Recently there was a Scientific American article (here) about a paper by A. Casali (abstract here) on a method of measuring whether consciousness is present in patients. There was an implied question in the Sc Am article as to whether the method was actually measuring consciousness.
P. Mitra says, “The scientists report that their measure performs impressively in distinguishing states of consciousness within subjects, as well as across subjects in different clinically identified consciousness stages. These promising results will no doubt attract further study. However, the claim that the measure is theoretically grounded in a conceptual understanding of consciousness deserves a closer look. It is tempting to think that a concretely grounded clinical study of consciousness naturally advances our scientific understanding of the phenomenon, but is this necessarily the case? It is common in medicine to see engineering-style associative measurements, measurements which aid pragmatic actions but do not originate from a fundamental understanding.” In other words, he asks if this study gets us any closer to a neural correlation of consciousness.
One thing we know about consciousness is that it depends on the thalamus-cortex conversation. When this loop stops functioning, consciousness disappears. The other thing we know is that activity in the cortex that is present during some states of unconsciousness is very local.
Imagine a pond with a light rain falling on it. Each drop sets off a train of ripples that travel a long way before they fade away. The disturbance is not just local (integrated). The ripples of many rain drops interact and give complex patterns to the surface of the water. The disturbance is complex (information rich). All that Casali is saying is that he can measure, by using a disturbance, whether the brain is in a state where it is complex and integrated. His measurement is straightforward – EEG records after the disturbance by transcranial magnetic stimulation (TMS) are put through a mathematical procedure like image compression to give his perturbational complexity index (PCI). There is nothing questionable here and all the pieces are trusted methods. The compression algorithm is simple to imagine. Think of a picture on a TV screen. If a block of colour is all the same colour then every pixel need not be transmitted in order to reproduce that block and if some part of the picture stays exactly the same for some period of time then the information for that block need not by re-transmitted over and over. How much an image can be compressed is a measure of its spatial and temporal complexity. TMS does perturb the activity of the brain by causing electrical fields to change and EEGs do reflect the activity in the brain. This seems to be expected and was shown by trials.
But what is the connection between the thalmo-cortical loops and the loss of integration and complexity? I ask you, for a few moments, to consider an idea; suspend disbelief for a while to take in the idea and then bring your critical facilities back. The thalamus is a group of small regions tucked up under the cortex. Almost all the information that goes to the cortex gets there through the thalamus – it gets to the spots that the thalamus sends it to. In effect the cortex knows very, very little that the thalamus has not told it. The thalamus sits on top of the end of the spinal cord (if you see the brain stem and the reticular formation as extension of the spinal cord). It receives information from most parts of the brain like a sort of Grand Central Station. Sleep starts when a place down in the brain stem signals to the thalamus. The thalamus then shuts down the thalmo-cortical loops and the cortex is left on its own. Shortly after this the cortex loses its waking state sort of activity. What if the each local region of the cortex cannot communicate with other parts of the cortex without the thalamus opening a gate? Now the cortex is not only isolated from the rest of the world but it is also isolated into little local systems separate from each other. Consciousness, which is all about the global integration of information, would disappear without the information and the integration. If we must use a computer metaphor: we can think of the thalamus as having its own computer called the cortex that it uses all day (for things like the stream of consciousness) and at night it turns it off (or lets it do its cleanups, refreshes and backups). Now you can come back to your critical persona and see if you like the idea.