The Map Of Smell

The perceptual apparatus we understand the least is smell. And that is a problem, because smell is extremely important not only for getting information about our surroundings, but also for our emotional well-being. Loss of smell can lead to severe problems with emotion regulation and emotional connection with loved ones. One major question about olfaction (the sense modality of smell) is how it represents. When we smell coffee or burnt pizza, what is the difference in the encoding of these two very different smells? This is the question that got at least partially answered.

A bit of background: When it comes to the other sense modalities — seeing, hearing, touching, taste — we have a fairly clear idea about how they represent. Take vision. Vision is retinotopic, which means that even the earliest visual representations of the world around us are homomorphic with the retina. This just means that the visual cortex have something like an inexact copy of the retinal image. In other words, the visual cortices are organized in the same way as the retina: If there is a small triangle projected at the top left corner of the retina, there will be a roughly triangle-shaped activation at the corresponding corner of the primary visual cortex. If the triangle then moves across the retina to the bottom right corner, the same thing happens in the primary visual cortex.

Hearing, in contrast, is tonotopic, which means that there is a correspondence between the pitch we hear and the region of the auditory cortex where this stimulus is processed. The sense of touch is especially interesting. Here, the part of the brain that is responsible for processing tactile stimuli—the somatosensory cortex—has a distorted map of the human body, which, in terms of structure, is something like a distorted mirror image. So a pinprick on your left pinkie finger and another pinprick on your left pinkie finger are localised very close together in your somatosensory cortex and a pinprick on your left ring finger only a tiny bit further from the first two. The map of the human skin in the somatosensory cortex is quite distorted, though, as more area is devoted to those parts of the body that have more tactile receptors (especially more pain receptors).

But when it comes to smell, the assumption has long been that the arrangement of the receptors in the nose and the corresponding arrangement of where they are processed in the brain might be plain random. As the especially impressive research on more than 5 million neurons in more than 300 individual mice shows, this is not true. Both the receptors in the nose and the parts of the piriform cortex (where smell is processed) are organized in distinct (but overlapping) stripes, according to the type of receptors.

These findings are important not only because they dispel an old mystery about smelling. They are important because they can tell us how we can help those who have lost their sense of smell. As has become painfully clear in light of the overwhelming number of people who lost their smell as a result of COVID-19 infection, the sense of smell is intimately connected to our emotions. The loss of the sense of smell can, and often does, lead to weakening or sometimes complete loss of emotional connection to others, including loved ones. Loss of smell is also a serious emotional loss. And with these new findings in hand, we are in a better position to try to do something about it.

- Psychology Today