Tuesday, 10 May 2016
Dynamics of complex networks sheds light on loss of consciousness associated with sleep
When I read the abstract of the article “Hierarchical clustering of brain activity during human nonrapid eye movement sleep” published by Dr Habib Benali and his team in the April 2012 issue of the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS), I quickly realized that the phenomenon that these authors had observed was pretty much the opposite of the one observed in a study by Douglass Godwin and his team: the temporary breakdown of the brain’s functional networks when an individual becomes aware of a stimulus.
In the Benali study, the research team used functional magnetic resonance imaging to measure functional connectivity among various parts of the brain. These measurements showed that when the subjects fell into non-REM sleep, in which their consciousness was reduced, their brain activity became less connected and more fragmented (modular). The authors hypothesize that this reorganization of brain activity into smaller and smaller independent modules during non-REM sleep prevents the brain from performing the overall integration of information that seems to be necessary for consciousness. This idea was first advanced some time ago by Bernard Baars, with his theory of the global workspace, and has now received support from a number of recent studies, including the one by Godwin and his team.
Taken together, these two studies clearly seem to point to something like a pooling of brain activity into extensive networks when it comes to conscious phenomena and a fragmentation of brain activity into smaller modules that perform certain computations locally, when it comes to unconscious phenomena.
Benali’s study also sheds a bit of light on some apparent paradoxes, such as that when you are asleep and unconscious, your brain remains just as active as when you are awake. The difference is that when you are in deep sleep, this activity is divided into smaller pieces, instead of being shared on a large scale.
In addition, this study shows that an increase in complexity (in this case, more consciousness rather than less) is accompanied by an increase in the number and complexity of the interrelations among the elements of a network, just as was the case for the metabolic networks in the first living cells (or autopoietic systems), in the transition from single-celled to multi-celled organisms, and in the emergence of neurons—specialized cells capable of integrating information and forming networks in which nerve impulses can travel rapidly.
In short, whatever aspect of life we look at, we see networks, and it is no accident that the increasingly complex manifestations of life (such as human consciousness) are accompanied by the formation of increasingly large and complex networks.
Complex reorganization of brain integration during human non REM sleep
Hierarchical clustering of brain activity during human nonrapid eye movement sleep
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