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Penrose-Hameroff Orchestrated Objective-Reduction Proposal for Human Consciousness Is Not Biologically Feasible

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Penrose-Hameroff Orchestrated Objective-Reduction Proposal for Human Consciousness Is Not Biologically Feasible PHYSICAL REVIEW E 80, 021912 ͑2009͒ Penrose-Hameroff orchestrated objective-reduction proposal for human consciousness is not biologically feasible Laura K. McKemmish,1 Jeffrey R. Reimers,1,* Ross H. McKenzie,2 Alan E. Mark,3 and Noel S. Hush4 1School of Chemistry, The University of Sydney, New South Wales 2006, Australia 2School of Mathematics and Physics, The University of Queensland, Queensland 4072, Australia 3School of Chemistry and Molecular Biosciences and Institute for Molecular Biosciences, The University of Queensland, Queensland 4072, Australia 4School of Molecular and Microbial Biosciences, The University of Sydney, New South Wales 2006, Australia ͑Received 15 November 2008; revised manuscript received 29 May 2009; published 13 August 2009͒ Penrose and Hameroff have argued that the conventional models of a brain function based on neural networks alone cannot account for human consciousness, claiming that quantum-computation elements are also required. Specifically, in their Orchestrated Objective Reduction ͑Orch OR͒ model ͓R. Penrose and S. R. Hameroff, J. Conscious. Stud. 2,99͑1995͔͒, it is postulated that microtubules act as quantum processing units, with individual tubulin dimers forming the computational elements. This model requires that the tubulin is able to switch between alternative conformational states in a coherent manner, and that this process be rapid on the physiological time scale. Here, the biological feasibility of the Orch OR proposal is examined in light of recent experimental studies on microtubule assembly and dynamics. It is shown that the tubulins do not possess essential properties required for the Orch OR proposal, as originally proposed, to hold. Further, we consider also recent progress in the understanding of the long-lived coherent motions in biological systems, a feature critical to Orch OR, and show that no reformation of the proposal based on known physical paradigms could lead to quantum computing within microtubules. Hence, the Orch OR model is not a feasible explanation of the origin of consciousness. DOI: 10.1103/PhysRevE.80.021912 PACS number͑s͒: 87.15.RϪ, 03.67.Lx, 07.05.Mh, 87.16.Ka I. INTRODUCTION even spread to include many neurons ͓7͔. Quantum gravita- tional effects are proposed to decohere the system on the The nature of human consciousness is perhaps the most physiological time scale ͓4,6͔. Decoherence causes the quan- important of unsolved scientific problems. It is widely ac- tum state of each tubulin-dimer qubit in each participating cepted that the consciousness is linked to the neural activity microtubule to be reduced to one of its classical forms ͑step in the brain; however, the chemical interactions between neu- rons have been argued by Penrose ͓1,2͔ to set restrictive limits on processing capacity. The most developed extended model of cognitive function is the Penrose-Hameroff orches- trated objective reduction ͓3–8͔͑Orch OR͒ model, which proposes that the quantum computation occurs in microtu- bule assemblies within the neurons of the brain. Currently, there is great interest in both this and other proposals for nontrivial quantum effects in biology ͓9–11͔. Figure 1 de- FIG. 1. The physiological elements of a qubit and the opera- picts the qubit in the Orch OR model and shows a proposed tional cycle of the quantum computation within the Penrose- quantum computational cycle. Coherent superpositions of al- Hameroff model for cognitive function by orchestrated objective ternative conformational states of individual tubulins dimers reduction, taken from Ref. ͓8͔; reproduced with permission of MIT are proposed to form the quantum computational elements Press. Left: tubulin dimers within microtubules exist in two confor- ͑qubits͒. The coherent oscillation depicted in the figure inter- mational states ͑shown black or white͒ and it is proposed that reso- converts the classical chemical conformational states in situ nance coupling between these conformations acts to form qubits to facilitate the resonant superpositions ͑step 1͒, with coher- from quantum superpositions of the local vibronic wave functions ence originating as a result of Fröhlich condensation ͓4,5͔. ͑shown gray͒. Such superpositions require coherent motion in the This nuclear motion is then coupled to electronic motion, vibrational modes, proposed to originate via Fröhlich condensation allowing the qubits to interact with each other while main- ͓4͔, that interchange the two conformational states and move inter- taining coherence, thus forming the quantum computer nal electrons ͑marked “e”͒. Right: four steps in the operational ͑ ͒ across an extensive network of microtubules. Figure 1 shows cycle starting at 1 superpositions start to appear from among clas- ͑ ͒ the formation of these extended superpositions and their sically localized states in the tubulin-dimer qubits, 2 coherent mo- spread to encompass many microtubules ͑steps 2 and 3͒ and tions and superpositions grow to encompass large regions of this and neighboring microtubules, ͑3͒ superposition reaches critical mass inducing objective reduction to form ͑4͒ an ensemble of tubu- lin dimers throughout many microtubules containing the results of *Author to whom correspondence should be addressed. FAX: the quantum computation specified through their ͑classical͒ ϩ61͑2͒93513329; [email protected] conformations. 1539-3755/2009/80͑2͒/021912͑6͒ 021912-1 ©2009 The American Physical Society MCKEMMISH et al. PHYSICAL REVIEW E 80, 021912 ͑2009͒ 4͒. This classical state then interacts with its chemical envi- retained ͓26,27͔, although its function may manifest differ- ronment, possibly through the actin sol-gel cycles that con- ently ͓4,8͔. Here, we concentrate on the intradimer motion: trol the microtubule network structure. As microtubules can ͑i͒ does it exist as envisaged in Fig. 1, and ͑ii͒ if it does not influence the electrochemical properties of neurons ͓12͔, the exist could any possible alternative process actually maintain results of the quantum computation could by this means in- coherence on the required time scale. The third critical issue fluence neural processing. concerning maintenance of coherence over the microtubule The basic physical principle underlying the resonance- network is much more difficult to treat authoritatively and based chemical qubit sketched in Fig. 1 is a widely known the only issue to have been widely discussed ͑see, e.g., motif throughout chemistry and biology. Originally ͓6–8,10,23,25,26͔͒; hence, it is not addressed herein. quantum-mechanical “resonance” was conceptualized to de- Specifically, we consider the feasibility of the Orch OR scribe the “aromatic” nature of benzene compared to the ex- proposal in light of recent developments in the understanding pected properties of its classical form, the hypothetical cy- of microtubule structure and function ͓12,28–45͔, as well as clohexatriene molecule. For benzene, the electron-phonon ␯ ͑ in regard to the likelihood of extensive coherent oscillations coupling involving the critical vibrational mode, 14 see, ͓ ͔ ͓ ͔ e.g., ͓13͔.͒, is insufficient to overpower the resonance stabi- in biology 46–49 and in microtubules 50–53 . We show lization and so a fully delocalized structure results ͓14͔.A both that the original biochemical scenario envisaged in related problem that is more directly relevant to quantum Orch OR is incorrect and that no possible modification of computing ͑see, e.g., ͓15,16͔͒ and Orch OR is the structure this proposal consistent with the biological facts can, using and dynamics of ammonia in the gas phase. For ammonia, known physical padadigms, produce the required physical the electron-phonon coupling in the relevant mode, the properties. Hence, if there are any nonelectrochemical con- umbrella-inversion mode, does overpower the resonance in- tributions to cognitive function then they must arise by some teraction and so two mirror-image pyramidal classical mechanism other than the Penrose-Hameroff proposal. This chemical equilibrium structures result ͓14͔. Coherent super- gives hope to the vision that digital computing could achieve positions of the resulting vibronic states is a known process truly significant levels of artificial intelligence. that in fact leads to the operation of the maser. Also pertinent to the Orch OR proposal are quantum phenomena in the ͓ ͔ II. EXISTENCE OF THE PROPOSED TUBULIN-DIMER Creutz-Taube ion 17 , a mixed valence bisruthenium com- VIBRATIONAL MOTION plex of pyrazine in which charge transport is controlled through coherent motion ͓18͔ involving long-range solvent The observed biochemical dynamics of tubulin dimers vibrational modes ͓19͔. This system is also very important in and their conformational states within microtubules is de- that it provided the model through which primary charge picted in Fig. 2; a review of this dynamics, particularly in separation, quantum coherence, and charge transport was un- regard to microtubules in neurons, has recently been pro- derstood in bacterial and plant photosynthetic reaction cen- vided by Conde and Cáceres ͓12͔. This figure is presented ters ͓14,20͔. More generally, all biological and chemical for comparison with the biochemical processes as envisaged charge-transport processes are now known to be controlled in the Orch OR proposal shown previously in Fig. 1. Micro- by analogous interplays between vibrational motions and tubules are cylindrical structures formed by the self- resonance couplings ͓21,22͔, with a key feature being the association of tubulin heterodimers. Primarily involved
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