Interview with Brice Bathellier

Following the publication of the article “Olfactory modulation of barrel cortex activity during active exploration and passive vibrissae stimulation”, can you tell us more about how the brain elaborates sensory percepts?

The elaboration of sensory precepts by the brain relies on areas of the cortex (the ‘grey matter’ on the surface of the brain), which can be considered as specialised channels, primary cortices, each processing a sensory modality (vision, hearing, touch, olfaction, gustation, proprioception). However, it is rare for a single sense to be involved in our sensations. To deal with this, there are both nerve links between these specialised centres and so-called “associative” higher areas that integrate the information coming from them. These are responsible for high-level cognitive and executive functions such as memory, language and planning. In the human species, 75 u cortex, particularly in the frontal and parietal lobes, are dedicated to these associative areas, which are responsible for our representation of the world and ourselves.

The general mechanism of multisensory perception is therefore based on the integration of different messages in the associative cortices. But what about the influence of one sense on the other? In other words, apart from the bottom-up integration that takes place in the associative areas, followed by possible top-down feedback, are there any ‘short circuits’ that enable faster or more optimised processing?

How would you define the originality of your team’s work?

The originality of my team’s work lies in having found neurons in the somato-sensory cortex of the mouse (tactile cortex) that respond to olfactory stimulation. In other words, the channel dedicated to touch is ‘infiltrated’ by olfactory ‘moles’. This may be a peculiarity of rodents. These nocturnal animals make extensive use of their sense of smell and their vibrissae (whiskers) to explore their environment. The vibrissae are veritable “fingers”. They are highly sensitive, thanks to the large surface area devoted to them in the somato-sensory cortex (barrel cortex in the title). Like the whiskers, the nose is the animal’s outpost, and coordination between the two senses has undoubtedly been selected for rapid exploration of the environment, food and fellow creatures. And this is exactly what the researchers have shown. Certain neurons in the cortex, sensitive only to stimulation of the vibrissae, see their response increased or decreased in the presence of odorant.

Inter-sensory modulations had already been found in animals between hearing and vision: in this case, the sound highlights the appearance of an object in the visual field. Olfaction-touch or vision-hearing, these processes enable bi-sensory messages to be integrated at the level of the primary cortices, even before moving on to the associative areas. The result would be better detection of multisensory coincidences, favouring rapid recognition and response, and possibly better learning in the longer term.

Finally, do similar mechanisms also exist in humans?

We don’t know whether such mechanisms exist in humans, but we do know about synesthesia, in which one or more senses are associated. The most common seems to be seeing the letters of the alphabet in colour. This may be the result of incomplete pruning of communications between the cortical area responsible for colour vision and the cortical area responsible for character vision. This pruning normally occurs in early childhood. It depends on experience and favours the performance of the circuits. In any case, even if nerve impulses travel more slowly through the brain, human babies are already capable of detecting these congruences and drawing lessons from them to support their curiosity and approach subsequent learning.