The second law of thermodynamics assists the study of the brain.
The principles of thermodynamics are applicable to any system including the study of brain states.
Deco's group and international collaborators study how brain dynamics change in different states, for example, sleep, wakefulness, coma, or neurological diseases. The idea is that the brain works like an orchestra, with a conductor and section leaders (violins, wind, strings...) and depending on our state, these hierarchies change and transform. A hierarchical system is where one or more zones take the baton, to lead other areas.
This hierarchy was quantified, not only by measuring the interactions between parts of the brain using fMRI, which allows seeing brain activity, but also by using a trick based on the second law of thermodynamics that says a system tends to go from ordered to more random behaviour or entropy as time passes.
With time, if entropy increases, it means that the system is not in equilibrium and is non-reversible in time, so, we cannot know at what point it is. Conversely, if entropy does not grow or does not occur, then it is a system in equilibrium and reversible in time.
An example of the increase in entropy, where the system is irreversible, would be a video of a breaking glass. If we see the film backward or forward we easily know if it is before or after the glass breaks.
In contrast, a system where entropy does not increase, which is reversible, would be a video of a billiards game at the moment when two balls collide and rebound away from each other. Playing the video forward or backward provides identical images, so we cannot know at which point we are.
By quantifying the relationship between different parts of the brain, symmetry is broken. By moving the series of signals back and forth, and compare them as with the films, we know which system it is: a reversible one is not hierarchical, a non-reversible one is hierarchical.
The technique is a good biomarker of brain consciousness. This is very practical as the classification of states helps define patient treatments and expectations.