Online Book 3a

Online Book 3a

3 MARCH 2012
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4.17: Penrose & Hameroff 2011
In their latest joint paper published as a chapter in Consciousness and the Universe (2011) (22.) Penrose and Hameroff deal with aromatic rings and proposed hydrophobic channels within microtubules that could be crucial for a quantum theory of consciousness. They point to unexpected discoveries in biology. The most important change since Penrose and Hameroff first propounded their ideas in the 1980s and 1990s is the recent discoveries in biology relative to higher temperature quantum activity. In 2003 Ouyang & Awschalom showed that quantum spin transfer in phenyl rings (an aromatic ring molecule like those found in protein hydrophobic pockets) increases at higher temperatures. In 2005 Bernroider and Roy (23.) researched the possibility of quantum coherence in K+ neuronal ion channels. A more crucial discovery came in 2007 when it was demonstrated that quantum coherence was functional in efficiently transferring energy within photosynthetic organisms (Engel et al, 2007). Subsequent papers showed functional quantum coherence in multicellular plants and also at room temperature. In 2011 papers by Gauger et al and Luo and Lu dealt with higher temperature coherence in bird brain navigation and in protein folding. Work by Anirban Bandyopadhyay with single animal microtubules showed eight resonance peaks correlated with helical pathways round the cylindrical microtubule lattice. This allowed 'lossless' electrical conductance.

Tubulin & aromatic rings: building blocks of consciousness? Each tubulin protein contains the amino acids tryptophan and phenylalanine with aromatic rings. Each hydrophobic pocket in the tubulin is suggested to be composed of four such aromatic rings, with the hydrophobic pockets being arranged in channels. Van der Waals London forces operate in the hydrophobic pockets in tubulin, based on the π electron rings of tryptophan and phenylaline. This concept derives originally from Fröhlich, who suggested that proteins are synchronised by the oscillations of dipoles in the electron clouds of these amino acids. Anaesthetic gases are similarly suggested to work through their action on aromatic amino acids in hydrophobic pockets in neuronal proteins, including membrane proteins.

Hydrophobic channels and long-range van der Waals:. A paper published in 1998 (Nogales et al, 8.) described the structure of the tubulin protein and identified the existence and location of the non-polar aromatic amino acids tryptophan and phenylamine in tubulin. These are located in hydrophobic pockets, but these pockets are within 2 nanometres of one another, and collectively they can be interpreted as hydrophobic channels or pathways rather than mere pockets. This is suggested to allow linear arrays of electron clouds capable of supporting long-range van der Waals London forces. The quantum channels in individual tubulins are seen as being aligned with those in neighbouring tubulins within the microtubule lattice, and these provide helical winding patterns.

The authors also make a direct reply to one critic in particular (McKemmish et al, 2010) McKemmish claimed that switching between two states of the tubulin protein in the microtubules would involve conformational changes requiring GTP hydrolysis which in turn would involve an impossible energy requirement. The authors however claim that electron cloud dipoles (van der Waals London forces) are sufficient to achieve switching without large conformational changes.

4.18: CRITICISMS OF THE HAMEROFF SCHEME

Where the Hameroff version of quantum consciousness remains ambitious relative to existing scientific knowledge is in the proposed link to the global gamma synchrony, the brain's most obvious correlate of consciousness. He proposes that coherence within dendrites connects via gap junctions to other neurons and thus to the neuronal assemblies involved in the global gamma synchrony. He thus proposes the existence of quantum coherence over large areas of the brain, sometimes including multiple cortical areas and both hemispheres of the brain.

Hameroff pointed to gap junctions as an alternative to synapses for connections between neurons. Neurons that are connected by gap junctions depolarise synchronously. Cortical inhibitory neurons are heavily studded with gap junctions, possibly connecting each cell to 20 to 50 other. The axons of these neurons form inhibitory GABA chemical synapses on the dendrites of other interneurons. Studies show that gap junctions mediate the gamma synchrony. On this basis, Hameroff suggested that cells connected by gap junctions may in fact constitute a cell assembly, with the added advantage of synchronous excitation. In this scheme computations are suggested to persist for 25 ms, thus linking them to the 40Hz gamma synchrony.

The attempt to extend a proposal for quantum features from single neurons out to neuronal assemblies of millions of neurons resurrects the nay-sayer objections about time to decoherence. The photosynthetic states that have been demonstrated persist for only over femtosecond and picosecond timescales. Where the decoherence argument still stands up is in dealing with a system that needs to be sustained for 25 milliseconds. Further to this the Hameroff's gamma wide theory involves difficult arguments about the ability of coherence to pass from neuron to neuron via the gap junctions.

Danko Georgiev, a researcher at Kanazawa University also criticises Hameroff's requirement for microtubules to be quantum coherent for 25 ms. This has been generally regarded as an ambitious timescale for quantum coherence, and Georgiev objects on the grounds that enzymatric functions in proteins take place on a very much quicker 10-15 picosecond timescale. Georgiev wants to base his version of OR consciousness on this 10-15 picosecond timescale. Such a rapid form of objective reduction would also remove the necessity for the gel-sol cycle to screen microtubules from decoherence, as it does in the Hameroff version of objective reduction.

Axons, dendrites and synapses: Georgiev also criticises Hameroff's emphasis on conscious processing as being concentrated in the dendrites. He claims that Hameroff's does not allow any consciousness in axons, and this creates a problem in explaining the problematic firing of synapses. Only 15-30% of axon spikes result in a synapse firing, and it is not clear what determines whether or not a synapse fires. He discusses the probabilistic nature of neurotransmitter release at the synapses, and the possible connection this has with quantum activity in the brain. The probability of the synapse firing in response to an electrical signal is estimated at only around 25%. Georgiev points out that an axon forms synapses with hundreds of other neurons, and that if the firing of all these synapses was random, the operation of the brain could prove chaotic.

He suggests instead the choice of which synapses will fire is connected to consciousness, and that consciousness acts within neurons. Each synapse has about 40 vesicles holding neurotransmitters, but only one vesicle fires at any one time. Again the choice of vesicle seems to require some form of ordering. The structure of the grid in which the vesicles are held is claimed to be suitable to support vibrationally assisted quantum tunnelling. P. Georgiev's emphasises the onward influence of solitons (quanta propagating as solitary waves) from the microtubules to the presynaptic scaffold protein, from where, via quantum tunnelling, they are suggested to influence whether or not synapses fire in response to axon spikes. Jack et al (1981) suggested an activation barrier, restricting the docking of vesicles and the release of neurotransmitters. The control of presynaptic proteins is suggested to overcome this barrier, and to regulate the vesicles that hold neurotransmitters in the axon terminals. This is suggested to be the process that decides whether a synapse will fire in response to an axon spike (a probability of only about 25%), and if it does, which of a choice of 40 or so vesicles will release its neurotransmitters.

The system he describes involves the neuronal cytoskeleton, and particularly the pre and post-synaptic scaffold proteins. Here, it is suggested that consciousness arises from the objective reduction of the wave function within these structures. The timescale of the system is argued to be defined by changes in tubulin conformations within the cytoskeleton and by the enzyme action in the scaffold proteins, which involves a timescale of 10-15 picoseconds, and thus implies a decoherence time on the same scale. Georgiev points out that it is much easier to suppose a decoherence time of this length in the brain than the 25 ms demanded by the Hameroff proposals.

Continued in Online Book 4