Dendritic synchrony moves through the brain to mediate consciousness
Depts. of Anesthesiology and Psychology, Centre for Consciousness, University of Arizona
Journal of Biological Physics, 2009
Introduction: The paper argues that rapidly shifting networks of dendrites connected by gap junctions instantiate the conscious processing of the brain, and focus on particular but also continually changing areas of the underlying non-conscious processing of axons.
Hameroff emphasises that conscious and non-conscious processing in the brain cannot always be regarded as separate and distinct. He points out that we can perform many routine functions unconsciously, but when a novel or hazardous situation arises, we flip over into conscious handling of the situation.
The best correlate of consciousness is phase synchrony at specific EEG frequencies recorded from different electrodes on the scalp or brain surface. Phase synchrony at a particular frequency can occur within one brain region, between neighbouring brain regions, between distant brain regions or between many spatially separated brain regions. Wolf Singer et al (1.) found that phase synchrony was connected to conscious perception within the (30-90 Hz) gamma band. Subsequent studies have shown gamma synchrony in a number of locations correlating with conscious perception, motor control, language, working memory, face recognition and sleep/dream cycles. The binding problem, as to why consciousness of different modalities is perceived as a unified whole can also be addressed via the gamma synchrony (2-9.). Specific types of consciousness content are correlated with with particular distributions of gamma synchrony in the olfactory bulb dendrites, and conscious experience of pleasure with gamma synchrony in the ventral tegmentum and nucleus accumbens (10-13.). Moreover, the distribution of gamma synchrony within the brain can change in a matter of hundreds of milliseconds and sometimes faster. Hameroff envisages the gamma synchrony as something that focuses on particular areas of the underlying non-conscious processing in neurons.
Hameroff is critical of conventional views that have seen dendritic and somatic processing of inputs from synapses as a passive affair. He points to complex signalling within and between dendrites, and in cytoskeletal somatic processing (14-16.). As an anaesthetist, Hameroff points out that anaesthetics act almost exclusively on dendritic and somatic proteins rather than on axonal proteins.
Experiments in the 1990s showed that the gamma synchrony seen in various brain regions required dendrite-to-dendrite gap junctions binding together groups of neurons. In the cortex, interneurons have a large number of dendrites and form connections with up to 70 neurons or glia cells. Interneurons often form both a synapse and a gap junction with another neuron. In layers 2 to 6 of the cortex, interneurons form gap junctions with neurons in all other layers, meaning that a gap junction mediated web pervades the cortex. The dynamics of such a web are determined by whether the gap junctions involved are opened or closed. A number of elements in the brain act to determine this opening/closure, including G protein activity, calcium ions and microtubules, the last of which relate to the Penrose/Hameroff model of consciousness. The synchronised dendritic web activity changes as the gap junctions open and close. The number of different possible dendritic webs formed amongst billions of neurons and glia is described as near-infinite. Axonal firing is found to be different when neurons are synchronised from when they are acting individually, suggesting that involvement in the dendritic webs described is efficacious for neurons. This last is relevant in consciousness studies, where it is often asserted that consciousness cannot be efficacious in the brain.
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