"How Does the Brain Generate Consciousness?"

Susan Greenfield

Abstract

This talk will explore a new approach to the well established ‘Hard Problem’ of how the brain generates an inner first-hand experience, consciousness. One of the stumbling blocks to date has been that neuroscience arguably has lacked a cohesive framework for linking micro- and macro- events in the brain. However, the concept of ‘neuronal assemblies’ is gradually gaining recognition: large scale (tens of millions) neuronal coalitions that are highly transient (sub-second) and which thereby could provide the essential missing link between molecular/cellular neurobiology and the cognitive neuroscience of non-invasive imaging of active brain regions. This all-important basic subjective state of consciousness does not appear to be an exclusive property of the human brain. There is no obvious, qualitative transformation in either the anatomy or the physiology of the central nervous system of human or non-human animals, no phylogenetic Rubicon in the animal kingdom. Similarly, there is no clear ontogenetic line that is crossed as the brain grows in the womb - no single event, nor change in brain physiology, and certainly not at birth, when consciousness might be generated in an all-or-none fashion. A more plausible, and scientific, view of consciousness might be therefore that it is not a qualitatively different property of the brain, some magic bullet, but that it is a consequence of a quantitative increase in the complexity of the human brain: consciousness will grow as brains grow. Hence consciousness is most likely to be a continuously variable property of the brain, in both phylogenetic and ontogenetic terms. Hence we shall see whether consciousness might vary in degree from one moment to the next, and the extent of a conscious state at any one time is correlated with the extent of a transient assembly of neurons. Voltage-sensitive dye imaging enables us for the first time, to characterize this as yet poorly understood but fundamental brain mechanism. We shall see how neuronal assemblies could account for the diverse features of the phenomenology of consciousness and therefore have the potential to constitute the much sought after ‘neural correlate of consciousness’. Once these assemblies are fully characterized physiologically, they could be used to generate, for the first time ever, experimentally falsifiable hypotheses that link subjective experience with physico-chemical events in the brain.

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