06 November 2012

Brain Networks in Anesthesia, Sleep, and Waking

Recent research collaboration between MIT, MGH, and Harvard Medical School has substantiated earlier findings on the mechanism of loss of consciousness in the human brain during general anesthesia.

Using sophisticated research tools including EEG and fMRI, they found that as consciousness was lost, different parts of the brain became "disconnected" from each other, even though particular areas of the brain continued to function.
...the time when consciousness was lost did coincide with a significant change in the overall structure of brain activity. While electrical activity in the conscious brain appears to be disorganized with no apparent regular patterns, at the point when study participants lost consciousness, their brain activity began to show regular oscillations between states of activation and deactivation.

..."These deactivated or silent periods of brain activity occur at different times in different brain regions, so communication between regions is interrupted" says Laura Lewis, co-lead author of the report. "It's as if one brain region is in Boston and the other is in Singapore – they can't make phone calls to each other because one is asleep when the other is awake." While this slow oscillation pattern has been previously observed in humans who are asleep or under ansethesia, this is the first study to record neuronal activity during the transition to unconsciousness, so it is the first to match the onset of this pattern with the loss of consciousness, she adds. _MXP

Quick synopsis of study:
The team asked each patient to respond to a sound as they drifted off. At the moment they stopped responding, Lewis and Purdon saw a dramatic change in neuronal activity across the cortex. Slow wave oscillations – the brainwaves that occur in deep, non-dreaming sleep – grew almost immediately.

Locally, these slow waves were in sync and neurons near each other coordinated their activity to correspond with the peaks and troughs of the waves they encountered, meaning continued communication. However, the slow waves were not in sync across the entire cortex.

While conscious, different regions of the cortex fire at the same time, so neurons can communicate over long distances if necessary... out-of sync slow waves make long distance communication near impossible. _NewScientist


PNAS study abstract for above research

More reading on brain networks in general anesthesia:

The Mystery Behind Anesthesia -- an article published almost a year ago in Technology Review, describing earlier research by the same team

Going Under -- a nice article from Science News published about 18 months ago, looking at earlier research from the above MIT/MGH/Harvard team and other competing research teams from the UK and U. Michigan / Cornell.

One cannot help considering the parallels between general anesthesia and normal sleep. Here are two articles that look at brain network activity in relation to sleep and consciousness:

The Process of Awakening

Development of Large Scale Functional Brain Network During Human Non-REM Sleep -- looks at activity of brain networks around the time of onset of sleep

You may ask why one should bother trying to understand how brain networks are put under, fall asleep, or wake up -- and how all that relates to normal brain network function during consciousness. The answer to that question comes along with all the things we will be able to do, once we understand.

It might be easier to understand how the human brain works if we had more sophisticated analytical and synthetic tools than the human brain to work with. As it is, we will have to muddle along trying out different theories until something "clicks into place." At that point, we should be able to design some pretty amazing brain assistants and brain substitutes.

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