Is the human brain, at the levels directly relevant for analysis of cognition, best modeled as a classical or quantum system?
(For instance, a baseball in some sense needs to be modeled as a quantum system -- in the sense that the way its molecules hold together can be described only using quantum not classical physics; but classical physics can be used to explain the normally relevant aspects of its macroscopic behavior. So at the levels directly relevant for analysis of a baseball game, a baseball is best modeled as a classical system. OTOH, at the levels directly relevant for analysis of the electromagnetic behavior of a Superconducting Quantum Interference Device (SQUID) -- a small but macroscopic device, used in magnetoencephalography machines, and demonstrating macroscopic quantum coherence in its magnetic field -- the SQUID is best modeled as a quantum system. Classical physics models just won't explain why the SQUID, a device you can hold and pinch between your fingers (though it only works when supercooled, which would freeze your fingers!), makes MEG machines work.)
Current brain theory indicates that for understanding its role in giving rise to the mind, the brain is most effectively modeled as a classical system (i.e. the brain is more like a baseball than a SQUID) ... but of course current brain theory could be incomplete.
(Even if the brain is a macroscopic quantum system, this of course doesn't prove that quantum dynamics are necessary for intelligence or consciousness or anything like that. Those are bigger and deeper questions, and I've argued in the past that sufficiently complex "classical" systems might need to be treated using quantum logic ... but this gets into a lot of deep issues that I don't want to digress onto here.)
Stuart Hameroff is one of the more vocal proponents of the "quantum brain" idea, and he has a new paper reporting a new theory in this direction, arguing that dendro-dendritic synapses are mediated via macroscopic quantum dynamics, thus posing a quantum neural net that operates in complex coordination with the classical neural net formed by axonal-dendritic synapses.
I don't have a strong opinion on that particular theory of Hameroff's. I look forward to discussing it with him at the Toward a Science of Consciousness conference in Hong Kong next month.
But I was struck by one of the references at the end of his paper, a Nature paper entitled
To quote part of the abstract:
Here we extend previous two-dimensional electronic spectroscopy investigations of the FMO bacteriochlorophyll complex, and obtain direct evidence for remarkably long-lived electronic quantum coherence playing an important part in energy transfer processes within this system. The quantum coherence manifests itself in characteristic, directly observable quantum beating signals among the excitons within the Chlorobium tepidum FMO complex at 77 K. This wavelike characteristic of the energy transfer within the photosynthetic complex can explain its extreme efficiency, in that it allows the complexes to sample vast areas of phase space to find the most efficient path.
In the comments to an earlier edit of this blog post, someone pointed out this more recent paper
Quantum Zeno Effect Underpinning the Radical-Ion-Pair Mechanism of Avian Magnetoreception
whose abstract says
The intricate biochemical processes underlying avian magnetoreception, the sensory ability of migratory birds to navigate using earths magnetic field, have been narrowed down to spin-dependent recombination of radical-ion pairs to be found in avian species retinal proteins. The avian magnetic field detection is governed by the interplay between magnetic interactions of the radicals unpaired electrons and the radicals recombination dynamics. Critical to this mechanism is the long lifetime of the radical-pair spin coherence, so that the weak geomagnetic field will have a chance to signal its presence. It is here shown that a fundamental quantum phenomenon, the quantum Zeno effect, is at the basis of the radical-ion-pair magnetoreception mechanism. The quantum Zeno effect naturally leads to long spin coherence lifetimes, without any constraints on the systems physical parameters, ensuring the robustness of this sensory mechanism. Basic experimental observations regarding avian magnetic sensitivity are seamlessly derived. These include the magnetic sensitivity functional window and the heading error of oriented bird ensembles, which so far evaded theoretical justification. The findings presented here could be highly relevant to similar mechanisms at work in photosynthetic reactions. They also trigger fundamental questions about the evolutionary mechanisms that enabled avian species to make optimal use of quantum measurement laws.
This of course is even more intriguing than the green sulphur bacteria stuff, because it has to do with perception in an intelligent macroscopic animal.
Hameroff's point in citing the paper on green sulphur bacteria (and it's a good one) seems to be: if long-lived quantum coherence can play an important role in photosynthesis, couldn't it also play a role in the brain somehow ... e.g. maybe via dendro-dendritic synaptic gap junctions?
The extrapolation from these other results to neuroscience is speculative, sure.... But this kind of result does make the possibility of quantum coherence impacting human cognition seem a bit less fanciful.
After all, I often recall that in the late 90's all the neuroscientists I talked to told me there was no neurogenesis nor synaptogenesis in adult mammals. Oops. Now they've got new data and changed their mind. My point isn't that quantum coherence is related to neuro or synapto genesis (though, who knows...), but rather that neuroscientists -- simultaneously with displaying the usual humility of biologists regarding the complexity of the systems they're studying -- have a long-standing habit of assuming the concept-set underlying their current understanding is much more adequate than it really is.
Our ignorance of the brain is why my own AI work is not based on trying to closely model the brain. Of course, it's possible that intelligence is fundamentally based on some freaky neuroquantum phenomenon, so that all digital-computer AI work is doomed by some intrinsic limitations ... but I doubt it. My own guess is that, even if the brain does involve macroscopic quantum coherence in some interesting sense, one can still make transhumanly intelligent systems using digital computers. And of course, if this doesn't work -- or if these transhumanly intelligent systems turn out to lack some crucial aspect of self-awareness as the quantum-consciousness advocates argue -- then we can always add some funky quantum computing chips into our AGI server farm!