Noses and Neuroscience: The Olfactory System

The neuroscience department at Bowdoin has been hosting a handful of guest speakers lately, and last week, Ian Davison came to Bowdoin to talk a little about his research in the olfactory system (a.k.a. your nose) and its connections to the brain. The field of neuroscience has figured out a lot about the other senses — vision, sense of touch, hearing — but truthfully, it’s not quite understood how your nose is able to identify a chemical odorant, categorize it as a “good” or “bad” smell, and then use the memory of that smell to affect your future behavior. 

That’s essentially what Davison is trying to figure out with his research. How does the brain recognize and then categorize different odors? Since smells convey important cues such as attraction or repulsion and odors contribute significantly to how memories are formed, it’s absolutely essential that our noses and brains are able to do this. Some smells, like rotting food, are inherently repulsive to humans. We can understand that this protects our health, but how does our brain know to tell us to avoid it?

Here's a quick fun neuroscience fact: mice are inherently terrified of the smell of cat urine, and mice without this olfactory recognition (due to a mutation) are eaten much sooner than their cat-urine-odor-fearing playmates. Thus we see how important it is that your brain properly interpret what you’re smelling and what that means for you!

In his first goal to understand how the brain processes different smells, Davison presented mice with different odors: fresh strawberries, slightly old strawberries, and rotten strawberries. By using some pretty cool/complicated techniques, he mapped the different patterns of neuronal activity that was associated with each smell. That’s another reason why this research is so complicated — when you smell a particular odor, it’s not one neuron or even one group of neurons that’s being activated. You’re activating multiple different groups of neurons (in this case called 'glomeruli') that are in different subsections of your brain. The cortical patterns of activated glomeruli didn’t show any particular patterns, and Davison is currently working to tweak some aspects of his experiment to work around some of these tricky variables.

In his second objective to see how social experiences can shape how organisms’ perception of odors, Davison gave the example of mating behaviors. If a female mouse is exposed to the smell of other unfamiliar males (rather than the male that she has spent the most time and mated with), she will not reproduce, even when the other males mate with her. This might seem strange, but it actually really makes sense when you find out that male mice have a tendency to eat pups that are not their own offspring.

So, how does the female prevent a pregnancy from happening when exposed to the smell of an unfamiliar male? Abridged answer: because the female mouse’s nose and brain can do something called rapid sensory imprinting, where each unique smell is like a fingerprint that identifies an individual animal. The female can distinguish different males by their smell, and these different smells induce hormonal circuit changes in her body that either enable or prevent her from becoming pregnant.

Here's another quick fun neuroscience fact: this form of birth control does not apply to humans, so don't even think about it!

Much more research has to be done on the olfactory system to figure out how odors are encoded into cortical circuits and how pheromonal memory formation affects circulating pheromone levels. That being said, it’s pretty cool to learn about research on the forefront of figuring out the mechanisms behind something that we can all do (but don’t know how or why we do it). That’s what makes neuroscience so unique!