A step towards correlates of consciousness

There are only a few ways to watch the brain in action and one is to follow its electromagnetic waves. Doesburg and his group have looked at the waves that accompany conscious awareness. They use binocular rivalry to mark when the content of consciousness changes (If different images are sent to the two eyes, we are not conscious of a mixed image but of one or the other alternatively and the changes can be reported.) and a key press to indicate a change in the conscious perception. The work confirms previous EEG measurements of conscious processes including:

  • Low gamma-band (30-50Hz) synchronization between neural groups codes the various features of objects and binds them into the perceptual experience. Neurons in synchrony communicate.

  • Remote gamma-band EEG phase synchronization index the onset of coherent visual perception and is associated with conscious rather than unconscious processes.

  • Gamma-band synchronization between the hemispheres is required for integration of sense input from the two hemispheres into consciousness.

  • Synchronized gamma waves between the thalamus and cortex as well as within the cortex is associated with consciousness. Disruption of thalamo-cortical communication is the nature of anesthesia.

  • Gamma-band synchronization increases and decreases in an oscillation. This gives the attentional blink when the synchronization is low and accounts for the ‘frames’ of working memory.

 

The study also adds valuable insights:

  • Large-scale gamma-band synchronization constitutes an oscillatory substrate for the stream of consciousness. The oscillation is due to the gamma waves being found in only one portion of the theta-band waves (4-7Hz). “onset of a new percept … time-locked bursts of gamma-band activation that recur at a theta rate.”

  • The prefrontal cortex and parietal lobe are essential parts of the ‘consciousness network’. Probably the prefrontal cortex is relevant for integration and self-awareness and the parietal cortex supplies the multimodal representation of space. The primary visual cortex was not generating time-locked gamma rhythms. “This supports the view that the large-scale oscillatory network detailed here is essentially related to perceptual experience itself, and not to those unconscious functions to give rise to changes within it.” In other words, the gamma synchronization that is local to the visual cortex is not to be confused with the remote synchronization that occurs from the parietal lobe.

  • The content of consciousness need not be completely new in each ‘frame’. “continuous viewing of a single unchanging stimulus will yield a procession of theta cycles in which the content remains the same.” When there is a change of content, the inferior temporal cortex is involved in the synchrony and when motor response is required so is the motor cortex.

  • There are differences in the phase of the theta wave during which area pairs become synchronous. In other words along a part of the theta wave various areas join and leave the gamma synochrony. This is probably not a single event but an ordered process.

 

Here is their abstract:

Consciousness has been proposed to emerge from functionally integrated large-scale ensembles of gamma-synchronous neural populations that form and dissolve at a frequency in the theta band. We propose that discrete moments of perceptual experience are implemented by transient gamma-band synchronization of relevant cortical regions, and that disintegration and reintegration of these assemblies is time-locked to ongoing theta oscillations. In support of this hypothesis we provide evidence that (1) perceptual switching during binocular rivalry is time-locked to gamma-band synchronizations which recur at a theta rate, indicating that the onset of new conscious percepts coincides with the emergence of a new gamma-synchronous assembly that is locked to an ongoing theta rhythm; (2) localization of the generators of these gamma rhythms reveals recurrent prefrontal and parietal sources; (3) theta modulation of gamma-band synchronization is observed between and within the activated brain regions. These results suggest that ongoing theta-modulated-gamma mechanisms periodically reintegrate a large-scale prefrontal-parietal network critical for perceptual experience. Moreover, activation and network inclusion of inferior temporal cortex and motor cortex uniquely occurs on the cycle immediately preceding responses signaling perceptual switching. This suggests that the essential prefrontal-parietal oscillatory network is expanded to include additional cortical regions relevant to tasks and perceptions furnishing consciousness at that moment, in this case image processing and response initiation, and that these activations occur within a time frame consistent with the notion that conscious processes directly affect behaviour.

 

ResearchBlogging.org
Doesburg, S., Green, J., McDonald, J., & Ward, L. (2009). Rhythms of Consciousness: Binocular Rivalry Reveals Large-Scale Oscillatory Network Dynamics Mediating Visual Perception PLoS ONE, 4 (7) DOI: 10.1371/journal.pone.0006142

3 thoughts on “A step towards correlates of consciousness

  1. I wonder if a kind reader could respond to the following thought from a non-scientist?

    ‘Synchronized gamma waves between the thalamus and cortex as well as within the cortex is associated with consciousness. Disruption of thalamo-cortical communication is the nature of anesthesia.’

    Is it therefore reasonable to suggest that, similar to laying down memories, some sort of emotional mediation might be required to bind a feature into conscious awareness?

    JK: Thank you for your comment. I think that the involvement of the thalamus does not indicate emotional involvement (although there may be some from other sources). The thalamus has input from the sight, hearing, touch and minor senses but not smell (which directly enters the cortex). It also has input from the basal areas involved in movement and from the brain stem. Without input from the brain stem, the thalamus does not maintain its input to the cortex and the cortex soon is without consciousness. So the thalamus is very much about senses and perception rather than emotion. It may even be that conscious awareness of emotion is indirect or partially so through sense input from the body like heart rate and the like.
    (The hypothalamus is a very different region from the thalamus and is important to emotion, drives, bodily status and the like, but not consciousness.)

  2. Will, I’m not sure what you’re reading into, but there’s nothing in that quote that implies any kind of emotional component. Nor do memories require an emotional component for formation.

    Emotions do enhance the subjective vividness of memories, but not their accuracy. Dr. Liz Phelps has done a bunch of work on emotions and memories, if you want to learn more on that.

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