Smell and memory: how the olfactory system links to memory centers.

Proust's Madeleine Moment gives us a vivid description of the link between odor, vision, and event memory and, most vividly, place memory. How and why are odor and memory so closely joined?

The how is relatively easy to understand. Between the neurons in the olfactory bulb in the nose and the memory centers in the brain is a mere three steps.

Odorants enter the nose, where they are captured by the endings of nerve cells (olfactory cells) located in the olfactory epithelium at the roof of the nose. These nerve endings pass through holes in the part of the skull called the cribriform plate to reach the olfactory bulb. These nerve endings are shown in yellow in the image from Wikipedia, above.

Once inside the olfactory bulb, these nerves meet up the next nerve cells in line to pass the message along. These next nerve cells compose the olfactory tract. The picture below, from Gray's Anatomy (1918) shows the base of the brain, with the olfactory bulb (the Q-tip shaped pair of structures in yellow at the top), and the olfactory tract leading directly into the brain substance.

Once the olfactory tract nerves plunge into the substance of the brain, they meet up with the first set of important cells for memory, located in the entorhinal cortex. Cells in this part of the brain are responsible for coordinating input from vision as well as smell. In addition, cells in this part of the brain keep track of our movements and turns via input from the the vestibular system in our inner ear.  In this way they help make navigation and place memory possible.

This combined representation is sent, second by second, to the hippocampus, which records our experiences of action, event and place in memory; and to the amygdala, which gives memories their emotional color; and finally to the orbitofrontal cortex, right above the eyes, to evaluate our overall situation, compare it with our emotions and memories, and decide what to do. 

Why all this coordination focused on smell, vision, and motion? For a very practical reason: we have to remember where we went to get food and water so we can go back for more, and we have to remember where and when we met up with either danger or comfort, in other words where our friends and enemies (human, plant, and animal) were, so we can find them back or avoid them.

An aside that demonstrates the power of the odor-emotion connection, so richly described by Proust: a very recent study showed that if you present people with a pleasant odor followed by a picture of a face with a neutral expression (no emotion), people will consider that person to be pleasant and friendly. Conversely, if you precede the same face with an unpleasant odor, the person will be judged unfriendly and unpleasant!* 

Makes one wonder about the subliminal influence of odors in everyday life — if you meet someone over a deliciously aromatic cup of tea, would you automatically find the person to be so very nice?

* Stephanie Cook, Nicholas Fallon, Hazel Wright, Anna Thomas, Timo Giesbrecht, Matt Field, and  Andrej Stancak. Pleasant and Unpleasant Odors Influence Hedonic Evaluations of Human Faces: An Event-Related Potential Study. Front Hum Neurosci. 2015; 9: 661. Published online 2015 Dec 1. doi:  10.3389/fnhum.2015.0066