All right, everyone. Break into groups and discuss, and we’ll reconvene in, oh, about 8 hours.

Just read a 2013 study published in NeuroImage by Enzo Tagliagucchi and colleauges, working out of Goethe University Frankfurt am Main. They used some fascinating modern analytic tools to study the way the brain’s functional network architecture changes as we move from wakefulness, through the different stages of sleep (from light sleep to deep sleep).

Functional network what, you say? Right. Well. Let me give you some background. Neuroscience research in the last five to ten years has put a huge emphasis on the links between brain areas, and the participation of brain areas in collective action. Gone are the days when we ask what part of the brain does what, as if the parts are separately responsible for different functions. Instead, we ask what parts of the brain are coupled to what other parts, and in what way does their collective action relate to function (i.e., to the generation of sensory or emotional experience, the processes of thought, the execution of actions, etc.). It’s all about connectivity now. Using cool techniques, researchers have been able to show that the brain consists of a relatively small number of networks, each of which has dense connections among the structures that participate in it, and relatively sparse connections with structures from other networks. Moreover, these networks become (collectively) active at different times, depending on what the brain is up to.

Using these techniques, Tagliagucchi and his colleagues explored how the arrangement of the connections among brain areas changes as people fall asleep and move from light sleep toward deep sleep. They found that as sleep deepens, there is an increase in modularity, which means that the brain’s various networks become increasingly segregated from one another. The connections within each network remain the same or are strengthened, while the connections between networks are weakened. There’s even a little bit of swapping that happens, wherein brain regions that participate strongly in one network during wakefulness are “reassigned” to other networks during deep sleep. So, basically, as the brain moves further and further away from consciousness, its functional subdivisions tend to go off and do their own thing, while dropping their connections with one another. Why they do this is still a matter for speculation, but it does support the idea proposed by some that consciousness is a function of whole-brain integration, and departures from consciousness represent a breakdown in this integration.

There’s an interesting EEG connection, too, in that this movement toward increased functional segregation of networks correlates highly with the amount of EEG activity in the Delta frequency band (between 1 and 4 Hz). The amount of Delta activity effectively indexes the amount of network segregation. This raises the intriguing possibility that the presence of Delta activity even during wakefulness for some clinical populations might correspond to a weakening of long-range, cross-network communication.

Leave a Reply