When odor brings back memories…

Every one of us has certainly had the intense experience of diving headlong into a memory triggered by a smell. Generally, it’s an enjoyable moment that brings back the place where we first smelled the odor, and particularly the emotions we were experiencing at that time. Neurologists have identified the brain mechanisms that bring about this flood of memories induced by odor. So how exactly are smell and memory linked in the brain?

The studies (along with Proust‘s apt description of the episode of the Madeleine from Remembrance of Things Past) have already shown that smell and taste are more evocative of memory than the other senses. Led by neurologist Kei M. Igarashi and his colleagues, and published in Nature, the research highlighted the appearance of brain waves of similar frequencies in two areas of the brain: the entorhinal cortex and hippocampus. The first is involved in the processing of olfactory information and the second plays a critical role in memory (and spatial skills). In the case of an olfactory stimulus associated with a memory that we are capable of describing or verbalizing, we’re talking about declarative memory. How were the researchers able to observe the retrieval of declarative memory through this interactive activity between the entorhinal cortex and the hippocampus in rats?

The experiment was conducted as follows: the researchers placed 17 rats in a maze and trained them to associate a smell with food. Specifically, the rats were taught to memorize a path (through the maze to reach the place where the food had been placed) based on a particular odor. After 3 weeks of training, the percentage of correct food-smell associations continued to increase, finally leveling out at 85%. Neuronal activity between the entorhinal cortex and hippocampus was then measured in 5 well-trained rats. The scientists then observed that the ability of the rats to correctly associate a smell to a response site was correlated with the appearance of perfectly synchronized brain waves (20-40 Hz) in these two areas of the brain. In this experiment, the harmony of frequencies thus allowed olfactory and spatial memory to become synchronized. The rats were thus able to recall a context, here the location of the food, based on a smell.

Nevertheless, as the authors of the study point out, if the relationship between the entorhinal cortex and hippocampus is established, its links to the formation of memory still require further research. Still, this experiment offers interesting information about learning and memorization techniques. Will we soon be introducing smell and taste in schools as learning tools?
Source: Kei M. Igarashi, Li Lu , Laura L. Colgin, May-Britt Moser & Edvard I. Moser, Coordination of entorhinal–hippocampal ensemble activity during associative learning


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