Consciousness and Neuronal Microtubules: The Penrose-Hameroff Quantum Model in Retrospect

Abstract

The microtubule-based model of consciousness, proposed simultaneously in the mid-1990s by the British physicist Roger Penrose and the American anesthesiologist Stuart Hameroff, is probably the best known, least understood, and most controversial among the various hypotheses that aim to account for such puzzling faculty in terms of quantum physics. In this theory microtubules —i.e., long fibrous structures that form an intracellular framework supporting the shape of every neuron— are able to integrate information in a non-computable way through transient conformational changes of the tubulin protein molecules that constitute them. Switching between two alternative states tubulin dimers allegedly may, apart from providing a huge capability for standard binary computation, reach a third quantum superposition state with its own specific wave function. As the number of tubulin dimers in the quantum superposition state gradually increases, coherence in this state is believed to build up and spread intracellularly and transcellularly through the population of microtubules across relatively wide regions of the brain. It is postulated that this process goes on up to a point when a spontaneous abrupt reduction of quantum coherence takes place, a collapse that amounts to an instant of awareness calculated to occur every 20–30 ms. The subjective impression of a continuous stream of consciousness would thus be the result of a succession of such self-reduction events. The biological and physical components of this model are briefly outlined here in terms intended to be accessible to both quantum physicists and neuroscientists, as background for a summary of the basic premises and the development of the theory from its beginning. The main criticisms and responses in the ensuing debate are then presented, followed by a few concluding comments after a brief review of historical precedents of the main notion of information handling by tiny biological fibers, going back to the early eighteenth century.