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L'émergence de la conscience

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Roger Penrose - beyond algorithmsStuart HameroffNiels Bohr

A famous mathematical theorem, the incompleteness theorem, or Gödel’s theorem, proposed by Kurt Gödel (1906-1978), plays a role in Penrose’s theory. This theorem shows that no system of axioms is powerful enough to generate all mathematical truths. According to Penrose, this shows that the human mind must have some non-algorithmic capabilities that go beyond axioms and rules. And indeed, the human mind can recognize as true those parts of arithmetic that transcend the systems of axioms.

Not all logicians agree with this interpretation, but Penrose nevertheless suggests that this non-algorithmic aspect of consciousness comes from its connections with quantum physics.

What makes quantum phenomena such as the superposition of states and the collapse of the wave function seem so strange and counter-intuitive? Many authors believe that it is not because these phenomena are fundamentally harder to understand than anything else, but mainly, to put it somewhat crudely, that the human brain originally evolved so that one person could tell others where good food and water could be found, and not to study elementary particles.

Karl Pribram developed a “holographic” model of the operation of the human brain, inspired by the discovery of the first holograms in the mid-1960s. In a hologram, all of the pieces of information recorded in the form of interference patterns on each fragment of the photographic medium can be used to reconstruct the entire image and to provide a three-dimensional view of it as a whole. By analogy, Pribram believes that memories are not stored in cells at specific sites in the brain, but are rather contained in the wave-interference patterns the run through it.

Developed by Pribram in collaboration with quantum physicist David Bohm, this model suggests that consciousness may emerge through the processing of information among dendrites and that the action potentials propagated along axons might in contrast be the substrate for the brain’s non-conscious activities.

Lien : THE HOLOGRAPHIC BRAINLien : Consciousness ReassessedLien : Karl Pribram, cerveau et univers holographiquesChercheur : Karl Pribram
Lien : Bohm's Gnosis: The Implicate OrderLien : Interview with David Bohm 1917-1992Lien : La fabrication des hologrammesLien : Holography

Physics can be said to offer two main levels of explanation: the familiar level that is used every day to describe large-scale objects, and the quantum level, which is used to describe the infinitely small and is governed by Schrödinger’s equation. Both of these levels are completely deterministic and computable. But at the quantum level, superposed states are possible, whereas at our familiar, macroscopic level, only one of these multiple states can exist. This explains why, when we make an observation at our familiar level, the superposed states must collapse into a single, unique possibility.

On the basis of this classical interpretation of quantum mechanics, physicists such as Eugene Wigner have come up with a startling hypothesis: that consciousness can cause this collapse of the wave function and thus determine the conscious contents of the brain. A number of authors, such as Henry Stapp, have gone on to build sophisticated models around this idea. But others, such as English physicist and mathematician Roger Penrose, have distanced themselves from these conventional interpretations of the collapse of the wave function. For Penrose, these interpretations are only approximations that will have to be refined through future developments in quantum theory.

Penrose has therefore proposed his theory of objective reduction. He describes objective reduction as a process that is gravitational in nature, but not local: in other words, it may exert effects from a distance. For these reasons, it could connect things that are distant in space, thus opening the possibilities of quantum coherence on a large scale and of “non-computable”phenomena that our brains could put to use.

But where exactly might these phenomena occur in the human brain? To answer this question, Penrose turns to the hypothesis offered by American anaethesiologist Stuart Hameroff, that consciousness emerges from quantum coherence at the level of the microtubules. As their name implies, microtubules are tiny tubes, composed of proteins, that are found in all cells of the human body, including the neurons. The microtubules form a sort of cytoskeleton for the cells and play a role in cell division as well as in the transport of organelles inside the cells.

Microtubules are composed of tubulin dimers formed into spirals to produce small tubes about 24 nanometres in diameter. These molecules of tubulin may be in two different states—extended or contracted—which might, according to Hameroff, result from a superposition of quantum states.

Penrose and Hameroff investigated microtubules because their small size and spiral protein structure would provide the essential conditions to orchestrate quantum collapses. For quantum coherence processes to occur, they must remain reasonably isolated from the external environment, and according to these authors, microtubules fulfil this condition.

Penrose and Hameroff state that this model might explain several of the fundamental characteristics of consciousness. Thus, the possibility of objective reduction effects from a distance might account for the unity of consciousness, and quantum indeterminism might be the source of free will.

Penrose adds that his theory might even help to explain the strange results of Libet’s experiment, because in Penrose’s view, when conventional reasoning about the temporal sequence of events leads us to contradictory conclusions (such as the impression of influencing the past, on time scales on the order of half a second), this constitutes a strong indication that quantum effects are at work.

Penrose and Hameroff’s model has received serious criticism, notably from philosophers Rick Grush and Patricia Churchland, who raise several objections that lead them to reject this model in its entirety. First of all, Grush and Churchland point out that microtubules are found in all plant and animal cells, and not only in the neurons of the human brain. These authors also state that some chemicals that are known to destroy microtubules do not seem to have any major effects on consciousness. They point out that a number of anaesthetics act without affecting the microtubules. And lastly, they note that there is no evidence that microtubules are involved in other phenomena that cause major changes in states of consciousness, such as the sleep/dream/wake cycle.

Strictly in terms of physics, one classic criticism is that offered by Max Tegmark, who states that the temperature of the brain is too high for elementary particles to remain in superposed states long enough for neuronal processes to be affected by them. Grush and Churchland add that microtubules cannot achieve the conditions of purity and isolation required by Penrose’s theory; they also argue that quantum effects cannot be transmitted from one microtubule to another as they would have to be to explain the unity of consciousness. In Grush and Churchland’s view, Penrose’s theory also fails to provide any valid explanations of the way that quantum effects could interact with neurons or neurotransmitters when microtubules are supposed to be isolated from their environment.

Another objection addresses one of the strengths of Penrose and Hameroff’s model, which is, according to its authors, that it can account for the unity of consciousness. But if this impression of the unity of human consciousness should prove to be an illusion, as some other authors believe, then once again, explanations based on non-locality and quantum coherence would become irrelevant.

Other critics have questioned whether, in the end, Penrose and Hameroff’s theory really has anything to do with consciousness or whether it actually simply replaces one mystery—subjective experience—with another—quantum coherence in microtubules.

Grush and Churchland have even gone so far as to ask whether the reason that Penrose and Hameroff’s model became so popular might be that whereas reducing consciousness to mere neural correlates has always seemed somehow degrading to some people, the quantum alternative enables some of the comforting mystery surrounding consciousness to be preserved.

That said, it is worth recalling here what Danish physicist Niels Bohr, the father of quantum physics and a professor of Werner Heisenberg’s, said one day to a young physicist: “We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct.”

Another hypothesis about consciousness that draws on quantum physics dates back to research done by Ricciardi and Umezawa in the 1960s. This hypothesis treats mental states, and in particular memories, as “vacuum states of the quantum field”. It takes neurobiological research on consciousness into account, inasmuch as it regards these “vacuum states” as consisting of neuronal assemblies corresponding to memory contents, and posits that it is the activation of these assemblies, triggered by external stimuli, that results in states of excitation that enable the memory content encoded in the “vacuum state” to be recalled.

As if that were not already pretty complicated, in the mid-1960s, Italian physicist Giuseppe Vitiello began to improve upon this hypothesis by adding the concepts of dissipative structures, chaos, and quantum noise. In Vitiello’s model, the fact that the brain is an open system in constant interaction with its environment is what accounts for its vast memory capacity. But this model still has some conceptual ambiguities that need to be clarified, in particular between mental and material states.

Lien : Quantum Field TheoryLien : PHYSIQUE QUANTIQUE DES CHAMPSLien : États comprimés - Bruit quantique standardLien : My Double Unveiled. The dissipative quantum model of brainLien : Trying to understand Giuseppe Vitiello's ideas on the dissipative quantum brain

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