Recovery of consciousness is mediated by a network of discrete metastable activity states,
Andrew E. Hudson et al, PNAS, May 13 2014
Summary and review of the above paper
INTRODUCTION: Recovery of consciousness after anaesthesia is argued to involve particular brain hubs that are active for periods of minutes before processing moves to another brain hub in an orderly sequence. The findings here can be related to recent consciousness studies, which points towards conscious processing being the function of intense activity in restricted regions or hubs of the brain.
Studies undertaken by the authors are claimed to demonstrate that recovery of consciousness after anaesthesia involves a series of activity configurations in different regions of the brain. Only certain brain states that are thought to be particular hubs occur on the way to recovering consciousness. The studies discussed here looked at the cingulate cortex and a region of the thalamus sometimes related to consciousness. During recovery from anaesthesia neuronal activity takes the form of discrete areas of activity lasting for some minutes. The transitions between these states involves brain hubs that connect otherwise separate areas of activity. The hubs are activated in an orderly process during the recovery of consciousness.
The authors argue that recovery of consciousness by a random process could not occur within a realistic timescale, and that therefore there must be a structured process by which consciousness was recovered. When the level of anaesthesia was gradually reduced, neural activity switched between several areas, each one persisting for several minutes. Activity could appear simultaneously in separate brain regions suggesting a general change in a wide thalamocortical area, although the activation of the thalamus appears separable from the activation of the cortex.
Eight distinct clusters of activity were detected, seen as attractor states of the thalamocortical activity. The system does not spend much time between the separate densely occupied states, while it may spend several minutes in each of these densely occupied states. Moreover, activity as a whole stabilises when one of the densely occupied states are reached. The location of energy minima determines the existence of the densely occupied states, with the energy maxima producing the boundaries between the states. The densely occupied states are referred to as metastable because spontaneous transitions do occur between the states, and stabilisation does not trap neural activity permanently in any one of the particular states.
Fluctuations between clusters are seen as related to thalamocortical fluctuations en route to regaining consciousness in the course of which there is a sequence of activity in clusters. Some of these cluster states are seen as forming hubs, which have to be activated before the process can move onto the next hub. There is claimed to be an orderly progression from anaesthesia to arrival in consciousness. This process is marked by an increase in high frequency oscillations in the intralaminar thalamic structure, although this is seen as necessary rather than sufficient by itself. The hubs represent only a small subset of the regions that are potentially available to help restore consciousness, but it is these hubs which actually support the restoration of consciousness. The brain is here seen as having a self-tuning process that facilitates the restoration of consciousness after a major disturbance such as anaesthesia.
The research reported here is supportive of other work, such as that or Tononi, which suggests that conscious processing is related to the convergence of neural activity on particular hubs in the brain. A solution to the problem of consciousness looks to need to centre on the high levels of activity in these hubs, which are very distinct from brain processing as a whole. This requires the discarding of the naïve 1990s argument that consciousness is just what it is like to have a brain, rather than as it is now seen as being related to very specialised functioning in a limited sub-set of the brain.