Consciousness Blog 19 October 2013
Monti and his colleagues used functional magnetic resonance imaging (fMRI) to study how the flow of information in the brains of 12 healthy volunteers changed as they lost consciousness under anesthesia with propofol. The participants ranged in age from 18 to 31 and were evenly divided between men and women.
To state that consciousness is dependent on the connected functioning of many parts of the brain is perhaps by now scarcely mould-breaking, but we have to start somewhere in understanding the neural correlates of consciousness, and there are still hardly any detailed accounts of how consciousness actually functions. One could also complain that the characteristics of a brain which has been attacked with a foreign chemical (propofol in this case) may not tell us much; it's like trying to describe the functioning of a clock after pouring sulphuric acid into it.
At all events, Dynamic Change of Global and Local Information Processing in Propofol-Induced Loss and Recovery of Consciousness, reporting on research carried out at Belgium's University Hospital of Liege, and published in PLOS Computational Biology, does reach useful conclusions. Here is the abstract:
"Whether unique to humans or not, consciousness is a central aspect of our experience of the world. The neural fingerprint of this experience, however, remains one of the least understood aspects of the human brain. In this paper we employ graph-theoretic measures and support vector machine classification to assess, in 12 healthy volunteers, the dynamic reconfiguration of functional connectivity during wakefulness, propofol-induced sedation and loss of consciousness, and the recovery of wakefulness. Our main findings, based on resting-state fMRI, are three-fold. First, we find that propofol-induced anesthesia does not bear differently on long-range versus short-range connections. Second, our multi-stage design dissociated an initial phase of thalamo-cortical and cortico-cortical hyperconnectivity, present during sedation, from a phase of cortico-cortical hypoconnectivity, apparent during loss of consciousness. Finally, we show that while clustering is increased during loss of consciousness, as recently suggested, it also remains significantly elevated during wakefulness recovery. Conversely, the characteristic path length of brain networks (i.e., the average functional distance between any two regions of the brain) appears significantly increased only during loss of consciousness, marking a decrease of global information-processing efficiency uniquely associated with unconsciousness. These findings suggest that propofol-induced loss of consciousness is mainly tied to cortico-cortical and not thalamo-cortical mechanisms, and that decreased efficiency of information flow is the main feature differentiating the conscious from the unconscious brain."
"All regions of the brain appear to be functionally further apart, reducing the efficiency with which information can be exchanged across different parts of the network," says Professor Monti.
The distinction between thalamo-cortical and cortico-cortical connectivity is particularly interesting, suggesting that consciousness is a feature of comparatively late-developing brains (in evolutionary terms). Of course that is not to say that thalamic input, including most types of sensory input, is not essential to the 'remembered present' which is surely the basis of higher forms of consciousness; and it doesn't prove that consciousness as observed in humans necessarily occupies the same neural structures as it might have done in 'lower' animals. But it is highly suggestive.
Read more in Chapter Two of Agent Human by Michael Bell, The Future of Groups and Self-Awareness.