Criticism
The main objection to the Hameroff side of the theory is that any quantum feature in the environment of the brain would undergo wave function collapse (reduction), as a result of interaction with the environment, far too quickly for it to have any influence on neural processes. The wave or superposition form of the quanta is referred to as being quantum coherent. Interaction with the environment results in decoherence otherwise known as wave function collapse. It has been questioned as to how such quantum coherence could avoid rapid decoherence in the conditions of the brain. With reference to this question, a paper by the physicist, Max Tegmark, refuting the Orch-OR model and published in the journal, Physical Review E is widely quoted. Tegmark developed a model for time to decoherence, and from this calculated that microtubule quantum states could exist, but would be sustained for only a femtoseconds (fs) timescale at brain temperatures, far too brief to be relevant to neural processing. A recent paper by Engel et al. in Nature does indicate quantum coherent electrons as being functional in energy transfer within photosynthetic organisms, but the quantum coherence described lasts for 660 fs rather than the 25 milliseconds required by Orch-OR, and this is compatible with Tegmark's calculations. More recent papers involving Guerreshi, G., Cia, J., Popescu, S. and Briegel, H. are looking to improve their model of entanglement in protein, a test which could falsify theories of non-trivial coherence or entanglement in protein.
In their reply to Tegmark's paper, also published in Physical Review E, the physicists Scott Hagan, Jack Tuszynski and Hameroff claimed that Tegmark did not address the Orch-OR model, but instead a model of his own construction. This involved superpositions of quanta separated by 24 nm rather than the much smaller separations stipulated for Orch-OR. As a result, Hameroff's group claimed a decoherence time seven orders of magnitude greater than Tegmark's, but still well short of the 25 ms required if the quantum processing in the theory was to be linked to the 40 Hz gamma synchrony, as Orch-OR suggested. To bridge this gap, the group made a series of proposals. It was supposed that the interiors of neurons could alternate between liquid and gel states. In the gel state, it was further hypothesized that the water electrical dipoles are oriented in the same direction, along the outer edge of the microtubule tubulin subunits. Hameroff et al. proposed that this ordered water could screen any quantum coherence within the tubulin of the microtubules from the environment of the rest of the brain.
Each tubulin also has a tail extending out from the microtubules, which is negatively charged, and therefore attracts positively charged ions. It is suggested that this could provide further screening. Further to this, there was a suggestion that the microtubules could be pumped into a coherent state by biochemical energy. Finally, it is suggested that the configuration of the microtubule lattice might be suitable for quantum error correction, a means of holding together quantum coherence in the face of environmental interaction. In the last decade, some researchers who are sympathetic to Penrose's ideas have proposed an alternative scheme for quantum processing in microtubules based on the interaction of tubulin tails with microtubule-associated proteins, motor proteins and presynaptic scaffold proteins. These proposed alternative processes have the advantage of taking place within Tegmark's time to decoherence.
A number of other criticisms have come to the fore over the years. Papers by Georgiev, D. point to a number of problems with Hameroff's proposals, including a lack of explanation for the probabilistic firing of axonal synapses, an error in the calculated number of the tubulin dimers per cortical neuron, and mismodelling of dendritic lamellar bodies (DLBs) discovered by De Zeeuw et al., which showed that the DLBs are located micrometers away from gap junctions. Further Hameroff's hypothesis that cortical dendrites would be shown to contain mainly 'A' lattice microtubules was experimentally disproved by, which showed that all in vivo microtubules have a 'B' lattice and a seam.
Recently the debate has focused round papers by Reimers et al. and McKemmish et al. and Hameroff's replies to these, which is not regarded as being independently reviewed. The Reimers paper claimed that microtubules could only support 'weak' 8 MHz coherence, but that the Orch-OR proposals required a higher rate of coherence. Hameroff, however, claims that 8 MHz coherence is sufficient to support the Orch-OR proposal. McKemmish et al. makes two claims; firstly that aromatic molecules cannot switch states because they are delocalised. Hameroff, however, claims that he is referring to the behaviour of two or more electron clouds; secondly McKemmish shows that changes in tubulin conformation driven by GTP conversion would result in a prohibitive energy requirement. Against this, Hameroff claims that all that is required is switching in electron cloud dipole states produced by London forces.
Read more about this topic: Orchestrated Objective Reduction
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