Smell and memory split apart along the brain’s pathway
You can recognise a smell, and you can also tell whether you have encountered it before. How do the brain’s systems separate these two things? A mouse study traces the route.
Researchers recorded the activity of individual neurons at five points along the olfactory pathway, from early cortex to deep within the hippocampus. They did this while mice smelled odours that were either familiar or entirely new. The researchers published their findings in eLife.
Two types of information, one system
In early brain regions, such as the anterior olfactory nucleus (the first station after the olfactory nerve), neurons responded broadly to many different odours. They also signalled whether an odour was novel or familiar. As information travelled further through the brain, selectivity increased: neurons responded more specifically to particular odours.
In the hippocampus, the brain region central to memory formation, responses were no longer dependent on novelty or familiarity. The population-level representations of odours had become stable there. At the same time, the neural codes for odour identity and for novelty detection had progressively separated from one another.
What does this tell us about memory and ageing?
The study describes a mechanism by which sensory recognition and implicit memories are processed along separate routes. That is relevant for understanding memory processes in general. The connection to ageing is currently indirect: both the olfactory pathway and the hippocampus are vulnerable in neurodegenerative conditions. Loss of smell recognition is an early sign in both Alzheimer’s and Parkinson’s disease. Understanding how this system normally functions is a step toward understanding what goes wrong in disease.
The study was conducted in mice and represents basic neuroscience research. Direct conclusions about humans cannot be drawn from these findings alone.