neurotaylor

You’re breaking up … anaesthesia and the brain

Lewis, L. D., V. S. Weiner, et al. (2012). “Rapid fragmentation of neuronal networks at the onset of propofol-induced unconsciousness.” Proceedings of the National Academy of Sciences.

A fascinating paper in the journal PNAS, this. OK, it’s in patients (pretty much has to be, given they’re using intracranial electrodes), and only three of them, but it offers tantalising ground for speculation, which is always fun. And it shows how influential the notion of brain oscillations is becoming. ‘Tweren’t always thus.

Behaviour: epilepsy patients were tested during loss of consciousness by being given the anaesthetic propofol and asked to press a button when they heard a (frequent) tone. Two consecutive non-responses were taken to mean that the anaesthetic had successfully knocked them out. Propofol boosts GABAergic activity.

Brain recordings: these were taken of the intracranial electrocorticogram (i.e. global electrical activity, if one can call the brain a globe), single neurons, and the intermediate level of the local field potential (LFP), all in the same region, temporal cortex. Nice.

Findings: what the authors found, in their own words, was ‘a functional isolation of cortical regions while significant connectivity is preserved within local networks’, slow oscillations in the local field potential, and ‘short periods of normal spike dynamics still can occur during unconsciousness’. Initially neuronal spiking is suppressed, but though cells’ activity may recover to pre-anaesthetic levels, the patterns are different — short bursts, interspersed with silence, the activity coupled to the slow oscillations. And the patient remains unconscious.

Conclusions: so to put it crudely, consciousness doesn’t seem to depend on the number of spikes, so much as on the long-range connections between active areas. The local slow oscillations, which look a bit like the slow waves of slow-wave sleep (but more fragmented and with faster onset), appear to have different phases in different areas, suggesting that coupling between areas is interrupted. Apologies if this is starting to sound like a bad neuro take-off of Fifty Shades of Grey, but in consciousness, it seems, coupling’s what it’s all about.

Having said which, as the authors point out, they don’t yet know ‘whether the slow oscillation is sufficient to produce unconsciousness’. And this kind of functional breaking up may not be the only way in which we humans can be rendered temporarily insensate. MRN, of course (more research needed).

Still, it’s interesting to think about implications: for attempts to create artificial conscious entities, for what may be going on in Alzheimer’s, and for the science of dreams, to name but three. All those patients’ reports of strange sensations and half-remembered awareness while supposedly unconscious seem less neurotic now. Fascinating stuff!