Ongoing Debate
Science
The main argument against the quantum mind proposition is that the structures of the brain are much too large for quantum effects to be important. It is impossible for coherent quantum states to form for very long in the brain and impossible for them to exist at scales on the order of the size of neurons.
This does not imply that classical mechanics can explain consciousness, but that quantum effects including superposition and entanglement are insignificant. Quantum chemistry is required to understand the actions of neurotransmitters, for example.
One well-known critic of the quantum mind is Max Tegmark. Based on his calculations, Tegmark concluded that quantum systems in the brain decohere quickly and cannot control brain function, "This conclusion disagrees with suggestions by Penrose and others that the brain acts as a quantum computer, and that quantum coherence is related to consciousness in a fundamental way"[11].
Proponents of quantum consciousness theories have sought to defend them against Tegmark's criticism. In respect of QBD, Vitiello has argued that Tegmark's work applies to theories based on quantum mechanics but not to those such as QBD that are based on quantum field theory. In respect of Penrose and Hameroff's Orch OR theory, Hameroff along with Hagan and Tuszynski replied to Tegmark[36]. They claimed that Tegmark based his calculations on a model that was different from Orch OR. It is argued that in the Orch OR model the microtubules are shielded from decoherence by ordered water. Energy pumping as a result of thermal disequilibrium, Debye layer screening and quantum error correction, deriving from the geometry of the microtubule lattice are also proposed as possible sources of shielding. Similarly, in his extension of Bohm's ideas, Bernroider has claimed that the binding pockets in the ion selection filters could protect against decoherence[19]. So far, however, there has been no experimental confirmation of the ability of the features mentioned above to protect against decoherence.
Philosophy
Another line of criticism is that no physical theory is well suited to explaining consciousness, particularly in its most problematical form, phenomenal consciousness or qualia, known as the hard problem of consciousness. It is not so much that colours and tastes and feelings --qualia or secondary qualities -- have been deliberately banished, but more that they cannot be captured in any mathematical description, which means they cannot be explicitly represented in physics, since all physical theory is expressed in mathematical language (as explained in Eugene Wigner's famous paper The Unreasonable Effectiveness of Mathematics in the Natural Sciences). If no physical theory can express qualia, no physical theory can fully explain consciousness. Replacing the mathematical apparatus of classical physics with the mathematical apparatus of quantum mechanics is therefore of no help in understanding consciousness, and indeed there is no known example of a quantum equation which encapsulates a taste or colour.
As David Chalmers puts it:
Nevertheless, quantum theories of consciousness suffer from the same difficulties as neural or computational theories. Quantum phenomena have some remarkable functional properties, such as nondeterminism and nonlocality. It is natural to speculate that these properties may play some role in the explanation of cognitive functions, such as random choice and the integration of information, and this hypothesis cannot be ruled out a priori. But when it comes to the explanation of experience, quantum processes are in the same boat as any other. The question of why these processes should give rise to experience is entirely unanswered. [12]
Other philosophers, such as Patricia and Paul Churchland and Daniel Dennett[13] reject the idea that there is anything puzzling about consciousness in the first place.
