Neuroscience: Joined-up thinking

Journal name:
Nature
Volume:
505,
Pages:
615–616
Date published:
DOI:
doi:10.1038/505615a
Published online

Chris Frith explores a masterful model of how consciousness plays out in the theatre of the brain.

Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts

Stanislas Dehaene Viking Books: 2014. ISBN: 9780670025435

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In 1874, Thomas Henry Huxley gave a prescient lecture on mind and brain. The biologist argued that subjective experience depends on the brain's “anterior divisions”, and that consciousness has as little effect on behaviour as a steam whistle has on a locomotive's progress — rendering humans little more than “conscious automata”. He raised two questions that remain key in contemporary studies of the neural basis of consciousness: what is special about the neural processes that underlie consciousness, and what, if anything, is consciousness for?

The 1870s seemed a likely time for a concerted research effort to answer those questions. Herman von Helmholtz had made the distinction between conscious and unconscious brain processes, and Gustav Theodor Fechner's 'psychophysics' had begun to allow the experimental study of the relationship between subjective experience and physical stimulation. But it was not until the 1970s that three-dimensional imaging of the living human brain became possible through physicist Peter Mansfield's work in magnetic resonance imaging. Among the first to realize the importance of this breakthrough for the study of mind and brain was cognitive neuroscientist Stanislas Dehaene. In his brilliant Consciousness and the Brain, Dehaene conveys the excitement of developing paradigms that such technologies have made possible.

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For Dehaene, consciousness is simply this: we are conscious of whatever we choose to focus our attention on. He details many experiments, and presents the best attempt yet to answer the two questions raised by Huxley.

Regarding the first, on neural processes, the brain does a lot of work before we become conscious of a stimulus, as Helmholtz pointed out. When you read these words, you are rarely aware of the individual letters — yet you must have analysed them to have understood the meaning. How much unconscious analysis happens before what we are looking at emerges into consciousness? Dehaene relates how clever techniques have been developed to answer this question.

In backward masking, for example, a word (such as 'five') is presented, followed by a mask (a meaningless series of letters, for example). It has been found that the brain begins to analyse the word as soon as it appears, but that this analysis ceases when the mask appears. If the switch from word to mask is very rapid, there is no consciousness that the word was presented. Yet, as Dehaene has shown, the unconscious neural processing that goes on before the mask appears is enough for meaning to be extracted.

Combined with brain imaging, such studies show that activity in the region concerned with word recognition is not sufficient for consciousness. Instead, Dehaene reveals, conscious experience depends on interactions between sensory regions and the parietal and frontal areas of the brain. This is one of four neural signatures of consciousness that he lists.

These findings could be key in diagnosing locked-in syndrome, a state resembling coma in which a person is fully conscious, but unable to demonstrate it. Using brain-imaging techniques, it should soon be possible to detect consciousness in suspected cases: if a person with the syndrome imagines making a movement, for example, changes in brain activity linked to that could be detected.

Dehaene's special contribution is his global-workspace theory, the first step in a complete account of why some neural processes lead to conscious experience. The brain contains a number of discrete modules specialized for specific tasks, such as visual perception and motor output. Dehaene shows that for advanced cognitive processes — such as seeing things from the viewpoint of others — information generated by these modules must be maintained, manipulated and understood by several or all of them. The 'global workspace' is the virtual arena, created by long-range, synchronized neural connections, in which this happens. Only information that can be shared between modules enters consciousness. Effectively, without such conscious access, higher cognitive abilities would not be possible: consciousness is, Dehaene argues, no steam whistle.

I am not completely convinced that a global workspace is sufficient for consciousness. I believe that the ability to tell people about our experiences, as when tasting wine for example, is a crucial feature. However, our reportage is often erroneous, and that does not seem compatible with the precision needed for the information broadcast in Dehaene's global workspace. Nevertheless, Dehaene's account is the most sophisticated story about the neural basis of consciousness so far. It is essential reading for those who want to experience the excitement of the search for the mind in the brain.

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