By Elaine Zhu
As summer blends into autumn, the air acquires a crisp and almost earthy scent. A deep breath in—the smell of cinnamon, hot cocoa, firewood crackling after a school bonfire—and memories come flooding through me, as if I’m transported to a familiar autumn day. It’s not just the smells of a cool autumn day that make me nostalgic: the smell of chlorine brings me back to my childhood swimming lessons and the smell of bread brings me back to baking with my family. With so many distinct smells in the world, how can one scent trigger such particular memories?
First, we must look at how our nose works. High up inside your nose are tissues with specialized sensory cells called olfactory sensory neurons. These neurons have odor receptors that are stimulated by small molecules in the environment. After these molecules are detected by the receptors, the neurons send an electric signal to the olfactory bulb in the forebrain, which is the anterior part of the brain. The forebrain then sends the signal to be deciphered by other regions of the brain, like the thalamus. To organize all the smells, the olfactory sensory neurons wire to an “olfactory map” within the brain’s olfactory bulb, which is the brain structure responsible for smell. The thalamus also sends these signals to the amygdala, where emotions are processed, as well as the hippocampus, where memories are consolidated and stored. This whole network results in a close relationship between what we smell and what we experience in our memories and emotions.
Scientists are currently exploring the connection between memory and smell. In a 2018 Nature Communications study on vivid memory storage, neurobiologists from the University of Toronto discovered a novel pathway between the anterior olfactory nucleus (AON), a region of the brain in the forebrain that is involved with olfaction, and the hippocampus. To investigate the connection between Alzheimer’s disease and smell, the scientists recreated the memory loss displayed by Alzheimer’s patients in the mice by severing this pathway between the AON and the hippocampus. In mice where the pathway between the AON and hippocampus was disrupted, the mice continued to smell odors that they had previously been exposed to for longer amounts of time, signifying that the mice could not remember the scent. This new finding could also help explain why an early symptom of Alzheimer’s disease is the loss of smell. While smell tests are being developed for early detection of Alzheimer’s disease, there remains much to be discovered about the link between Alzheimer’s and olfactory problems.
The intimate connection between our nose and our brain creates a strong relationship between what we smell and what we remember. This link can even illuminate how certain diseases like Alzheimer’s disrupt specific pathways in our brain and bodies. We still don’t have all the answers regarding this connection between our sense of smell and our memories, but further research on the AON and hippocampus pathway and others like it can shed light on the complicated relationship between smell and memory.