Quantum physical interpretations of the universe are not essential or in any way fundamental to the empirical approach to mind. The empirical approach is about stressing that observation is more important than theory, it does not depend on any particular physical theory.
However, along with Relativity, the mere existence of quantum theory shows that a simple model of the world where discrete lumps of matter mediate interactions is incomplete. If simple materialism is not a universal physical theory then the regress arguments in the philosophy of mind are not incontrovertible and mind cannot be dismissed simply because early materialism implies a homunculus (the little man within a little man within a..). Indeed the opposite is true, the homunculus that is implied by materialism means that the observation of mind should be respected and nineteenth century materialism rejected.
Although quantum physics neither validates nor invalidates New Empiricism it does, however, have interesting consequences for the analysis of mind that may or may not be supported by Empiricism. Modern advances in quantum physics, such as decoherence theory, show that there are several problems that have a direct impact on our idea of mind.
Before addressing the role of quantum theory in the philosophy of mind it is essential to distinguish between two different problems, the first problem is whether or not the brain could contain a superposition of quantum states adjacent to our normal environment and the second problem is the nature of the environment itself.
The possibility that the brain may exist in a superposition of states like a quantum computer is interesting but fraught with difficulties (see Tegmark 2000). Electromagnetic fields may be able to sustain a superposition of states (Anglin & Zurek 1996) but I will not discuss superpositions of this type any further here.
The second problem, the problem of the nature of the environment is far more interesting. Decoherence theory has illuminated this problem so that in the 21st century we can get a clearer idea of the problem than ever before.
The problem of the nature of the environment arises because when a measurement is made on an isolated system that has two possible superpositions of state there are two possible outcomes of the measurement, one being the measuring instrument plus system in one state and the other the measuring instrument plus system in the other state. The measuring instrument and the system are said to be "entangled" and form a new conjoint system with its own superpositions of state. But we only ever see the measuring instrument in one state. Zurek analysed this problem and realised that if the measuring instrument and system combination could be isolated it would form a superposition of states and this superposition would be extended if we added more measurements to the conjoint system. For instance, if a beam of light struck the measuring instrument it would create a triple entanglement and if a scientist looked at light photons coming from the instrument it would create a conjoint, four component entanglement. If this four part system were isolated it would exist in a superposition of quantum states, each with its own probability. This predicts that there would be as many copies of the scientist as there were possible states. Zurek took this analysis yet further and demonstrated that the entire environment becomes a conjoint state because of the interactions between its components. This "environment" has the same properties as the world of classical (non-quantum) physics.
Although decoherence theory predicts that there would be as many copies of the scientist as there are states of the system it also predicts that an individual scientist, ie: an individual copy, would only observe one state of the system. This scientist is surrounded by a classical world without any significant superpositions of state. Decoherence theory is essentially a "many worlds" interpretation of quantum theory with many copies of an individual being possible in the multiverse but each individual observing a classical universe. So far so good, but what about the observer, you and I?
If I consider you or your brain there is some, but little scope within decoherence theory for a superposition of states. If your brain has a synapse that is briefly in a superposition of firing and not-firing I will discover that it will rapidly adopt one of the two states. So I probably see your brain as a classical device. But what of my own observation?
It could be claimed that if my own observation is the same as a single outcome, if it is not a superposition of states, then it is localised to a single branch, a single, entangled, conjoint state and hence dependent upon the classical state of the brain. However, there is a "sleight of hand" in this argument because there is no way within the argument to distinguish between an environment that is created by the possibility of conscious observation and one that creates conscious observation. According to decoherence theory it would be possible to introduce an observer-system into a non-entangled universe and after a second or two an "environment" would form around the observer-system. A few minutes later the environment that is due to the presence of the observer would be indistinguishable from any other classical environment.
So how can we distinguish between an environment that is created by the possibility of conscious observation and an environment in which this is not the case? If we are to believe the theory that the environment creates our observation then our experience must be fully encompassed by the theory - as scientists we are not entitled to reject observation on the basis of incomplete or flimsy theories. If we are to believe the theory that the environment creates our experience then our experience cannot contain phenomena that are not encompassed by decoherence theory. In fact there is a vast difference between my experience and decoherence theory because my observation contains time as an observable. My own observation is not just like synapses firing, it is composed of objects that are connected in time as well as space (See Time and conscious experience). It has time as a preeminent "observable" and hence does not conform to the Schrödinger equation used in the development of decoherence theory (See Horwitz (2005)). Furthermore, my conscious experience appears to be passive and as Zeh (2000) pointed out, if our conscious experience is passive the rules of decoherence may not apply and it could well be the spacetime point of our observation that selects the universe where we find ourselves. On this model the environment would consist of those events that are compatible with the spacetime form of conscious observation. This would be consistent both with decoherence theory as a limited quantum description of the classical environment and with modern cosmology (See for instance "Hawking's reflections on spacetime and the existence of humans in: Quantum Cosmology, M-theory and the Anthropic Principle ). Our environment would be that part of the multiverse that is consistent with conscious experience ie: that part that has 3 spatial and 1 or more temporal extensive dimensions
This is a different view from the conventional idea of quantum physics and mind. For instance, Zurek (2003) explicitly assumes from the outset that the conscious observer is like a computer to avoid much of the problem of observation. This assumption that we are like a computer leads to the tautology that a Newtonian environment created by decoherence creates a conscious experience that has previously been defined as something created by a Newtonian environment. This "built-in" assumption has misled many commentators into thinking that decoherence proves that the conscious observer is a product of decoherence like a digital computer and hence immaterial to quantum physics.
If Zurek is right and experience is classical then it cannot contain time extended events. If observation is right then our experience is non-classical from the start, embedding the energy-time form of the Heisenberg Uncertainty Principle within it and the classical world originates in observers.
Proving that Zurek is wrong will require experiments. I would suggest analysing delayed choice experiments, these seem to show that decoherence originates at the position of the observer rather than being a property of the average state of the environment. Kent (2005) points out that if wavefunction collapse were to occur some time after events occur (ie: wavefunction collapse occurs in the brain) then there are ways of testing this using entangled particles. He notes that the qm experiments performed to date cannot distinguish between non-local and local collapse of the wavefunction in the brain but could be amended to do so. Unfortunately Kent presumes that conscious experience occurs at 0.1 sec after sensory events whereas neuroscientists know that the gap is more like 0.5 secs, let us hope that no physicist wastes a year doing this experiment only to be told that the time gap should have been 0.4 secs greater.
Some further reading
Anglin, J.R. & Zurek, J.H. (1996). Decoherence of quantum fields: decoherence and predictability. Phys.Rev. D53 (1996) 7327-7335 http://arxiv.org/abs/quant-ph/9510021
Baker, D (2006) Measurement Outcomes and Probability in Everettian Quantum Mechanics. http://philsci-archive.pitt.edu/archive/00002717/
Bachtold, M (2008). Five Formulations of the Quantum Measurement
Problem in the Frame of the Standard Interpretation. J Gen Philos Sci (2008) 39:17–33
Bitbol, M (2008) Consciousness, Situations and the measurement problem of quantum mechanics. NEUROQUANTOLOGY, 6, 203-213, 2008
Horwitz, L.P. (2005) On the Significance of a Recent Experiment
Demonstrating Quantum Interference in Time. http://www.arxiv.org/pdf/quant-ph/0507044
Janssen, Hanneke (2008) Reconstructing Reality: Environment-Induced Decoherence, the Measurement Problem, and the Emergence of Definiteness in Quantum Mechanics.
Kent, A (2005). Causal Quantum Theory and the Collapse Locality Loophole. Physical Review A 72:11, 12107, American Physical Society, 7/2005. http://www.citebase.org/abstract?id=oai%3AarXiv.org%3Aquant-ph%2F0204104
Saunders, S (1996). Time, Quantum Mechanics and Probability. http://philsci-archive.pitt.edu/archive/00000465/00/Part3uj(S).pdf
Tegmark, M. (2000)The importance of quantum decoherence in brain processes. Phys.Rev. E61 (2000) 4194-4206
Wallace, D (2007). The Quantum Measurement Problem: State of Play http://arxiv.org/abs/0712.0149
Zeh, H.D. (2000) The Problem of Conscious Observation in Quantum Mechanical Description http://arxiv.org/abs/quant-ph/9908084
Zurek, W.H. (2003). Decoherence, einselection and the quantum origins of the classical. Rev. Mod. Phys. 75, 715 (2003) http://arxiv.org/abs/quant-ph/0105127