In the most recent issue of Nature (March 30) Christof Koch and Klaus Hepp offer a critique of theories, such as Roger Penrose and Stuart Hameroff's, that human consciousness invoke quantum principles. Most interestingly, they suggest a new thought experiment:
We challenge those who call upon consciousness to carry the burden of the measurement process in quantum mechanics with the following thought experiment. Visual psychology has caught up with magicians and has devised numerous techniques for making things disappear. For instance, if one eye of a subject receives a stream of highly salient images, a constant image projected into the other eye is only seen infrequently. Such perceptual suppression can be exploited to study whether onsciousness is strictly necessary to the collapse of the wave function. Say an observer is looking at a superimposed quantum system, such as Schrödinger’s box with the live and dead cat, with one eye while his other eye sees a succession of faces. Under the appropriate circumstances, the subject is only conscious of the rapidly changing faces, while the cat in the box remains invisible to him. What happens to the cat? The conventional prediction would be that as soon as the photons from this quantum system encounter a classical object, such as the retina of the observer, quantum superposition is lost and the cat is either dead or alive.This is true no matter whether the observer consciously saw the cat in the box or not. If, however, consciousness is truly necessary to resolve the measurement problem, the animal’s fate would remain undecided until that point in time when the cat in the box becomes perceptually dominant to the observer. This seems unlikely but could, at least in principle, be empirically verified. The empirical demonstration of slowly decoherent and controllable quantum bits in neurons connected by electrical or chemical synapses, or the discovery of an efficient quantum algorithm for computations performed by the brain, would do much to bring these speculations from the ‘far-out’ to the mere ‘very unlikely’. Until such progress has been made, there is little reason to appeal to quantum mechanics to explain higher brain functions, including consciousness.
The end of quantum theories of consciousness? Well, I suspect a rebuttal from Hamroff is forthcoming!
Koch, C. & Hepp, K. (2006): Quantum mechanisms in the brain. Nature 440: 611-612.