale once on command.

Each of the odors was presented eight different times in a random order, and a control pen with no odor was also used. A belt placed near the diaphragm measured the inhalation of the participants and allowed the researchers to correlate breathing patterns with brain activity during olfactory neuronal activity.

The experiment consisted of two tasks — the first where the participants were asked to indicate whether they liked or disliked the odor, and the second, where they were asked to identify the odor by selecting one out of four options.

These tasks allowed the researchers to determine how neurons responded to olfactory cues in subjective preferential terms as well as in the recognition and identification of odors.

Results

The researchers found that neurons that specifically responded to odors also helped decode the identity of the odors. The study found that the amygdala plays a distinct role in the emotional aspects of olfactory senses, and the hippocampus is involved in identifying odors.

Close to 40% of the neurons in the amygdala, piriform cortex, hippocampus, and entorhinal cortex displayed specific firing responses to different odors and also showed increased firing rates when exposed to odors as compared to exposure to the control with no odor. This confirmed their role in processing olfactory information.

While the neurons in the amygdala, piriform cortex, and entorhinal cortex showed neuronal activity related to odor identification, fewer neurons were required in the piriform cortex to identify the odor as compared to the other regions accurately.

Furthermore, the odor identification-related neuronal activity occurred faster in the amygdala and the piriform cortex than in the hippocampus and entorhinal cortex.

The neurons in the hippocampus, amygdala, and piriform cortex also showed reduced activity when the same odor was repeatedly presented to them. This process is called repetition suppression and was found to be independent of any inhalation changes, indicating that the neurons adapted to repeated stimuli over time.

The neurons in the amygdala responded more to odors that were liked than those that were disliked, indicating an emotional response to olfactory senses in the region.

The hippocampal neurons exhibited increased firing rates when odors were identified correctly, indicating that this region of the brain was associated more with successful odor recognition.

Interestingly, the piriform cortex and the amygdala responded to olfactory and visual cues, such as images corresponding to the odors, suggesting that these regions were involved in integrating sensory stimuli from different modalities.

Some areas also responded to different representations of the same object, such as odor, word, and image, indicating a higher order of processing semantic cues.

Conclusions

Overall, the study presented a detailed view of how the human brain processed olfactory stimuli and highlighted the roles of the piriform cortex and amygdala in the rapid and accurate processing of olfactory cues and recognition and emotional processing of odors.