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Is quantum mechanics relevant to the scientific understanding of the mind-brain problem?

Author: Javier Sánchez-Cañizares
Published in: F.J. Soler Gil - M. Alfonseca (coords.), "60 questions on science and faith answered by 26 university professors". Madrid: Stella maris, pp. 245-51.
Date of publication: 2014


Quantum models of consciousness

Criticisms of the relevance of CM for understanding the brain

In what sense is CM relevant to the mind-brain problem?



In recent decades, advances in the field of neuroscience have renewed interest in understanding the relationship between the mind and the human brain. Quantum Mechanics (QM) has been present in this discussion since its beginnings through the well-known "paradox of measurement". The standard interpretation of QM assumes the existence of two mutually irreducible processes: (a) the deterministic evolution according to the Schrödinger equation of the wave function representing the physical state of the system, once its initial conditions have been established; and (b) the indeterministic "collapse" of this wave function into a state compatible with the result of the measurement of a particular observable.1. In this way, the CM would be pointing out the limits of a purely deterministic view of nature and, in particular, of brains. It is to be expected, therefore, that the neuroscientific research of the brain will at some point encounter quantum phenomenology.

Over the years, different theoretical models have attempted to explain the specific way in which CM is said to play a significant role in brain physiology. However, these theories have generally not been held in high esteem by neurologists due to a lack of scientific plausibility. Before giving a brief description of the models, it should be noted that there is currently a line of research that uses the CM formalism to describe some phenomena of human consciousness and behaviour.2. The aim is to apply certain formal features of CM to certain mental phenomena, but without going into the underlying physics of these phenomena, on which judgement is suspended. Certainly, the direct application of the formalism of QM to mental states allows for a valuable adjustment of many empirical data, but it says very little about the reality that causes them. Nevertheless, this line of research could offer a determination of the relevance of CM to the mind-brain problem, insofar as it shows the inability of classical conditional probability models to explain some of the currently available results.3 to explain some of the results currently available.

Quantum models of consciousness

Throughout the history of CM, scientists have delved into the behaviour of the brain at the microscopic scale and its amplifications at the macroscopic level in search of a possible substrate for quantum phenomena. Such phenomena could explain the properties of the human psyche more convincingly than traditional cognitive neuroscience. Among the most active representatives of this line of research are: (a) Stuart Hameroff and Roger Penrose, for whom consciousness would be intimately linked to the goal and structured collapse of the wave function in the microtubules4 of neurons, caused by gravitational interaction; (b) Stuart Kauffman, who considers the brain as a system that continuously moves from decoherence to quantum recoherence; and (c) the team led by Giuseppe Vitiello, who applies a dissipative formalism of quantum field theory to explain the various patterns of coherent activity in the brain.5 to explain the various patterns of coherent activity that would occur in the human brain at contact with multiple external stimuli. The fundamental problem with these theories is that, even if they were to succeed from an empirical point of view (by explaining and predicting new phenomena beyond the reach of a classical theory), they would result in only a small advance in the understanding of the mind-brain problem, as they still do not provide an answer to how the transition or conversion from the physical to the mental (and vice versa) takes place.

There are other models that resort to CM to explain this last transition, considering consciousness and mental activities as primary realities, which would have manifestations in the physical world only understandable from CM. For example, the following can be cited at degree scroll : (a) Friedrich Beck and John Eccles, who proposed a model quantum enhancement of the speech through synapses; (b) Henry Stapp, who resorts to the quantum Zeno effect to explain how conscious attention is able to fix relations between physical and mental states; and (c) Efstratios Manousakis, for whom the activities of our brain, the perceptual flow of events and the CM itself would emerge from primary operations of consciousness. However, how it is possible for certain physical events to have effects on our consciousness remains unexplained in this subject of models, which also fail to provide an explanation of the paradox of measurement in CM.6.

Criticisms of the relevance of CM for understanding the brain

The most important criticisms of the relevance of CM for scientific understanding of the mind-brain problem come from the experimental field. Despite the existence of the aforementioned models and the promising results of Hameroff's and Penrose's theory, the fundamental criticism of neuroscientists is that no experiments have been presented so far.7theory, the fundamental complaint of neuroscientists is that no experiment has so far been presented that shows unambiguous signs of quantum effects in the brain. One could say that, de facto, there is no answer final about the empirical relevance of QM in the brain and that none of the proposed models seems to enjoy a priori plausibility from a neurobiological point of view. At the same time, resorting to pre-quantum physics for the substantiation of neural processes would not be adequate to address the mind-brain problem in all its complexity.

Critics of the relevance of QM ultimately rely written request on the effect of decoherence processes at different levels to ensure classical brain behaviour. Quantum decoherence is currently the most common resource to try to explain the transition from the world of quantum entangled possibilities to the classical world of real events. Decoherence theory postulates that when a system interacts with a sufficiently large physical environment, the interference terms in the wave function of the former tend to cancel out, because of the interaction with the latter. In such a status, quantum interference does not occur in the system and the classical regime emerges from the various quantum possibilities. The interaction of the system with its environment thus resembles the process of a classical measurement, according to the standard interpretation of QM. The system is partially measured by its environment through a gradual process of decoherence, which takes the system from a coherent superposition of possible states to a "mixed" state, reflecting only the probabilities of each measurement.8. The existence of decoherence processes is well known experimentally and is one of the major difficulties in constructing, for example, quantum computers.

Nevertheless, the specific way in which decoherent processes act in physical and biological systems is only partly understood. On the one hand, decoherence theory does not provide a consistent ontology of the reality of the world, offering only a procedure for practical purposes. Decoherence depends on the representation chosen for the wave function (on its contextualisation according to the measurement to be made), so that the reduced density matrix may be diagonal in one representation, but not in another.9. On the other hand, the theory of decoherence does not explain how the collapse of the wave function would occur in isolated systems, nor the nature that an isolation would have to have for the environment not to be involved. And, above all, it says nothing about which part of a general physical system should be considered as an environment and which should not, since it does not provide any definite limit of any variable of the system that would ensure its quantum or classical behaviour.

In what sense is CM relevant to the mind-brain problem?

While neuroscience as such does not need to delve into these conceptual problems for the time being, limiting itself to empirical evidence, philosophers of mind should draw some conclusions. In particular, the simple reference letter to classical complexity as a future explanation of mental phenomena leads to a plea of principle. CM is the fundamental physical theory underlying the physiology of the brain. In it, classical behaviour is recovered thanks to decoherence models. But these models call for an ad hoc treatment that makes CM an epistemologically non-unified theory. For the resource to work, it is necessary to invoke a priori a different attention of the parts that make up the system. This has to be divided into a subsystem (the brain or the part of it whose study is considered relevant to consciousness) and a thermal bath (a mathematical idealisation of the environment), whose Degrees of freedom are averaged and eliminated from the problem. In this sense, decoherence as the ultimate explanation of the emergence of the classical world in the brain and of a mental activity caused by complexity turns out to be an incomplete and essentially dualistic theory.

On the other hand, it is B that decoherence occurs when a physical system is defined a priori in order to obtain information about it through some subject measurement. In other words, the decision about the system to be studied and the observation to be made are irreducible parts of the process. We must decide a priori which physical subsystem is going to be relevant and how (under which observables) it is going to be relevant, since decoherence theory implies the selection of subsystems by the observer. Thus, the standard interpretation of CM sample is the limit beyond which separation from nature and human access to it is no longer possible. In CM, logic, the knowledge and its neural correlates assume the same importance as the characteristics of what is being described. We are dealing with levels of reality where cognitive statements about the dynamic variables of nature become themselves part of the problem. It must be emphasised, therefore, that the philosophical framework of CM is significant for the mind-brain problem not simply because it provides randomness versus determinism, but because it claims an irreducible influence of the observer's choice of relevant information on the evolution of physical reality.


(1) While in classical mechanics a physical system is described by the values taken by the Degrees of freedom or relevant variables of the system, in CM a physical system is described by a vector of complex numbers which is called the wave function. As long as no measurement is made, the evolution of the wave function follows the Schrödinger equation (a differential equation analogous to the deterministic equations of classical physics). However, when a measurement of a particular physical quantity (an observable) is made, the value of the wave function is no longer obtained from the Schrödinger equation, but "jumps" or "collapses", in an indeterministic way, to a value compatible with the result of the measurement according to probability rules that are computed from the wave function. This is the standard or Copenhagen interpretation of the CM.

(2) A list of the most relevant groups can be found in Atmanspacher, H. [online]: "Quantum approaches to consciousness" in Zalta, E.N. (ed): The Stanford Encyclopedia of Philosophy (2011), [Accessed: 27/02/2014].

(3) Conditional probability is the probability that event A occurs, knowing that another event B also occurs. CM predicts outcomes (actually measured in other contexts) that violate the composition rules of conditional probability and prohibit a purely cognitive interpretation of the wave function (as if it only refers to the state of knowledge of the observer). In other words, CM gains access to a reality that is not describable by the employment of classical (statistical) mechanics and the laws of conditional probability.

(4) Microtubules are Structures of cells, consisting of two proteins (alpha and beta tubulin), which extend throughout the cytoplasm.

(5) Quantum field theory is a generalisation of QM when the physical system of interest needs to be described by a continuous and infinite number of Degrees of freedom or relevant variables.

(6) A more general discussion of each of these models can be found in Sánchez-Cañizares, J.: "The Mind-Brain Problem and the Measurement Paradox of Quantum Mechanics: Should We Disentangle Them?", in NeuroQuantology 12/1, 2014, in press.

(7) Hameroff, S. - Penrose, R. [online]: "Consciousness in the universe: A review of the 'Orch OR' theory", in Physics of Life Reviews, 2013, DOI: 10.1016/j.plrev.2013.08.002 [Accessed 27/02/2014].

(8) Prior to a measurement, the wave function includes interference between all possible states of the system. It is in a "coherent superposition" of states. In general, such interferences have appreciable effects on the measurement probabilities of the observables (generating novel quantum effects). However, through interaction with a sufficiently large second system, the interference terms may eventually cancel out, and the initial system could become a "mixed" state (describable by a mere set of probabilities for each possible measurement), becoming "classical" (describable by non-quantum physics).

(9) The density matrix is another MC formalism equivalent to the wave function. It is especially useful when dealing with composite systems. It is called reduced when it describes a subsystem after having weighted the influence of the environment on it. If the reduced matrix has only non-zero elements on its main diagonal, then decoherence has occurred. However, the specific form of the density matrix depends on the particular observables to be measured.