Penrose-Hameroff orchestrated objective-reduction proposal for human consciousness is not biologically feasible

Penrose and Hameroff have argued that the conventional models of a brain function based on neural networks alone cannot account for human consciousness, claiming that quantum-computation elements are also required. Specifically, in their Orchestrated Objective Reduction (Orch OR) model [R. Penrose and S. R. Hameroff, J. Conscious. Stud. 2, 99 (1995)], it is postulated that microtubules act as quantum processing units, with individual tubulin dimers forming the computational elements. This model requires that the tubulin is able to switch between alternative conformational states in a coherent manner, and that this process be rapid on the physiological time scale. Here, the biological feasibility of the Orch OR proposal is examined in light of recent experimental studies on microtubule assembly and dynamics. It is shown that the tubulins do not possess essential properties required for the Orch OR proposal, as originally proposed, to hold. Further, we consider also recent progress in the understanding of the long-lived coherent motions in biological systems, a feature critical to Orch OR, and show that no reformation of the proposal based on known physical paradigms could lead to quantum computing within microtubules. Hence, the Orch OR model is not a feasible explanation of the origin of consciousness.

Figure 1

The physiological elements of a qubit and the operational cycle of the quantum computation within the Penrose-Hameroff model for cognitive function by orchestrated objective reduction, taken from Ref. [8]; reproduced with permission of MIT Press. Left: tubulin dimers within microtubules exist in two conformational states (shown black or white) and it is proposed that resonance coupling between these conformations acts to form qubits from quantum superpositions of the local vibronic wave functions (shown gray). Such superpositions require coherent motion in the vibrational modes, proposed to originate via Fröhlich condensation [4], that interchange the two conformational states and move internal electrons (marked “e”). Right: four steps in the operational cycle starting at (1) superpositions start to appear from among classically localized states in the tubulin-dimer qubits, (2) coherent motions and superpositions grow to encompass large regions of this and neighboring microtubules, (3) superposition reaches critical mass inducing objective reduction to form (4) an ensemble of tubulin dimers throughout many microtubules containing the results of the quantum computation specified through their (classical) conformations.

Figure 2

Current understanding of the biochemical processes that modify the conformational states of tubulin dimmers. A more modern symbol for the dimers is used compared to that used by Penrose and Hameroff (see Fig. 1) to facilitate a more complete description of the biochemical processes; while the microtubule-$B$ lattice is shown [28], the depicted processes are lattice independent [12, 28, 29, 30].