This paper points out that photosynthetic complexes are adapted to capture light, and put its energy into long-term storage. This process has normally been described in classical terms, and quantum coherence has been to a good extent ignored in the traditional analysis. However, the possibility of quantum coherence has been predicted, and in this paper the authors describe evidence for long-lived quantum coherence being involved in energy transfer within photosynthetic systems. The wavelike process is thought to account for the efficiency of the sytem, because it allows the sampling of large areas of phase space, in order to find the most efficient path, or to transfering energy to the area in the lowest energy state.
The Engel et al experiment involved electronic spectroscopy to observe the evolution of electronic coherence. Quantum beating was found to last for 660 fs, which was much more than the 250 fs estimated for conventional models. Conventional models had assumed that quantum coherence would be rapidly destroyed, and had therfore not factored it into their models of photosynthetic systems.
By contrast, the authors conclude that long-lived quantum coherence must play an active role in photosynthetic systems. A quantum coherent system allows sampling in order to direct energy to the lowest energy state. The system is viewed as performing a quantum computation, in which it senses many states simultaneously and from these selects the correct answer. This is seen as analogous to Grover's algorithm, allowing both the discovery of the lowest energy state and the transfer of coherence. This is more efficent than any classical
search engine.
Protein is seen as providing the structure in which coherence can be preserved and at the same time modulating the coherence as a result of the local dielectric environment.
3.)
Taming the Quanta
Martin Plenio, Imperial College London
Lecture to the Royal Society, 14 October 2008
Martin Plenio’s lecture of October 14 2008 has provided an interesting footnote to the Engel paper reviewed immediately above. In photosynthesis, the chlorophyll molecule is 98% efficient in transporting energy. Energy is absorbed in the form of light. The molecule supports excitation and oscillation of electrons, and allows the exploration of pathways in the molecule. A classical system would only be 60-70% efficient in transporting energy, but the chlorophyll molecule is 98% efficient. The molecule is at 300 degrees Kelvin or room temperature. Given the high temperature, Plenio thinks that there is likely to be some dephasing of the light quanta, but contrary to the normal view that this would be the end of any quantum processing, he considers that the efficiency of energy transportation could actually be enhanced by some limited dephasing. To illustrate his point, he referred to a well known experiment in which a beam of light is split as it passes through one beam splitter, and is later rejoined at a second beam splitter. In this situation, only one or two possible detectors beyond the second beam splitter will be activated. However, if one part of the split light beam is measured, either of the detector may subsequently be activated. Plenio thinks that the analogous situation of the activation of extra ‘detectors’ within chlorophyll could allow even more paths to be explored and even greater efficiency of energy transport.
4.)
Protons as Neurotransmitters
Nature, March 2008 and Beg et al in Cell
Professor Eric Jorgensen of the University of Utah and his team have demonstrated that free protons can bind to receptor sites on muscles. The relevant study was carried out on nematode worms (c.elegans). The authors studied the defecation cycle of c. elegans.
The muscle contractions involved in this cycle are not directed by neurons but by signalling using protons or hydrogen ions, the positively charged part of the hydrogen atom. Instead of using neurotransmitters, the PBO-4
protein in the intestinal cell membrane facing muscle cells exchanges H+ protons for sodium ions (Na+). The protons thus released act as chemical signals to activate ion channels in muscle cells. The PBO-5
protein expressed in other muscle cells is seen as a likely receptor for protons. The protons are released in pulses, and the fluctuations in the concentration of protons controls the defecation cycle of c. elegans. This is correlated to calcium ion release, and the two processes could be linked by the sodium ion concentration in the cytoplasm.
The proton-sodium ion exchanges found in the intestines of c. elegans are also expressed in the mammalian brain, and Beg et al suggest that these exchanges could be involved in learning and memory. Jorgensen comments that mice that lack proton receptors cannot learn, suggesting that proton signalling may be involved in the learning process. This study is considered likley to open up research into other non-classical forms of neurotransmission. Jorgensen's findings were published in the journal 'Cell'.
5.)
A Neural Mechanism that Randomnises Behaviour
R.H.S. Carpenter
Physiology Laboratory, University of Cambridge
Journal of Consciousness Studies, vol. 6, No. 1, 1999, pp. 13-22
The paper starts by pointing out that the time taken to react voluntarily to stimulis is far longer than can be accounted for by known nervous system processing. The strength of response is shown to rise in proportion to the incoming sensory data, until a critical level, at which action is taken. However, the rate of rise fluctuates randomly from trial to trial. This claim is based on studies of neurons in the frontal eye field, and the time taken between presenting a visual stimulus and making a saccade (an eye movement). The average gap between presentation of the stimulus and the saccade is 200ms. Normal processing in the nervous system is claimed to account for at most one third of this time. The shortest route from the retinal receptors to the eye muscles passes through the superior colliculus and should take only 60ms. However, the colliculus receives input that comes ultimately from the parietal cortex and the frontal eye fields. The control is inhibitory, otherwise the eyes would be constantly darting towards each and every stimulus. The blanket of inhibition has to be lifted for a saccade to be made. The colliculus lacks the information to make useful decisions, because it registers only where things are in space, but not what they are.
The biggest problem is that in a series of trials the response time varies over a surprisingly large range. While the average saccadic latency is 200ms, on some 5% of trials the latency is either less than 150ms or more than 300ms. In the first stage of the latency period, neurons distinguish between a target stimulus and distractors. This takes about the same period of time, about 70ms, whether the eventual latency period is short or long, so the whole of the variability is concentrated in the latter part of the latency period.
The article suggests that this means that the variability is not due to noise in the sensory pathways, but to something actually introduced by the brain. The randomness of the reaction times is seen as a function of deliberate randomisation by neural processes in the brain.
Carpenter says that the underlying process is obscure, although he points out that its is consistent with the Penrose/Hameroff model.
6.)
Consciousness, Neurobiology & Quantum Mechanics: The Case for Connection
Stuart Hameroff
In: The Emerging Physics of Consciousness, Ed. J. Tuszynski Springer, 2006
ISBN-13 978-3-540-23890-4
Hameroff summarises his proposals in the early part of the chapter. He thinks that consciousness arises in the dendrites of neurons that are connected by gap junctions to form 'hyperneurons', and that these are related to the gamma synchrony. Axonal spikes and synapses are seen as making inputs to and receiving outputs from the microtubular process as part of an interactive systems.
In general, Hameroff argues that the emerging evidence of neurobiology has moved in favour of the ORCH OR model over the last decade, not withstanding the continued unpopularity of the theory.
Hameroff classifies all the
mainstream approaches to consciousness as 'classical functionalism'. Functionalism takes no account of what the brain is made of or of anything finer grained than the level of neuron-to-neuron connections. It beleives that these connections could be copied in another material such as silicon, and that the resulting construct would be conscious.
However, Hameroff argues that although axonal spikes and synaptic connections play a key role in information processing in the brain, they may not be the main currency of consciousness. He argues that quantum processing in microtubules within the dendrites and gap junctions between dendrites are the basis of consciousness.
The main case against quantum processing in the brain has always been the view that it would decohere faster than the time taken for any useful biological process. Hameroff accepts that this is in principle a valid argument. However, he claims that a gelatinous non-liquid ordered state can arise in the neuronal interior, and provide screening for microtubules against the rest of the environment.
A further objection to quantum processing was that even if it arose in one neuron, it would difficult to communicate across the brain. This was countered by the suggestion that there could be quantum tunneling at gap junctions between neurons. In recent years, gap junctions have been discovered to be more widespread in the brain than was previously thought. They are also related to the 40 Hz gamma synchrony. This oscillation was at one time promoted by Crick and Koch as the most promising correlate of consciousness. However, the idea fell from favour with
mainstream neuroscience, when it was discovered that the gamma synchrony correlated with dendritic activity rather than axonal spiking.
Backward referral in timeThe recent evidence of neurobiology suggests periodicity in perception and reaction times, notably the gamma synchrony and the alpha and theta waves. The former has sometimes been promoted as a correlate of consciousness and the latter related to visual activity. Hameroff also touches on the famous Libet experiment that demonstrated a 500ms timelag between a stimulus and the perception entering consciousness. The subject is not aware of this time lag as a result of so-called backward referral in time.
Hameroff thinks that the backward referral in time should be taken seriously. The
mainstream has tended to favour something resembling the Dennett 'multiple drafts' concept, which would involve an after the event reconstruction of what had happened. Hameroff prefers an alternative suggestion by Roger Penrose. Penrose proposed that the bain sent information backward through time. This is not as far fetched as it sounds, since the laws of physics, as embodied in the Maxwell and Schrodinger equations, do not preclude backward travel in time. It is just that we do not experience objects or even rays of light arriving from the future, and there is a further argument in the form of the 'grandfather paradox' by which we could travel into the past and kill our own grandfather. This is presented as an argument against macroscopic objects travelling into the past. However, Einstein's equations have been shown to allow for a repeated return to certain points in the past. This is derived from the concept of tilting of light cones, as used in relativity, where a sufficiently strong gravitational field can tilt the light cones so that they comes full circle, making what is called a closed timelike curve. Penrose has suggested that while macroscopic objects may be prevented from going back in time, quantum states might be able to make a 500ms journey into the past. However, it does not appear to preclude a 500ms adjustment within the brain.
Learning & MemoryHameroff points out that changes in dendrites can lead to increased synaptic activity. This is basic to ideas about learning, memory and neural correlates of consciousness. The changes in dendrites involve the number and arrangement of receptors and the arrangement of dendritic spines and dendrite to dendrite connections. Axon potentials or spikes have been assumed to be the main basis of consciousness, but Hammerof suggests that there could be other candidates. Electrodes implanted into the brain detect mainly the activity of dendritic gap junctions plus inhibitory chemical synapses. Thus detected synchrony derives from dendrites rather than axonal spikes.
The main function of dendrites is seen to be the handling of input signal into the neuron which may eventually result in an axon spike. However, this is not the whole story, since many cortical neurons have dendrites but no axons. Here dendrites interact with other dendrites. Also there can be extensive dendritic activity with no spikes. The evidence suggests that there are complex logic functions in the dendrites, and these may oscillate over a wide area, while remaining below the axon spiking threshold. Many post-synaptic receptors send signals into the dendrite cytoskeleton.
Gamma SynchronyGamma synchronies, in the 30-70Hz range, have aroused interest as possible correlates of consciousness. Gray and Singer (1) found coherent gamma oscillations in the brain that were dependent on visual stimulation. It was suggested that this synchrony could solve the binding problem, which is the problem of how the different inputs into the brain are bound together into a single conscious experience. It was suggested that the synchrony reflected the activity of a relevant assembly of neurons. Varela (2) noted that synchrony operated whenever the processing of spatially separated parts of the brain were brought together in consciousness. Gamma synchrony has been demonstrated across cortical areas, hemispheres and the sensory/motor modalities. The synchrony is involved in a range of brain activities including perception of sound, REM dream sleep, attention, working memory, face recognition and somatic perception. Also gamma decreases during general anesthesia and returns on waking from this. Hameroff regards gamma synchrony as the best overall correlate of consciousness.
He further addresses the question of how the gamma synchrony is mediated. There is coherence over large areas of the brain sometimes including multiple cortical areas and both hemispheres of the brain, with zero or near zero phase lag. If the synchrony was based on the axon/synapse system a considerable lag would be expected.
Hameroff points to gap junctions as an alternative to synapses for connections between neurons. Neurons that are connected by gap junctions depolarise synchronously. Gap junctions play a more important role in the adult brain than was previously supposed. Numerous studies (3) show that gap junctions mediate the gamma synchrony. A neuron may have many gap junction connections, but not all of them are necessarily open at the same time. The opening and closing of the junctions may be regulated by the microtubules. Hameroff suggests that cells connected by gap junctions may in fact constitute a cell assembly, with the added advantage of snchronous excitation. Cortical inhibitory neurons are particularly studded with gap junctions, possibly connecting each cell to 20 to 50 others (4). The axons of these neurons tend to form inhibitory GABA chemical synapses on the dendrites of other interneurons.
Screening of MicrotubulesHameroff moves on to discuss the role of the cytoskeleton, which is seen to determine the structure, growth and function of neurons. Actin is the main constituent of dendritic spines and is present throughout the neuronal interior. Actin can depolymerise into a dense network, and when this happens the interior of the cell is converted from an aqueous solution into a gelatinous state. Furthermore, when this happens the whole of the cytoskeleton forms a negatively charged matrix around which water molecules are bound into an ordered state (5). It is particularly remarked that the neurotransmitter glutamate binding to NMDA and AMPA receptors cause gel states in actin spines (6). Cytoskeletal functions include microtubules and actin enabling the growth of axons and dendrites, and transport of molecules that maintain and regulate synapses.
DendritesThe cytoskeleton of the dendrites is distinctive from both non-neuronal cells and the axons of neuronal cells. The microtubules as opposed to the continous ones found in axons and have mixed polarity. This appears a sub-optimal arrangement from a normal structural point of view, and it is suggested that in conjunction with microtubule associated proteins (MAPs) this arrangement may be optimal for information processing. These microtubule/MAP arrangements are connected to synaptic receptors on the dendrite membrane by a variety of calcium and sodium influxes, actin and other inputs (7). Alterations in the microtubule/MAPs network in the dendrites correlate with the arrangement of dendrite synapatic receptors (8). Studies (9) demonstrate that the cytoskeleton is also involved in signal transmission. It is suggested that the microtubule lattice could be more than just convenient wiring. The lattice is well designed to represent and process information. The microtubule is thus comprised of pieces of tubulin capable of representing bits of information. Tubulin switches between two conformations. It is suggested that tubulin conformational states could interact with with neighbouring tubulin by means of dipole interactions. The dipole-coupled conformation for each tubulin could be determined by the six surrounding tubulins.
Hameroff describes
protein conformation as a delicate balance between contervailing forces. Proteins are chains of amino acids that fold into three dimensional conformations. Folding is driven by van der Waals forces between hydrophobic amino acid groups. These groups can form hydrophobic pockets in some proteins. These pockets are critcal to the folding and regulation of
protein. Amino acid side groups in these pockets interact by van der Waals forces. Nonpolar atoms and molecules can have instantaneous dipoles.
Hameroff discusses the process of anesthesia that erases consciousness, but leaves many non-conscious functions intact. Anesthetic gas molecules are soluble in a lipid-like hydrophobic environment. Such areas are present in the brain in the lipid regions of cell membranes and in hydrophobic pockets within proteins. It is suggested that anesthetic gas molecules interact with amino acid groups via London forces, altering the normal action of London forces on the conformation of
protein.
Hameroff further looks at quantum information processing. Quantum superpositions, where the quantum waves represent multiple possibilities for the state of a particle, are known to persist until quanta are either measured or naturally interact with the rest of the environment. Penrose suggests that this wave function collapse is a real event. He sees superposition as a separation in the underlying spacetime geometry. Each quanta is embedded in a bit of space, and as superpositions grow a blister or separation appears in spacetime. If the separation between superpositions grows to more than the Planck length collapse occurs. The normal quantum wave collapse is seen as an entirely random choice of the state of a quantum particle, from amongst the various superpositions of states. However, these collapses involve interaction with the environment.
Penrose suggests that a quanta, which does not interact with the environment will undergo objective reduction (OR) when the separation between superpositions begins to exceed the Planck length. He also suggests that while the normal collapse is totally random, OR is not totally random, but involves a non-computable process. This is suggested because Penrose thinks that the brain manifests a non-computational aspect, and that the wave function collapse is the only place in the universe where such a thing can exist.
Penrose also proposes that OR based quantum computation occurs in the brain. It is important to stress that
quantum computing as such is not expected to generate consciousness. In the quantum computers which many are now trying to develop quantum collapse will occur as a result of measurement or interaction with the environment. It is only in the event of OR that non-computability and consciousness could be brought into play.
Hameroff goes on to look at some of the detail of the theory that he and Penrose developed as to how consciousness could be based in microtubules in the brain. They suggest that quantum compuations take place in microtubules orchestrated by the inputs of synapse via MAPs. Hence the theory is often known as Orch OR, for orchestrated objective reduction. The computations are suggested to persist for 25ms, which would link them to the 40 Hz gamma synchrony. The computations are terminated by objective reduction. It is proposed that in dendrites, the tubulin sub-units of the microtubules interact by dipole coupling so as process information. The tubulin conformation is governed by quantum London forces, so that the tubulins can exist as quantum superpositions of different conformations. In superposition the tubulins would be qbits in a quantum computer, computing by means of non-local entanglement with other tubulin qbits. This entanglement would not just be with tubulins in the same microtubule, but other microtubules in the same dendrite, and in other dendrites connected by gap junctions. It should be remembered that neurons connected by gap junctions can be viewed as a single hyperneuron and the hyperneuron can be seen as a conventional neuron assembly.
Dendritic InteriorsThe dendritic interiors alternate between two states as a result of the polymerisation of actin
protein. In the depolymerised form the interior of the neuron is aqueous and microtubules signal and process information classically. There are synaptic inputs to the microtubules during this phase. When actin polymerises, the interior of the dendrite becomes quasi-solid of gelatinous, and water near to the proteins beomes ordered as a result of the actin gelation. Debye layers of counterions may also shield the microtubules, due to the charged C-termini tails on the tubulins.
This is suggested to make the microtubules sufficiently isolated from the environment for quantum superposition to occur in the tubulins. Coherent pumping of energy and quantum error correction may also help to prevent decoherence. Quantum error correction involves a code that can detect and correct decoherence in a quantum system. The geometry of a quantum computer lattice could be formed so as to be resistant to decoherence. Microtubules are suggested to have a structure which is particularly suitable for error correction. When the superpositions in the microtubules are reduced, the result can be used to regulate axon firing and changes in synapses.
Hameroff claims to refute Tegmark's attempt to disprove the Penrose/Hameroff model. This is significant as Tegmark's criticism of Orch OR has been widely accepted as a satisfactory dismissal of the theory, and responses to Tegmark are habitualy ignored. Tegmark calculated microtubule decoherence time as being 10^-13 seconds, which would certainly be much too short for any neural activity. However, he worked on the basis of his own model for quantum activity in microtubules, which was never proposed by Penrose/Hameroff. For some reason he did not choose to address the Penrose/Hameroff model. This invalidates his particular approach, whatever the truth is about decoherence, but it has not prevented his work from being quoted as an absolutely reliable refutation of Orch OR. Tegmark based his calculation on a 24nm separation of solitons from themselves along the microtubules, whereas Orch OR proposes a superposition separation distance six orders of magnitude smaller.
Capability of Being Tested or Falsified
The ability of a theory to be falsified is the acid test of a scientific theory. The Orch Or model is claimed to stand up well in this respect. In 1998 twenty testable predictions of Orch OR were published. Four of these have actually been validated over the last decade, being, signalling along microtubules (9), correlation of synaptic function and cytoskeleton (10), action of psychoactive drugs on microtubules (11) and gap junction mediation of gamma synchrony (12). Others are currently being tested. The 16 untested predictions are:-
1) Microtubule stabilising drugs could treat brain diseases (13).
2) Laser spectroscopy could demonstrate coherent oscillation in microtubules (14).
3) Correlation of vibrational states of microtubule networks with cell activity.
4) Correlation of cytoskeletal networks with memory and neural behaviour
5) Preponderance of 'A lattice' microtubules in dendrites, as these are more suited to information processing.
6) Demonstration of non-local correlations between tubulins on same and different microtubules.
7) Superconducting devices could detect quantum coherence in microtubules.
8) Coherent photons will be detected from microtubules.
9) Dendritic microtubules surrounded by cross linked actin gels.
10) Cycles of gelation linked to gamma synchrony.
11) Gelation cycles regulated by calcium ions associated with microtubules.
12) Quantum tunnelling across gap junctions.
13) Non-local correlation between tubulins in different neurons.
14) Neural mass involved in cognitive tasks is inversely proportional to preconscious time, as in E=hbar/t.
15) Gap junction connections with retinal glial cells.
16) Testing of objective reduction (OR). Will test if isolated quantum superpositions self-collapse according to E=hbar/t (15).
The theory is 'fundamentalist' in assuming that consciousness has to be a fundamental property of the universe that is accessed by the brain. Penrose's OR relates to the fundamental level of the universe, where at 10^-35 m, the continuity of spacetime breaks down and becomes quantised. This is proposed as the level where non-computable processes and possibly qualia are embedded. The rarity of consciousness in the universe is because only in a structure like the brain is it possible to isolate relatively large superpositions, such as nanograms of tubulin.
7.)