Opinion – Suzana Herculano-Houzel: There are good smells here!

I haven’t read Proust, but I know the power of the madeleines, the French fries, on his memory. Smell has this reputation as a sense with special powers over memory: just a little smell and there comes a flood of affective memories, as if each aroma opened a floodgate of memories and associated feelings.

In fact, it’s just like that, even in the brain. Even the smallest of mammals have an olfactory cortex, called “piriform” because of its respectable pear shape, directly associated with the olfactory bulb, at the front of the brain, on one side, and the amygdala, already in the subcortical part, on the other. . This amygdala, which is not the amygdala of the throat, has the entire physiology of the body at its command, and is therefore capable of relieving our hearts, turning our viscera, heating our bodies, making our faces red or making us livid.

In other words: it makes us feel in our skin, literally, what goes on in our brain. The sensations that the brain then evokes, according to the physiological state of the body, are what we call emotions.

I teach my students that smell is therefore a physiological sense, capable of directly triggering changes in the state of the body: everything that comes to mind when we respond HOW we are, not WHERE we are.

But I will have to revise my lessons, because the cortical representation of smell is also spatial, according to a study recently published in the journal Nature by a group of researchers at the Champalimaud Foundation in Lisbon, Portugal. The researchers used a simple but ingenious test that gives mice the opportunity to touch their noses at one end of an X-shaped maze to receive a whiff of a scent there and then fetch a liquid prize at the corresponding end of the maze. , according to a code of correspondence that animals learn: banana on this arm, lemon on that, rose on the other.

Using electrodes inserted into the animals’ piriform cortex, which allows them to hear the activity of neurons as the animals stick their noses where they want and go after their prize, the scientists discovered that different neurons in the piriform cortex represent not only the identity of each smell, but also but also where they happen. It’s as if, with each odor, the olfactory cortex tells the brain not just “banana”, but “here is where the banana smell happens”.

The difference is extremely important, as we live on a space-time continuum. It sounds like “Star Trek” stuff, and that’s right: we, like the things around us, don’t exist loose in the world, but rather anchored to places where objects and sensations happen. The hippocampus, right next to the olfactory cortex, is the part of the brain where an ever-moving merry-go-round stitches together the narratives of our lives in space-time, as neurons representing the current activity move up and down the merry-go-round. The sense of smell, by the way, contributes by marking each place with its scent.