Orch-OR

From Wikipedia, the free encyclopedia

Jump to: navigation, search
Unbalanced scales.svg
 The neutrality of this article is disputed. Please see the discussion on the talk page. Please do not remove this message until the dispute is resolved. (October 2009)
Unbalanced scales.svg
 This article may be inaccurate in or unbalanced towards certain viewpoints. Please improve the article by adding information on neglected viewpoints, or discuss the issue on the talk page. (October 2009)
Quantum mechanics
\Delta x\, \Delta p \ge \frac{\hbar}{2}
Uncertainty principle
Introduction · Mathematical formulation
This box: view • talk • edit

Orch OR (Orchestrated Objective Reduction) is a theory of consciousness, which is the joint work of theoretical physicist Sir Roger Penrose and anesthesiologist Stuart Hameroff. Mainstream theories assume that consciousness emerges from the brain, and focus particularly on complex computation at connections known as synapses that allow communication between brain cells (neurons). Orch OR combines approaches to the problem of consciousness from the radically different angles of mathematics, physics and anesthesia.

Penrose and Hameroff initially developed their ideas quite separately from one another, and it was only in the 1990s that they cooperated to produce the Orch OR theory. Penrose came to the problem from the view point of mathematics and in particular Gödel’s theorem, while Hameroff approached it from a career in cancer research and anesthesia that gave him an interest in brain structures.

Contents

[edit] Gödel's Incompleteness Theorem

Main article: Gödel's incompleteness theorem

In 1931, the mathematician and logician Kurt Gödel proved that any theory capable of expressing elementary arithmetic cannot be both consistent and complete. Further to that, for any consistent formal theory that proves certain basic arithmetic truths, there is an arithmetical statement that is true, but not provable in the theory.

In his first book on consciousness, The Emperor's New Mind (1989), Penrose made Gödel's theorem the basis of what quickly became an intensely controversial claim[1]. He argued that the theorem showed that the brain had the ability to go beyond what could be achieved by axioms or formal systems. This would mean that the mind had some additional function that was not based on algorithms (systems or rules of calculation). A computer is driven solely by algorithms. Penrose asserted that the brain could perform functions that no computer could perform. He called this type of functioning non-computable.

[edit] The quantum level

Penrose went on to consider what it was in the human brain that might not be driven by algorithms. The physical law is described by algorithms, so it was not easy for Penrose to come up with physical properties or processes that are not described by them. He was forced to look to quantum theory for a plausible candidate.

In quantum theory, the fundamental units, the quanta, are in some respects quite unlike objects that are encountered in the large scale world described by classical physics. When sufficiently isolated from the environment, they can be viewed as waves. However these are not the same as matter waves, such as waves in the sea. The quantum waves are essentially waves of probability, the varying probability of finding a particle at some specific position. (These probabilities apply to other states of the particle, such as its momentum, but for the sake of simplicity we will refer to position.) The peak of the wave indicates the location with maximum probability of a particle being found there. The different possible positions of the particle are referred to as superpositions or quantum superpositions. We are speaking here of the isolated form of the quanta. When the quanta are the subject of measurements or of interaction with the environment, the wave characteristic is lost, and a particle is found at a specific point. This change is commonly referred to as the collapse of the wave function.

When the collapse happens, the choice of position for the particle is random. This is a drastic departure from classical physics. There is no cause-and-effect process, and no system of algorithms that can describe the choice of position for the particle.

This provided Penrose with a candidate for the physical basis of the suggested non-computable process that he proposed as possibly existing in the brain. However, this was not the end of his problems. He had identified something in physics that was not based on algorithms, but at the same time, randomness was not a promising basis for mathematical understanding, the aspect of mind that Penrose particularly focused on.

[edit] Objective reduction

Penrose now proposed that existing ideas on wave function collapse might only apply to situations where the quanta are the subject of measurement or of interaction with the environment. He considered the case of quanta that are not the subject of measurements or interactions, but remain isolated from the environment, and proposed that these quanta may be subject to a different form of wave function collapse.

In this area, Penrose draws on both Einstein's general theory of relativity, and on his own notions about the possible structure of spacetime[1][2]. General relativity states that spacetime is curved by massive objects. Penrose, in seeking to reconcile relativity and quantum theory, has suggested that at the very small scale this curved spacetime is not continuous, but constitutes a form of network.

Penrose postulates that each quantum superposition has its own piece of spacetime curvature. According to his theory, these different bits of spacetime curvature are separated from one another, and constitute a form of blister in spacetime. Penrose further proposes a limit to the size of this spacetime blister. This is the tiny Planck scale of (10−35 m). Above this size, Penrose suggests that spacetime can be viewed as continuous, and that gravity starts to exert its force on the spacetime blister. This is suggested to become unstable above the Planck scale, and to collapse so as to choose just one of the possible locations for the particle. Penrose calls this event objective reduction (OR), reduction being another word for wave function collapse.

An important feature of Penrose's objective reduction is that the time to collapse is a function of the mass/energy of the object undergoing collapse. Thus the greater the superposition, the faster it will undergo OR, and vice versa. Tiny superpositions, e.g. an electron separated from itself, if isolated from environment, would require 10 million years to reach OR threshold. An isolated one kilogram object (e.g. Schrödinger’s cat) would reach OR threshold in only 10−37 seconds. However objects somewhere between the scale of an electron and the scale of a cat could collapse within a timescale that was relevant to neural processing.

The threshold for Penrose OR is given by the indeterminacy principle E=ħ/t, where E is the gravitational self-energy or the degree of spacetime separation given by the superpositioned mass, ħ is the reduced Planck constant, and t is the time until OR occurs.

There is no existing evidence for Penrose's objective reduction, but the theory is considered to be testable, and plans are in hand to carry out a relevant experiment [3].

From the point of view of consciousness theory, an essential feature of Penrose's objective reduction is that the choice of states when objective reduction occurs is selected neither randomly, as are choices following measurement or decoherence, nor completely algorithmically. Rather, states are proposed to be selected by a 'non-computable' influence embedded in the fundamental level of spacetime geometry at the Planck scale.

Penrose claimed that such information is Platonic, representing pure mathematical truth, aesthetic and ethical values. More than two thousand years ago, the Greek philosopher Plato had proposed such pure values and forms, but in an abstract realm. Penrose placed the Platonic realm at the Planck scale. This relates to Penrose's ideas concerning the three worlds: physical, mental, and the Platonic mathematical world. In his theory, the physical world can be seen as the external reality, the mental world as information processing in the brain and the Platonic world as the encryption, measurement, or geometry of fundamental spacetime that is claimed to support non-computational understanding.

[edit] 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 subunits of the protein, tubulin. Proteins constitute much of the driving machinery of living organisms. Proteins contain hydrophobic pockets. These pockets contain atoms with electrons called π electrons, which means electrons in the reactive outer part (outer shell) of the atom that are not bonded to other atoms. The tubulin protein subunits of the microtubules have hydrophobic pockets within two nanometres of one another. Hameroff claims that this is close enough for the π electrons of the tubulin 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.

[edit] 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 Reviews 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 Reviews 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.

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 been actually already 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 far away from actual gap junctions.

[edit] See also

[edit] References

  1. ^ a b Penrose, Roger (1989). The Emperor's New Mind: Concerning Computers, Minds and The Laws of Physics. Oxford University Press. pp. 480. ISBN 0-198-51973-7. 
  2. ^ a b Penrose, Roger (1989). Shadows of the Mind: A Search for the Missing Science of Consciousness. Oxford University Press. pp. 457. ISBN 0-19-853978-9. 
  3. ^ Marshall, W., Simon, C., Penrose, R., and Bouwmeester, D. (2003). "Towards quantum superpositions of a mirror". Physical Review Letters 91: 130401. doi:10.1103/PhysRevLett.91.130401. http://arxiv.org/abs/quant-ph/0210001. 
  4. ^ Hameroff, S.R., and Watt, R.C. (1982). "Information processing in microtubules". Journal of Theoretical Biology 98: 549-561. http://www.quantumconsciousness.org/documents/informationprocessing_hameroff_000.pdf. 
  5. ^ a b c d Hameroff, S.R. (1987). Ultimate Computing. Elsevier. http://www.quantumconsciousness.org/ultimatecomputing.html. 
  6. ^ Hameroff, Stuart. "Quantum Consciousness". http://www.quantumconsciousness.org. Retrieved November 10, 2009. 
  7. ^ Hameroff, S.R. (2006). "The entwined mysteries of anesthesia and consciousness". Anesthesiology 105: 400-412. 
  8. ^ Hameroff, S. (2009). "The “conscious pilot” - dendritic synchrony moves through the brain to mediate consciousness". Journal of Biological Physics. doi:10.1007/s10867-009-9148-x. 
  9. ^ Bennett, M.V.L., and Zukin, R.S. (2004). "Electrical Coupling and Neuronal Synchronization in the Mammalian Brain". Neuron 41: 495-511. doi:10.1016/S0896-6273(04)00043-1. http://dx.doi.org/10.1016/S0896-6273(04)00043-1. 
  10. ^ Buhl, D.L., Harris, K.D., Hormuzdi, S.G., Monyer, H., and Buzsaki, G. (2003). "Selective Impairment of Hippocampal Gamma Oscillations in Connexin-36 Knock-Out Mouse In Vivo". Journal of Neuroscience 23: 1013-1018. 
  11. ^ Dermietzel, R. (1998). "Gap junction wiring: a `new' principle in cell-to-cell communication in the nervous system?". Brain Research Reviews 26: 176-183. 
  12. ^ Draguhn, A., Traub, R.D., Schmitz, D., and Jefferys, J.G.R. (1998). "Electrical coupling underlies high-frequency oscillations in the hippocampus in vitro". Nature 394: 189-192. 
  13. ^ Fries, P., Schroder, J.-H., Roelfsema, P.R., Singer, W., and Engel, A.K. (2002). "Oscillatory Neuronal Synchronization in Primary Visual Cortex as a Correlate of Stimulus Selection". Journal of Neuroscience 22: 3739-3754. 
  14. ^ Galarreta, M., and Hestrin, S. (1999). "A network of fast-spiking cells in the neocortex connected by electrical synapses". Nature 402: 72-75. 
  15. ^ Gibson, J.R., Beierlein, M., and Connors, B.W. (1999). "Two networks of electrically coupled inhibitory neurons in neocortex". Nature 402: 75-79. 
  16. ^ Hormuzdi, S.G., Filippov, M.A., Mitropoulou, G., Monyer, H., and Bruzzone, R. (2004). "Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks". Biochimica et Biophysica Acta 1662: 113-137. 
  17. ^ LeBeau, F.E.N., Traub, R.D., Monyer, H., Whittington, M.A., and Buhl, E.H. (2003). "The role of electrical signaling via gap junctions in the generation of fast network oscillations". Brain Research Bulletin 62: 3-13. 
  18. ^ Velazquez, J.L.P., and Carlen, P.L. (2000). "Gap junctions, synchrony and seizures". Trends in Neurosciences 23: 68-74. 
  19. ^ Rozental, R., and de Carvalho, A.C.C. (2000). "Introduction". Brain Research Reviews 32: 1-2. 
  20. ^ Maudlin, T. (1995). "Between The Motion And The Act... A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2396/2325. 
  21. ^ Klein, S.A. (1995). "Is Quantum Mechanics Relevant To Understanding Consciousness A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2397/2326. 
  22. ^ McCullough, D. (1995). "Can Humans Escape Gödel? A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2398/2327. 
  23. ^ Moravec, H. (1995). "Roger Penrose's Gravitonic Brains A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2399/2328. 
  24. ^ Baars, B.J. (1995). "Can Physics Provide a Theory of Consciousness? A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2401/2330. 
  25. ^ Chalmers, D.J. (1995). "Minds, Machines, And Mathematics A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2402/2331. 
  26. ^ McCarthy, J. (1995). "Awareness and Understanding in Computer Programs A Review of Shadows of the Mind by Roger Penrose". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2403/2332. 
  27. ^ McDermott, D. (1995). "\star Penrose is Wrong". Psyche 2. http://journalpsyche.org/ojs-2.2/index.php/psyche/article/view/2406/2335. 
  28. ^ Grush, R., Churchland, P.S. (1995). "Gaps in Penrose's toilings". Journal of Consciousness Studies 2 (1): 10-29. http://mind.ucsd.edu/papers/penrose/penrose.pdf. 
  29. ^ Feferman, S. (1996). "Penrose's Gödelian argument". Psyche 2: 21-32. http://math.stanford.edu/~feferman/papers/penrose.pdf. 
  30. ^ Tegmark, M.. "Importance of quantum coherence in brain processes". Physical Reviews E 61: 4194–4206. 
  31. ^ Engel, G.S., Calhoun, T.R., Read, E.L., Ahn, T.-K., Mancal, T., Cheng, Y.-C., Blankenship, R.E., and Fleming, G.R. (2007). "Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems". Nature 446: 782-786. 
  32. ^ Hagan, S., Hameroff, S., and Tuszyński, J.. "Quantum Computation in Brain Microtubules? Decoherence and Biological Feasibility". Physical Reviews E 65: 061901. http://arxiv.org/abs/quant-ph/0005025. 
  33. ^ Hameroff, S. (2006), "Consciousness, Neurobiology and Quantum Mechanics", in Tuszynski, Jack, The Emerging Physics of Consciousness, Springer, pp. 193-253 
  34. ^ Hameroff, S.R. (1998). "Quantum Computation In Brain Microtubules? The Penrose-Hameroff "Orch OR" model of consciousness". Philosophical Transactions Royal Society London (A) 356: 1869-1896. http://www.quantumconsciousness.org/penrose-hameroff/quantumcomputation.html. 
  35. ^ Kikkawa, M., Ishikawa, T., Nakata, T., Wakabayashi, T., Hirokawa, N. (1994). "Direct visualization of the microtubule lattice seam both in vitro and in vivo". Journal of Cell Biology 127 (6): 1965-1971. doi:10.1083/jcb.127.6.1965. http://jcb.rupress.org/cgi/content/abstract/127/6/1965. 
  36. ^ Kikkawa, M., Metlagel, Z. (2006). "A molecular "zipper" for microtubules". Cell 127 (7): 1302-1304. doi:doi:10.1016/j.cell.2006.12.009. http://dx.doi.org/10.1016/j.cell.2006.12.009. 
  37. ^ Reimers, J.R., McKemmish, L.K., McKenzie, R.H., Mark, A.E., and Hush, N.S. (2009). "Weak, strong, and coherent regimes of Fröhlich condensation and their applications to terahertz medicine and quantum consciousness". Proceedings of the National Academy of Sciences 106: 4219-4224. doi:10.1073/pnas.0806273106. 
  38. ^ Georgiev, D.D. (2007). "Falsifications of Hameroff-Penrose Orch OR model of consciousness and novel avenues for development of quantum mind theory". Neuroquantology 5 (1): 145-174. http://philsci-archive.pitt.edu/archive/00003049/. 
  39. ^ Georgiev, D.D. (2009). "Remarks on the number of tubulin dimers per neuron and implications for Hameroff-Penrose Orch". Nature Preceedings. http://precedings.nature.com/documents/3860/version/1. 
  40. ^ De Zeeuw, C.I., Hertzberg, E.L., Mugnaini, E.. "The dendritic lamellar body: A new neuronal organelle putatively associated with dendrodentritic gap junctions". Journal of Neuroscience 15: 1587-1604. 
Retrieved from "http://en.wikipedia.org/wiki/Orch-OR"