- One of the riddles of consciousness may
have been solved by a team of scientists studying the crackle of electrical
activity in the brain at the instant a face is recognised.
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- "I think, therefore I am" was
Descartes's adage, but the new work suggests that "I am coherent,
therefore I think" may be closer to the mark.
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- The French study sheds light on how the
brain blends the flood of information from the senses into an overall thought,
the so-called "binding problem" that some neuroscientists think
may hold the key to consciousness.
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- Thinkers have grappled with the problem
over the centuries. Descartes believed that we have a single centre, the
pineal gland, where all sensory signals converge and are evaluated, a view
long rejected by scientists.
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- A paper published in the latest issue
of the journal Nature provides the first evidence that the brain actually
binds different flows of information together into a thought when assemblies
of nerve cells are synchronised, firing together with millisecond precision.
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- Dr Francisco Varela and colleagues in
a CNRS laboratory at the Hôpital de la Salpétrière,
Paris, showed binding at work while monitoring activity in the brain as
subjects studied ambiguous stimuli that looked meaningless when upside
down but appeared as faces when the right way up.
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- They found synchronisation in widely
separated regions of the brain only occurred when the pictures could be
recognised as a face, corresponding to the moment of perception, the first
time this has been demonstrated.
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- "This binding pattern, this shadow
of thought that goes through the brain as you do a perception is precisely
the correlate, not just of the visual scene, but also of the unity of the
state of consciousness," Dr Varela said.
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- During these "gamma oscillations",
in a band of frequencies around 40 beats per second, nerve cells scattered
across the brain synchronise to form a coherent group, rather like millions
of tiny lightbulbs switching on and off simultaneously.
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- This wave of synchronised switching may
be the mechanism that binds disparate activity in the brain into a conscious
thought.
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- The study also showed that when the thought
(recognising a face) was followed by a deed (in this case, when the subjects
responded with the push of a key), the firing neurons desynchronised strongly
before entering a new pattern of synchrony. This, the researchers argue,
marks the brain shifting from one cognitive state to another.
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- "That is a major discovery,"
he said. "Between one wave and the other, there has to be a complete
decrease in synchrony, an active 'unglueing' of the brain patterns before
the next moment of thought can come up," he said.
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- The peak of one wave corresponds to face
perception, and the next to the subject responding. The separation between
the peaks, and thus of each thought, marks the "frames of time for
cognitive life".
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