James Newman: Such intrusive thoughts, those are rather rare and episodic in a person who has been traumatised, but what is particularly common in psychological trauma is something called "psychic numbing". This is this tremendous effort by consciousness to keep those painful feelings and memories from intruding and so the person's whole conscious psychology becomes one of avoiding anything that will cause them to re-experience the trauma. So that's just one example of how you can apply this Global Workspace metaphor to, for example, to psychopathology in this case or to traumatisation. But the other thing is that this model is also very useful in trying to understand how the brain actually instantiates consciousness. Of course the first thing that's essential to understand is that most processes in the brain are not conscious. As Bernie said, the brain consists in about a 100 billion neurons, a significant percentage of which are firing at the same time. And they're massively and reciprocally interconnected, so that it's hard to see how a single stream of consciousness with this very limited capacity of only seven, plus or minus 2, what we call "chunks" of information, can arise out of such a system. That's sort of the aspect that I've been working on with Bernie.

He actually first came up with a likely candidate for generating this stream when he was developing Global Workspace theory in the 1980's. He called it the "extended reticular thalamic activating system" and it is precisely an extension of the idea of the Reticular Activating System that was brilliantly elucidated by Magoun and Moruzzi, back in 1949. What these two neuroscientists discovered was that if you stimulate a portion of the very core of the brain stem you get an immediate arousal of the entire cortex, and the animal becomes alert and oriented to their environment. If they're sleeping they'll immediately come out of their sleeping state and start looking around, or if they're drowsy or quiescent they will suddenly become alert, they'll orient to something. Further research showed that this area in the brain stem was merely the beginning of an extended system that ascends, much like a fountain, all the way to the cortex and serves, in the most simple form I could put it, as the "searchlight" or "spotlight" on the theater of consciousness. It is what illumines the "play" of images that is taking place. Of course this is a very simple metaphor, just as the global workspace idea is a simple metaphor but from that you can elucidate what the spotlight is doing, and it turns out that really it's an immensely complex bank of spotlights. The thing that's so adaptive about these spotlights is that they can be controlled, in varying ways, by all of the systems and processors of the brain to ensure the spotlight is indeed illuminating what is most relevant to the person at that moment. Of course, the range of what is relevant to our consciousness can extend from the simplest crack of a stick in the woods causing us to immediately turn and look and see what is behind us; upto, at the highest level, our staying focussed on a very complex subject like I'm doing right now. I don't want to be distracted by the birds that are singing in the trees, I want to keep on our subject: the nature of consciousness.

This extended system begins in the reticular formation, in the core of the brain stem and continues to form the core of the brain as it goes up. The next "waystation" is the thalamus in the center of the brain. The thalamus turns out to be the communications hub of the brain. It's been known for many years that the thalamus relays all sensory information from the outside world. What is less appreciated is that the thalamus is really the main outside source of activation for the entire cortex. There are some other sources, but basically external activation of the cortex is a product of thalamic circuits.