If we examine our brain activity as a whole, the first image that comes to mind is one of an untuned TV: a noisy image consisting of black and white dots. Therefore, the question is: how is it possible that the brain works in such an organized way? The research teams of Jordi García-Ojalvo, leader of the Dynamical Systems Biology laboratory and full professor at Pompeu Fabra University, and Maria V. Sánchez-Vives, leader of the Systems Neuroscience team and ICREA research professor at the August Pi i Sunyer Biomedical Research Institute (IDIBAPS), are studying the phenomenon of stochastic coherence in the brain in an article published in the journal Nature Physics.

The neurons hosted in our brains do not seem to follow an ordered pattern of behaviour. In fact, the activity of an individual neuron rarely shows regularity in its impulses. How, then, can we explain the marked cycles that govern the functioning of our brain, especially in periods like sleep? Researchers point to the so-called “white noise“, that is, the set of random signals that commonly occur in any system and are usually considered undesirable. Let us imagine a neuronal response that depends on the magnitude of the signal reaching a certain threshold. Perhaps the signal alone, despite being cyclical and regular, does not reach the threshold required to trigger the response. However, if we add a background noise to this cyclical signal, the value of the signal increases thus exceeding the necessary threshold that will give rise to the response.

This mechanism has been previously described in other systems. Earth glaciations, for example, have happened fairly regular over millions of years. However, the Earth’s axis is not enough to explain these periodical glaciations. It is stochastic resonance: random fluctuations (background noise) that are added to the weak oscillation of the rotational axis that give glaciations their regularity. The phenomenon has also been described in biological systems. Paddlefish detect their food, plankton, through weak electrical oscillations that plankton emit regularly. A study at the University of Missouri shows that adding background noise (random electrical oscillations) to the system, the paddlefish detects plankton more easily and therefore eats more.

The scientific teams of UPF and the IDIBAPS have studied the cerebral cortex in situations that simulate the state of deep sleep, and for the first have described time this stochastic coherence in the brain. In order to do so, they controlled the noise level by varying the excitability of the cortex and observed that the low waves characteristic of deep sleep become more regular when this excitability, and therefore randomness, increases. Thus, they have managed to detect that there is a noise level that is optimal for maximum regularity, but from here on, noise dominates over order. It turns out that white noise, which could be compared with the black and white dots that can be seen on out-of-tune TVs, is what allows such irregular signals generated by neurons to be converted into oscillations which often show clockwork regularity.

Reference work: Belén Sancristóbal, Beatriz Rebollo, Pol Boada, Maria V. Sanchez-Vives, Jordi Garcia-Ojalvo. Collective stochastic coherence in recurrent neuronal networks. Nature Physics, May 2016. DOI: 10.1038/NPHYS3739.