quantum consciousness, theories of
Theories which explore possible connections between quantum mechanical phenomena and consciousness. See also consciousness.

Since the publication of Roger Penrose's two books The Emperor's New Mind (1989), and Shadows of the Mind (1994), there has been a tremendous resurgence of interest in exploring possible connections between quantum mechanical phenomena and consciousness. But there have been those who have been exploring the connection for many decades. In this essay, I will first briefly run through Penrose's main ideas, especially as they have been developed in collaboration with Stuart Hameroff. I will then discuss some of the reasons that people have had for trying to defend some such connection since long before the Penrose-led resurgence.
Penrose's main argument
Penrose's main argumentative line can be summed up as follows (this summary is taken from Grush and Churchland (1995)):
Part A: Nonalgorithmicity of human conscious thought.
       A1) Human thought, at least in some instances, is sound , yet nonalgorithmic (i.e.
       noncomputational). (Hypothesis based on the Gödel result.)
       A2) In these instances, the human thinker is aware of or conscious of the contents of these
       A3) The only recognized instances of nonalgorithmic processes in the universe are perhaps
       certain kinds of randomness; e.g. the reduction of the quantum mechanical state vector.
       (Based on accepted physical theories.)
       A4) Randomness is not promising as the source of the nonalgorithmicity needed to account
       for (1). (Otherwise mathematical understanding would be magical.)
       A5) Conscious human thought, at least in some cases, perhaps in all cases, relies on
       principles which are beyond current physical understanding, though not in principle beyond
       any (e.g. some future) scientific physical understanding. (Via A1 - A4
Part B: Inadequacy of Current Physical Theory, and How to Fix It.
       B1) There is no current adequate theory concerning the 'collapse' of the quantum
       mechanical wave function, but an additional theory of quantum gravity might be useful to this
       B2) A more adequate theory of wave function collapse (a part, perhaps, of a quantum gravity
       theory) could incorporate nonalgorithmic, yet nonrandom, processes. (Penrose hypothesis.)
       B3) The existence of quasicrystals is evidence for some such currently unrecognized, 
       nonalgorithmic physical process.

       B4) Future theories of physics, in particular quantum gravity, can be expected to incorporate
       nonalgorithmic processes. (via B1 - B3)
Part C: Microtubules as the means of harnessing quantum gravity.
       C1) Microtubules have properties which make certain quantum mechanical phenomena (e.g.
       super-radiance) possible. (Hameroff/Penrose hypothesis.)
       C2) These nonalgorithmic nonrandom processes will be sufficient, in some sense, to account
       for A5. (Penrose hypothesis.)
       C3) Microtubules play a key role in neuron function.
       C4) Neurons play a key role in cognition and consciousness.
       C5) Microtubules play a key role in consciousness/cognition (by C3, C4 and transitivity).
       C6) Microtubules, because they have one foot in quantum mechanics and the other in
       conscious thought, provide a window for nonalgorithmicity in human cognition.
       D) Quantum gravity, or something similar,via microtubules, must play a key role in
       consciousness and cognition.
Part A takes up a large chunk of Penrose (1994), and although I think his argument ultimately fails, I cannot imagine anyone doing a better job of trying to make that argument. Part B gets into much of Penrose's non-consciousness-related thought, in particular a theory of quantum gravity that he has been working on for some time. Part C is due mostly to Stuart Hameroff (see, e.g. Hameroff (1994)), who has been conjecturing on the computational capacities of microtubules for a while, but was inspired by Penrose's work to work out a theory of quantum mechanical effects in microtubules.
Details on Penrose's argument are best found in his Shadows of the Mind (1994), and details of the criticisms can be found in Grush and Churchland (1995) [a draft of this paper is available here]. Penrose and Hameroff (1995) is a brief reply to the criticisms of Grush and Churchland. Furthermore, there are on-line papers by both Penrose and Hameroff, listed below, that will provide more details from the proponents themselves.
Motivations for quantum theories of consciousness
Apart from Penrose's work, there are many who have drawn connections between quantum mechanics and consciousness. There are a number of motivations that have driven people to look for such a connection. Here are a few of the more influential:
1. Free will. Many people are convinced that humans have free will, and yet are also convinced that the Newtonian-mechanical goings-on of things as large as neurons makes no room for free will. They thus turn to quantum mechanics in the hope that the non-determinism of the collapse of the wave function will provide a foot in the door for free will. Of course the wave function collapse is, according to current theory, random, and it is not clear that this is any better than determinism when it comes to explaining free will. Nevertheless, the hope seems to be that, at least in some cases, consciousness exerts its influence on the world through effecting some collapses, presumable some in the brain somewhere, in one way rather than another.
2. The unity of consciousness. It is claimed that consciousness has a unity, or wholeness to it, that cannot be explained by reducing consciousness to a scattered group of neurons. Rather, many think that quantum mechanical coherence (a phenomenon whereby many different objects can share a single wave function, and in some respects behave as a single particle) gives an explanation for this. Of course, it could be objected that this line of reasoning rests on a blatant content/vehicle confusion. From the fact that some of our introspections have a content with certain properties (we perceive our consciousness to be non-scattered, for example), it is concluded that the vehicle of this content must also have these properties (being non-scattered, for example). Of course this line of reasoning fails horribly. The bank's computer represents my checking account as a more or less unified entity, but the electromagnetic objects that constitute the vehicle of that representation are scattered widely, and could be scattered over a large geographic area -- perhaps even distributed with parts of record from other accounts --, depending on how their computer hardware is set up. One can write the word 'red' in blue ink. In general, there need be no match between the properties that characterize a content, and the properties of the vehicle that carry that content. Given this, there seems to be little motivation to try to explain the unity of consciousness via quantum mechanical coherence.
3. The mysteriousness of consciousness. Consciousness appears to be an extremely mysterious phenomenon. It is not clear how a collection of molecules whose chemical composition is not unlike that of a cheese omelet could be aware of anything, to feel pain, or see red, or dream about the future. Quantum mechanics also seems to be very mysterious -- particles going traversing two paths at the same time, for example. So perhaps they are the same mystery. Nobody phrases it that way, of course, but this seems to be a line of intuition that motivates many people. It is often argued that mere neurons could not be conscious or aware, and this seems to be because one can imagine all the working of of a neuron, or even a large group of neurons, without seeing how consciousness could be implicated. But because the mechanisms underlying quantum mechanical phenomena are less viaualizable, or comprehensible, or whatever, it seems not to be as clear that something as mysterious as consciousness couldn't work its way into the machine somehow. Clearly, this intuition survives only as long as the mechanisms of quantum mechanics are mysterious to the person making the argument.
Rick Grush