Why the brain is probably not a quantum computer

Based on a calculation of neural decoherence rates, we argue that that the degrees of freedom of the human brain that relate to cognitive processes sh ould be thought of as a classical rather than quantum system, i.e., that th ere is nothing fundamentally wrong with the current classical approach to n eural network simulations. We find that the decoherence timescales (similar to 10(-13)-10(-20) s) are typically much shorter than the relevant dynamic al timescales (similar to 10(-3)-10(-1) s), both for regular neuron firing and for kink-like polarization excitations in microtubules. 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. (C) 2000 Published by Elsevier Science Inc. All rights reserved.