Australasian Science: Australia's authority on science since 1938

Rebooting the Brain

By Tim Hannan

A study of recovery from anaesthesia finds that returning to consciousness is not a simple path.

One of the enduring mysteries of the human brain is how it transitions between the conscious and unconscious states. It is not well-understood how the brain, when recovering from a major disruption to its activity, is able to navigate its way back to consciousness.

Anaesthesia has been the most useful method of studying this process, and past observation of patients undergoing surgery has led to the popular intuition that, as the anaesthetic wears off, the brain simply wakes gradually, much like an energy-saving light bulb steadily increasing illumination over time. However, a new study has found that this theory is incorrect, and that the anaesthetised brain moves through a series of stable, discrete states en route to consciousness, in a manner more analogous to a computer being rebooted.

Anaesthesia is the practice of inducing unconsciousness, a state in which a person is neither awake nor aware. In this state, pain and other unpleasant sensations are not experienced, so an individual may undergo surgical or other medical procedures without distress or discomfort.

Of course, the brain is not completely inactive while anaesthetised, as neurons continue to generate electrical activity, which can be monitored in various regions as a local field potential. The nature of this low-level brain activity is not well-understood, and neither is the process by which the brain regains consciousness after having been anaesthetised. The popular assumption has been that the brain follows a steady, linear path to the waking state, reflected in a general increase in electrical activity across diverse regions of the brain.

In a recent issue of the Proceedings of the National Academy of Sciences, a group of American anaesthetists have presented data questioning this assumption. Andrew Hudson and colleagues anaesthetised rats while recording electrical activity in several brain regions associated with arousal and consciousness, including the anterior and posterior regions of the cingulate cortex, which surrounds the corpus callosum, and the intra­laminar nuclei of the thalamus. When the brain is anaesthetised, the oscillations in the electrical activity in these regions is much reduced in comparison with those observed in the normal waking state, both in human and rat brains. A similar pattern of reduced activity is seen when a human or rat is asleep.

The researchers then gradually reduced the amount of anaesthetic, and observed the electrical activity in the targeted regions of the brain. In contrast to the predicted pattern of a general, linear increase in activity, they found that the brain passed through a number of identifiable and discrete activity states during the return to consciousness. These states emerged in an ordered pattern, with certain states persisting for a matter of minutes at a time; the researchers identified these as somewhat stable “hub states” that organised the activity of otherwise disconnected neural processes.

Intriguingly, while every one of the anaesthetised rats passed through all of these hubs on the path to consciousness, the sequence in which they occurred varied across occasions and between animals. That is, while the recovery of consciousness required passing through each of these hub states at some point, there were a number of possible pathways by which this could be achieved.

In finding that recovery from anaesthesia is not a steady, linear process but a dynamic pathway through certain specific states, this study has important practical implications for monitoring patients following surgery. More broadly, it raises the prospect of a new way of understanding brain activity in patients who are in a coma following injury or neurological disease. It may be speculated that the brains of comatose patients may be unable to attain a specific hub state, or to transition between two of these states. It is possible that the depth of coma could be measured through the occurrence or transition between hubs, and the recovery of consciousness could be predicted by observing such changes in brain activity.

Tim Hannan is an Associate Professor of Clinical Psychology at Charles Sturt University, and the President of the Australian Psychological Society.