Based on a calculation of neural decoherence rates, we argue that the degrees of freedom of the human brain that relate to cognitive processes should be thought of as a classical rather than quantum system, i.e., that there is nothing fundamentally wrong with the current classical approach to neural network simulations. We find that the decoherence time scales $(\sim {10}^{-13}–{10}^{-20}\mathrm{s})$ are typically much shorter than the relevant dynamical time scales $(\sim {10}^{-3}–{10}^{-1}\mathrm{s}),$ both for regular neuron firing and for kinklike 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.

• Received 6 July 1999

DOI:https://doi.org/10.1103/PhysRevE.61.4194