Neural correlates of consciousness

21/03/2009 by admin.

A very important investigation has been reported. The original paper is here; it is reviewed in ScienceDaily here, The New Scientist  here, and Neurophilosophy here. The research was done by a French team led by R. Gaillard.

In the course of treating 10 epileptic patients, electrodes were placed in their brains to identify the focuses of their attacks. The patients agreed to take part in some research while the electrodes were in place. The data from electrodes placed inside the brain has resolution (both in timing and location) that neither brain scans nor EEGs have. On the other hand the researchers used only a few electrodes (176) and they are placed where they were needed for clinical reasons not for research reasons. The patients were shown words under conditions where some reached consciousness and some did not. The differences between the electrode signals in these two conditions were collected to give a picture of the difference between processing of sensory input that ends in consciousness and that which doesn’t. This is an attempt at the ‘holy grail’ of the NCC, neural correlates of consciousness.

The results were:

1.      No specific seat of consciousness was found but rather there was an involvement across most the cortex.

2.      The early stages (less the 200ms) of conscious and unconscious processing were very similar.

3.      “Conscious word processing was associated with the following four markers: (1) sustained iERPs within a late time window (>300 ms after stimulus presentation); (2) sustained and late spectral power changes, combining a high-gamma increase, beta suppression, and alpha blockage; (3) sustained and late increases in long-range phase coherence in the beta range; and (4) sustained and late increases in long-range causal relations.” In other words, when the word became conscious (1) activity relating to the word continued past 300 msec (2) the late activity lasted some time and involved an increase in high frequency, decrease in medium frequency and blocking of low frequency waves (3) the medium frequency waves became synchronized between distant parts of the cortex (4) changes in one part of the cortex caused changes in distant parts.

4.      The results were consistent with B. Baar’s Global Workspace model of conscious access. I believe it might be consistent with a number of other models as well.

5.      Activity started in the back of the brain, moved progressively forward and reached the frontal cortex in those events reaching consciousness. It then feed back to re-activate the areas in the back of the brain again.

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