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November 7, 2011
How Our Neurons Focus On The World Around Us


To be able to focus on the world, we need to turn a part of ourselves off for a short while, and this is precisely what our brain does. But, why do we "turn off" our neurons at times when we need them most? A team of researchers from Inserm, led by Jean Philippe Lachaux and Karim Jerbi (Centre de Recherche en Neurosciences de Lyon (Lyon Neuroscience Research Centre)), has just demonstrated that a network of specific neurons, referred to as "the default-mode network" works on a permanent basis even when we are doing nothing.

They demonstrate more specifically that when we need to concentrate, this network disrupts the activation of other specialized neurons when it is not deactivated enough. The results have just been published in the Journal of Neuroscience.

When we focus on the things around us, certain parts of the brain are activated: this network, well known to neurobiologists, is called the attention network. Other parts of the brain, however, cease their activity at the same time, as if they generally prevented our attention from being focused on the outside world. These parts of the brain form a network that is extensively studied in neurobiology, and commonly known as the "default-mode network," because, for a long time, it was believed that it activated itself when the brain had nothing in particular to do. This interpretation was refined through ten years of neuroimaging research that concluded by associating this mysterious network ("the brain's dark energy" as it was called by one of its discoverers, Marcus Raichle) with a host of intimate and private phenomena of our mental life: self-perception, recollections, imagination, thoughts…

A study carried out by a team at the Centre de Recherche en Neurosciences de Lyon (led by Tomas Ossandon and managed by Jean-Philippe Lachaux, Research Director at Inserm and Karim Jerbi, Research Leader at Inserm) has just revealed how this network interferes with our ability to pay attention, by assessing the activity of the human brain's default-mode network neurons on a millisecond scale for the first time ever, in collaboration with Philippe Kahane's epilepsy department in Grenoble.

The results unambiguously illustrate that whenever we look for an object in the area around us, the neurons of this default-mode network stop their activity. Yet, this interruption only lasts for the amount of time required to find the object: in less than a tenth of a second, after the object has been found, the default-mode network resumes its activity as before. And if our default-mode network is not sufficiently deactivated, then we will need more time to find the object. These results show that there is fierce competition for our attentional resources inside our brain which, when they are not used to actively analyse our sensorial environment, are instantaneously redirected towards more internal mental processes. The brain hates emptiness and never stays idle, even for a tenth of a second.


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