As neuroscientists continue to build a more accurate and sophisticated model of the human brain, finding the neurological correlate of conscious awareness remains a tough nut to crack. The difficulty stems partly from the fact that consciousness is likely not localized in any one specific brain region.
But as our technology advances and we are able to look at brain function in real time and in greater detail, researchers are starting to zero in on the hardwiring that produces consciousness.
In this context, consciousness is operationally defined as being aware of sensory stimulation, as opposed to just being awake. We are not conscious of everything we see and hear, nor of all of the information processing occurring in our own brains. We are aware of only a small subset of input and processing, which is woven together into a continuous and seamless narrative that we experience.
The New Scientist has a good review of this topic – in which they discuss the work of Bernard Baars who proposed in 1987 the “global workspace theory.” Essentially, he hypothesized that conscious awareness stems from a discrete network of neurons that are widely distributed throughout the cortex. This networks receives input from the various sensory regions of the brain and puts it all together – filtering out any contradictory or unnecessary information to create one unified picture of reality in a continuous stream that we experience.
According to this model sensory input that gets filtered out of the global workplace remains subconscious, as is any processing that occurs in other parts of the brain but is filtered or not presented to the global workplace.
Baars also proposes that the global workspace can explain the dichotomy between the slow serial functioning of the conscious brain and the fast parallel processing of the brain as a whole. He writes in his book on the topic:
The difference is, of course, that most psychologists work with the limited capacity component of the nervous system, which is associated with consciousness and voluntary control, while neuroscientists work with the “wetware” of the nervous system, enormous in size and complexity, and unconscious in its detailed functioning. But what is the meaning of this dichotomy? How does a serial, slow, and relatively awkward level of functioning emerge from a system that is enormous in size, relatively fast-acting, efficient, and parallel? That is the key question.
The global workspace seemed like a reasonable hypothesis from the point of view of explaining existing observations and data, but there wasn’t a way to really test it, and so it remained in science limbo. Until recently, that is, when the new tools of neuroscience enabled neuroscientists to look at brain function to find the potential correlates of the global workspace.
A team of researcher led by Stanislas Dehaene of the French National Institute of Health and Medical Research, beginning in 2005, looked at a phenomenon known as inattention blindness. (For a fun demonstration of this, take a look at Richard Wiseman’s Color Changing Card Trick.) Basically, inattention (or inattentional) blindness occurs when we fail to notice something which is right in front of us. The information simply does not become part of our stream of consciousness. This seemed like a good opportunity to test the global workspace theory.
In the study they presented subjects with two streams of four letters. In some cases the subjects had to answer a question after the first stream, which distracted them and caused them to miss the second stream of letters. In other cases they perceived both streams of letters.
In both cases for 270 milliseconds the streams of letters resulted in the same neuronal activity (as measured by a 128 lead EEG). In the case when the subjects perceived the second string this initial activity was followed by a synchronized burst of activity in parts of the brain (frontal and parietal lobes) thought to be part of the global workplace. In cases where the subjects did not consciously perceive the letters there was no such activity – the neurons quieted down after 270 milliseconds.
What this could mean is that the initial 270 milliseconds of activity represents the subconscious processing in the visual and visual association cortex, while the next phase of activity is conscious awareness of that stimulus by the global workspace. This experiment has been replicated with implanted electrodes as well.
So it seems there is about a 300 millisecond delay from perception to conscious awareness, and those stimuli we are consciously aware of result in activation of a distributed network of neurons, while those we are not aware of do not result in such activation. So far so good for the global workspace.
However, just being awake should result from some basic level of activity in any consciousness network. In fact, this is what researchers have found – the default mode network (DMN) is the baseline activity in the network thought to be part of the global workspace.
Steven Laureys (yes, the same Steven Laureys from facilitated communication infamy) hypothesized that if the DMN is part of the brain function that causes consciousness then we would expect the level of activity in the DMN to be greatest among healthy controls and those who are locked in (conscious but paralyzed), decreased in those with minimally conscious state, and decreased further in those in a vegetative state. That is in fact what he found when he studied 14 patients with disorders of consciousness and 14 healthy controls.
This is a fascinating area of research which seems to be progressing nicely. Although the results should be considered preliminary, it is not surprising that researchers are finding that brain activity correlates with levels of consciousness and awareness. The global workspace and the brain regions now associated with it are a good candidate for the neural substrate of consciousness.
The obvious clinical application is an improved ability to diagnose by direct functional brain scanning which unresponsive patients are conscious and which have impaired consciousness, and to what degree. This could also lead to an important criterion for prognosis – who is likely to recover and who will not.
But the basic neuroscience advance is interesting in its own right. Understanding why certain brain processes are conscious and others are not will take us a long way to building a model of overall brain function.
By Steven Novella | TheNess