Saturday, March 20, 2010

Roger Penrose - Conscious Understanding: What is its Physical Basis?

Roger Penrose is an interesting guy, even though I reject a lot of his theory of consciousness. This is the first I have heard from him since Shadows of the Mind and, later, his work with Stuart Hameroff on the quantum collapse theory of consciousness (the Orch-OR theory, Orchestrated Objective Reduction).

Wikipedia actually has a nice explanation of the theory and the objections to it (which are nearlt universal):

The creation of the Orch OR model

When he wrote his first consciousness book, The Emperor's New Mind in 1989, Penrose lacked a detailed proposal for how such quantum processes could be implemented in the brain. Subsequently, Hameroff read The Emperor’s New Mind and suggested to Penrose that certain structures within brain cells (neurons) were suitable candidate sites for quantum processing and ultimately for consciousness[4][5]. The Orch OR theory arose from the cooperation of these two scientists, and were developed in Penrose's second consciousness book Shadows of the Mind (1994)[2].

Hameroff's contribution to the theory derived from studying brain cells (neurons). His interest centred on the cytoskeleton, which provides an internal supportive structure for neurons, and particularly on the microtubules[5], which are the important component of the cytoskeleton. As neuroscience has progressed, the role of the cytoskeleton and microtubules has assumed greater importance. In addition to providing a supportive structure for the cell, the known functions of the microtubules include transport of molecules including neurotransmitter molecules bound for the synapses, and control of the cell's movement, growth and shape[5].

Hameroff proposed that microtubules were suitable candidates to support quantum processing[5]. Microtubules are made up of tubulin protein subunits. The tubulin protein dimers of the microtubules have hydrophobic pockets binding the drug taxol separated 8 nm away, which might contain delocalized π electrons. Tubulin has other smaller non-polar regions, for example 8 tryptophans per tubulin, which contain π electron-rich indole rings distributed throughout tubulin with separations of roughly 2 nm. Hameroff claims that this is close enough for the tubulin π electrons to become quantum entangled [6]. Quantum entanglement is a state in which quantum particles can alter one another's properties instantaneously and at a distance, in a way which would not be possible, if they were large scale objects obeying the laws of classical as opposed to quantum physics.

In the case of the electrons in the tubulin subunits of the microtubules, Hameroff has proposed that large numbers of these electrons can become involved in a state known as a Bose-Einstein condensate. These occur when large numbers of quantum particles become locked in phase and exist as a single quantum object. These are quantum features at a macroscopic scale, and Hameroff suggests that through a feature of this kind quantum activity, which is usually at a very tiny scale, could be boosted to be a large scale influence in the brain.

Hameroff has proposed that condensates in microtubules in one neuron can link with microtubule condensates in other neurons and glial cells via gap junctions[7][8]. In addition to the synaptic connections between brain cells, gap junctions are a different category of connections, where the gap between the cells is sufficiently small for quantum objects to cross it by means of a process known as quantum tunneling. Hameroff proposes that this tunneling allows a quantum object, such as the Bose-Einstein condensates mentioned above, to cross into other neurons, and thus extend across a large area of the brain as a single quantum object.

He further postulates that the action of this large-scale quantum feature is the source of the gamma synchronisation observed in the brain, and sometimes viewed as a neural correlate of consciousness [9]. In support of the much more limited theory that gap junctions are related to the gamma oscillation, Hameroff quotes a number of studies from recent years [10][11][12][13][14][15][16][17][18][19].

The Orch OR theory combines Penrose's hypothesis with respect to the Gödel theorem with Hameroff's hypothesis with respect to microtubules. Together, Penrose and Hameroff have proposed that when condensates in the brain undergo an objective reduction of their wave function, that collapse connects to non-computational decision taking/experience embedded in the geometry of fundamental spacetime.

The theory further proposes that the microtubules both influence and are influenced by the conventional activity at the synapses between neurons. The Orch in Orch OR stands for orchestrated to give the full name of the theory Orchestrated Objective Reduction. Orchestration refers to the hypothetical process by which connective proteins, known as microtubule associated proteins (MAPs) influence or orchestrate the quantum processing of the microtubules.

Objections to Orch OR

Penrose's interpretation of Gödel's first incompleteness theorem is rejected by many philosophers, logicians and artificial intelligence (robotics) researchers[20][21][22][23][24][25][26][27]. A paper by the philosophers Rick Grush and Patricia Churchland attacking Penrose has received widespread attention within consciousness studies[28]. Solomon Feferman, a professor of mathematics, logic and philosophy has made more qualified criticisms[29]. He faults detailed points in Penrose's reasoning in his second book 'Shadows of the Mind', but says that he does not think that they undermine the main thrust of his argument. As a mathematician, he argues that mathematicians do not progress by computer-like or mechanistic search through proofs, but by trial-and-error reasoning, insight and inspiration, and that machines cannot share this approach with humans. However, he thinks that Penrose goes too far in his arguments. Feferman points out that everyday mathematics, as used in science, can in practice be formalised. He also rejects Penrose's platonism.

The main objection to the Hameroff side of the theory is that any quantum feature in the environment of the brain would undergo wave function collapse (reduction), as a result of interaction with the environment, far too quickly for it to have any influence on neural processes. The wave or superposition form of the quanta is referred to as being quantum coherent. Interaction with the environment results in decoherence otherwise known as wave function collapse. It has been questioned as to how such quantum coherence could avoid rapid decoherence in the conditions of the brain. With reference to this question, a paper by the physicist, Max Tegmark, refuting the Orch OR model and published in the journal, Physical Review E is widely quoted[30]. Tegmark developed a model for time to decoherence, and from this calculated that microtubule quantum states could exist, but would be sustained for only 100 femtoseconds at brain temperatures, far too brief to be relevant to neural processing. A recent paper by Engel et al. in Nature does indicate quantum coherent electrons as being functional in energy transfer within photosynthetic protein, but the quantum coherence described lasts for 660 femtoseconds[31] rather than the 25 milliseconds required by Orch OR. This reinforces Tegmark's estimate for decoherence timescale of microtubules, which is comparable to the observed coherence time in the photosynthetic complex.

In their reply to Tegmark's paper, also published in Physical Review E, the physicists, Scott Hagan and Jack Tuszynski and Hameroff[32][33] claimed that Tegmark did not address the Orch OR model, but instead a model of his own construction. This involved superpositions of quanta separated by 24 nm rather than the much smaller separations stipulated for Orch OR. As a result, Hameroff's group claimed a decoherence time seven orders of magnitude greater than Tegmarks, but still well short of the 25 ms required if the quantum processing in the theory was to be linked to the 40 Hz gamma synchrony, as Orch OR suggested. To bridge this gap, the group made a series of proposals. It was supposed that the interiors of neurons could alternate between liquid and gel states. In the gel state, it was further hypothesized that the water electrical dipoles are orientated in the same direction, along the outer edge of the microtubule tubulin subunits. Hameroff et al. proposed that this ordered water could screen any quantum coherence within the tubulin of the microtubules from the environment of the rest of the brain. Each tubulin also has a tail extending out from the microtubules, which is negatively charged, and therefore attracts positively charged ions. It is suggested that this could provide further screening. Further to this, there was a suggestion that the microtubules could be pumped into a coherent state by biochemical energy. Finally, it is suggested that the configuration of the microtubule lattice might be suitable for quantum error correction, a means of holding together quantum coherence in the face of environmental interaction. In the last decade, some researchers who are sympathetic to Penrose's ideas have proposed an alternative scheme for quantum processing in microtubules based on the interaction of tubulin tails with microtubule associated proteins, motor proteins and presynaptic scaffold proteins. These proposed alternative processes have the advantage of taking place within Tegmark's time to decoherence.

Most of the above mentioned putative augmentations of the Orch OR model are not undisputed. "Cortical dendrites contain largely A­-lattice microtubules" is one of 20 testable predictions published by Hameroff in 1998[34] and it was hypothesized that these A­-lattice microtubules could perform topological quantum error correction. The latter testable prediction had already been experimentally disproved in 1994 by Kikkawa et al., who showed that all in vivo microtubules have B-lattice and a seam [35][36]. Other peer-reviewed critiques of Orch OR have been published in recent years. One of these is a paper published in PNAS by Reimers et al.[37], who argue that the condensates proposed in Orch OR would involve energies and temperatures that are not realistic in biological material. Further papers by Georgiev point to a number of problems with Hameroff's proposals, including the lack of explanation for the probabilistic firing of the axonal synapses[38], an error in the calculated number of tubulin dimers per cortical neuron[39], and mismodeling of dendritic lamellar bodies (DLBs) discovered by De Zeeuw et al.[40], who showed that despite the fact that DLBs are stained by antibody against gap junctions, they are located tens of micrometers away from actual gap junctions. Also it was shown that the proposed tubulin-bound GTP pumping of quantum coherence cannot occur neither in stable microtubules[41] nor in dynamically unstable microtubules undergoing assembly/disassembly[42].

With that foundation, here is the lecture.

Google Tech Talk
March 10, 2010


Presented by Sir Roger Penrose.

Powerful arguments can be given, to support the case that the quality of human understanding is not something that can be simulated in a trustworthy way, by any entirely computational system. If this case is accepted, it raises the question of what deep physical processes and what subtle brain structures might be involved in order that consciousness can come about. Some remarkable new observations concerning A-lattice microtubules will be briefly described, these having considerable relevance to this issue.

Sir Roger Penrose is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the Mathematical Institute, University of Oxford and Emeritus Fellow of Wadham College. He has received a number of prizes and awards, including the 1988 Wolf Prize for physics which he shared with Stephen Hawking for their contribution to our understanding of the universe. He is renowned for his work in mathematical physics, in particular his contributions to general relativity and cosmology. He is also a recreational mathematician and philosopher.

No comments: