27 December 2011

This Is Your Brain Under General Anaesthesia

Neuroscience has often benefited from natural experiments—patients who lose their ability to remember, produce language, or regulate their emotions after parts of their brains are damaged or have to be surgically removed. Anesthesiologists preside over an analogous experiment every day: they watch elements of consciousness disappear. Under general anesthesia, for instance, patients lose pain perception, awareness, memory, and the ability to move. An anesthesiologist can influence each of these changes in different ways by varying the dosages and types of drugs used.

"By taking away different functions that we associate with consciousness," Brown says, "we might be able to start piecing together parts of the jigsaw puzzle." Neuroscientists could begin to do for consciousness what they have done with memory and language. _TechnologyReview

EEG Spectrum Under General Anaesthesia

Real time EEG monitoring is a useful adjunct to other ways of monitoring the anaesthetised patient. With better spatial resolution, EEG imaging of brain activity can make every surgical procedure a natural experiment in the mechanisms of consciousness.
EEG-based brain monitors are already a common sight in operating rooms; some anesthesiologists track the brain activity of their patients with commercially available monitors that use algorithms to transform EEG signals into crude indexes. (Others track only physical signs such as heart rate and blood oxygen levels.) But few of them, he says, spend time looking at the raw EEG data.

...As patients enter an anesthetized state, the normal pattern of low-intensity but high-frequency waves shifts to one of less frequent but more intense pulses—as if the constant chatter of the brain had given way to a chant. The location of activity shifts from the back of the brain to the front. Although it's possible to take patients into such a deep state of unconsciousness that their EEG is essentially flat, in most cases bursts of EEG activity alternate with periods of relative inactivity that can last for minutes. The brain processes appear "highly organized," he says. "There are very regular patterns in time, and very regular patterns in space."

...some drugs will decrease the frequency of brain waves seen in EEG readings, resulting in slow, regular oscillating waves across large areas of the brain. Other drugs cause certain areas to show fast, regular oscillations. Because anesthesiologists usually give a cocktail of drugs to each patient, these effects can happen simultaneously. The result, says Brown, is like a jammed signal: "Either way, [the different parts of the brain] can't communicate."

Over the past few years, other EEG studies have supported the idea that anesthesia doesn't simply shut the brain down but, rather, interferes with its internal communication. Mashour's research, for instance, has shown that feedback between the front and back of the brain is interrupted during general anesthesia, leading to a disconnect between different brain networks. That feedback is thought to be important for consciousness. _TechnologyReview
Clearly there are many ways to disrupt brain networks, and normal waking consciousness. Hypnosis has been used as "anaesthesia" during painful surgeries for over a hundred years. It will be fascinating to review the EEG pattern of such a surgery, for comparison to other anaesthetic agents.

We are still quite a long ways from developing the perfect anaesthetic, and from understanding human consciousness in all its states and varieties. But if western civilisation can hold up for just a few more decades against all of its internal and external enemies, some marvelous tools for enhanced health, cognition, longevity, and more, are on the way.

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