In a healthy brain, the most apparent change in behaviour and consciousness occurs when we fall asleep, at the point when we shift from a state in which we are alert, awake and aware of our surroundings, to one of sleep. A study published in the printed version of Cerebral Cortex, led by Gustavo Deco, director of the Center of Brain and Cognition of the Department of Information and Communication Technologies (DTIC) at UPF, has shown the global and local changes that occur in the brain when a person falls asleep.

There is a region in the brain that has been mainly preserved during evolution - the reticular activating system - which includes the reticular formation and its connections, and which is responsible for the regulation of wakefulness and day/night oscillations.

A decline in the activity of the reticular activating system leads to slow-wave sleep, which is characterized by slow brain waves. At night, synchronized slow-wave sleep alternates with sleep paradoxical or desynchronized or sleep, so that most sleep is slow-wave, with intervals of paradoxical sleep.

Gustavo Deco, the director of the Center of Brain and Cognition, and Swiss and American scientist colleagues, have found , that this change is not sudden, but instead more gradual than previously thought. They have also shown that on waking, local slow waves  are recorded and that slow waves are rarely global.

Neuromodulation is responsible for the extent of the slow brain waves

Imaging studies using functional magnetic resonance imaging (fMRI) reveal changes in the functional connectivity when the brain is at rest, between the time of waking and slow-wave sleep. However, how the state of rest is altered during this transition period remains unclear.

According to Deco "in this study, we used computational models to simulate human cortico-cortical anatomical connections in order to evaluate the functional changes in the brain's resting state when the model " falls asleep" and that is how they were able to show that there is a progressive decline in excitation of neuromodulation.

This study shows that when cholinergic neuromodulation (via the neurotransmitter acetylcholine) decreases, local slow waves appear, while the general organization of neural networks in the resting state does not change.

At the macroscopic level, these local slow waves are structured in networks similar to the networks in the resting state. However, when the neuromodulator decreases further, to very low levels, the slow waves become global and the neural networks in the resting state merge into a single undifferentiated and broadly synchronized network.

For further reference, see:

Gustavo Deco, Patric Hagmann, Anthony G. Hudetz, Giulio Tononi (2014), " Modeling Resting-State Functional Networks When the Cortex Falls Asleep: Local ang Global Changes", Cerebral Cortex, doi.: 10.1093/cercor/bht176, desembre, 24, pp. 3180-3194.