Using optogenetic control, researchers have created an electrical signature that is perceived as an odor in the brain's smell-processing center, the olfactory bulb, even though the odor does not exist. Applied in the brains of mice, the approach proved useful in advancing our understanding of the neuronal logic of how mammalian brains perceive smells and distinguish one smell from the next. "Deciphering how the sense of smell works has recently received an interesting new twist for two reasons: a robust early symptom of COVID-19 is a loss of the sense of smell, and trained animals can potentially be trained to sniff out diseases," said lead author Edmund Chong. "Hence, a better understanding of the mechanisms of smell can potentially aid the design of powerful tools for disease detection and treatment during a pandemic." The science of sense seeks to understand the neural activity generated by sensory stimuli and how this activity creates and shapes the perception of the senses. Odors induce complex activity patterns in the olfactory bulb, a tiny structure located at the front of the brain. The combinations of individual neurons that respond to the stimuli - which can vary in both location and timing - are thought to underly how individual smells are perceived. However, untangling the complexities of such activity and understanding how it determines how a scent is perceived have been difficult. Edmund Chong and colleagues designed experiments based on the availability of mice genetically engineered by another lab so that their brain cells could be activated by shining light on them -- a technique called optogenetics. Next they trained the mice to recognize a signal generated by light activation of six glomeruli -- known to resemble a pattern evoked by an odor -- by giving them a water reward only when they perceived the correct "odor" and pushed a lever. If mice pushed the lever after activation of a different set of glomeruli (simulation of a different odor), they received no water. Using this model, the researchers changed the timing and mix of activated glomeruli. They found that changing which of the glomeruli within each odor-defining set were activated first led to as much as a 30 percent drop in the ability of a mouse to correctly sense an odor signal and obtain water. According to the results, the novel approach revealed key spatial and temporal neural features, which combined, offer a code of sorts for how the brain converts sensory information into perception of an odor.
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Journal
Science