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16577Where Does Consciousness Come From?

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  • medit8ionsociety
    Mar 19, 2009
      Where Does Consciousness Come From?
      ScienceDaily (Mar. 17, 2009) — Consciousness
      arises as an emergent property of the human mind.
      Yet basic questions about the precise timing,
      location and dynamics of the neural event(s)
      allowing conscious access to information are
      not clearly and unequivocally determined.
      Some neuroscientists have even argued that
      consciousness may arise from a single "seat"
      in the brain, though the prevailing idea
      attributes a more global network property.
      Do the neural correlates of consciousness correspond
      to late or early brain events following
      perception? Do they necessarily involve coherent
      activity across different regions of the brain,
      or can they be restricted to local patterns of
      reverberating activity?
      A new paper suggests that four specific, separate
      processes combine as a "signature" of conscious
      activity. By studying the neural activity of
      people who are presented with two different
      types of stimuli – one which could be perceived
      consciously, and one which could not – Dr. Gaillard
      of INSERM and colleagues, show that these four
      processes occur only in the former, conscious
      perception task.
      This new work addresses the neural correlates of
      consciousness at an unprecedented resolution,
      using intra-cerebral electrophysiological recordings
      of neural activity. These challenging experiments
      were possible because patients with epilepsy who
      were already undergoing medical procedures requiring
      implantation of recording electrodes agreed to
      participate in the study. The authors presented
      them with visually masked and unmasked printed
      words, then measured the changes in their brain
      activity and the level of awareness of seeing the
      words. This method offers a unique opportunity to
      measure neural correlates of conscious access with
      optimal spatial and temporal resolutions. When
      comparing neural activity elicited by masked and
      unmasked words, they could isolate four converging
      and complementary electrophysiological markers
      characterizing conscious access 300 ms after word
      perception.
      All of these measures may provide distinct glimpses
      into the same distributed state of long-distance
      reverberation. Indeed, it seems to be the convergence
      of these measures in a late time window (after 300 ms),
      rather than the mere presence of any single one of
      them, which best characterizes conscious trials.
      "The present work suggests that, rather than hoping
      for a putative unique marker – the neural correlate
      of consciousness – a more mature view of conscious
      processing should consider that it relates to a
      brain-scale distributed pattern of coherent brain
      activation," explained neuroscientist Lionel
      Naccache, one of the authors of the paper.
      The late ignition of a state of long distance
      coherence demonstrated here during conscious
      access is in line with the Global Workspace
      Theory, proposed by Stanislas Dehaene,
      Jean-Pierre Changeux, and Lionel Naccache.
      ________________________________________
      Journal reference:
      1. Gaillard et al. Converging Intracranial
      Markers of Conscious Access. PLoS Biology, 2009;
      7 (3): e61 DOI: 10.1371/journal.pbio.1000061
      Adapted from materials provided by Public Library
      of Science, via EurekAlert!, a service of AAAS.
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