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microtubules, coherence, consciousnessStuart Hameroff 's recent letter to psyche-dre: coherence Date: Mon, 15 May 1995
From: Stuart Hameroff
To: PSYCHE-D
In addition to the Goedel/noncomputability issues, there are other
puzzling features of consciousness for which quantum theory offers
possible explanations: binding/unitary sense, transition from
pre-conscious processing to consciousness, simultaneity and flow of
time, non-determinism, and Chalmers' (1994; 1996) "hard problem" of
what exactly consciousness IS - the subjective nature of experience.
Stan Klein claims that for quantum theory to be relevant to
consciousness "what is needed for the brain are superpositions of a
neuron firing |F> AND a neuron not firing |NF>". And indeed, the notion
of all-or-none, membrane mediated neural firing being the ONLY
significant level of information signaling and processing in the brain
IS the accepted conventional wisdom in cognitive science, neuroscience
and philosophy. Each neuron, however, is so incredibly complex that
this notion should be recognized as a gross oversimplification, if not
delusion. Future developments in technology will bear this out.
So the relevance of quantum theory to consciousness may be at a more
fundamental level: but where? Perhaps at the level of ordered water
throughout the cell, layered at surfaces of membranes, organelles, and
in particular the cytoskeleton (Jibu et al, 1994; and references
therein). Or perhaps at the level of proteins, the most versatile and
intelligent of biomolecules. Michael Conrad (1992; 1994) for example,
describes quantum coherence among dipoles and hydrogen bonds throughout
each protein coupled to its conformation (and thereby function).
Frohlich's (1968; 1970; 1975) model predicts that assemblies of
proteins may be bioenergetically pumped into quantum coherence (akin to
a Bose-Einstein condensate, as Scott Hagan has explained; cf. Marshall,
1989).
The point is that if quantum theory is relevant to consciousness, it
is superposition at a level of biostructures much smaller than neurons
that are important. Superpositions at the level of proteins and
surrounding water, in which the protein conformational shape and
associated function are coupled to quantum events, are the most likely
to be relevant. For example in the case of a protein capable of
switching between two different conformational states A and B, there
may also be a superposed quantum state of both A AND B. After a time T,
the protein will "reduce" to either A or B. If such proteins are
configured in a lattice so that coherence occurs among the superposed
states, "quantum computing" (e.g. Benioff,1982; Deutsch and Josza,1992;
Feynman, 1986) may occur whose outputs regulate neural firing. Issues
of isolation and bioenergetics required for biomolecular quantum
coherence are tricky, but feasible. (Frohlich coherence is the subject
of a weeklong conference in Prague, September 11-15, 1995.)
Microtubules, geometric lattices of proteins, seem particularly suited
for such a role. They have the following characteristics: 1) high
prevalence, 2) functional importance (for example regulating neural
connectivity and synaptic function), 3) periodic, crystal-like lattice
dipole structure with long-range order, 4) ability to be transiently
isolated from external interaction/observation, 5) functionally coupled
to quantum-level events, 6) hollow, cylindrical (possible waveguide),
and 7) suitable for information processing. Membranes, membrane
proteins, synapses, DNA and other types of structures have some, but
not all, of these characteristics. Cytoskeletal microtubules are the
most likely (but not necessarily the only) biomolecular quantum devices
in neurons.
Roger Penrose and I have completed two recent papers (Hameroff and
Penrose, 1996a; 1996b) which describe "orchestrated objective reduction
(Orch OR)" of quantum coherence in microtubules as a formal model of
consciousness. A brief summary follows:
We envisage that conformational states of microtubule subunits
(tubulins) are coupled to internal quantum events (dipoles,
delocalizable electrons in hydrophobic pockets, hydrogen bonds), and
cooperatively interact (compute) with other tubulins. We further assume
that macroscopic coherent superposition of quantum-coupled tubulin
conformational states occurs throughout significant brain volumes (for
example by a Frohlich type Bose-Einstein condensate, and/or quantum
optical coherence as described by Jibu , Yasue, Hagan et al,1994; etc)
and provides the global binding essential to consciousness.
We equate the emergence and quantum computing phase of microtubule
quantum coherence with pre-conscious processing which grows (for up to
500 milliseconds - Libet et al, 1979) until the mass-energy difference
among the separated states of tubulins reaches a threshold related to
quantum gravity. At that point, self-collapse, or "objective
reduction" ("OR" - Penrose, 1994) occurs. We thus relate consciousness
to the (self) collapse process itself (in agreement, for example, with
Stapp, 1993). Cascades of self-collapses give rise to the "stream" of
consciousness, and provide a "flow" of time.
According to the arguments for OR put forth in Penrose (1994),
superposed states each have their own space-time geometries (see
Shadows of the Mind, p. 338). When the degree of coherent mass-energy
difference leads to sufficient separation of space-time geometry, the
system must choose and decay (reduce, collapse) to a single universe
state [avoiding the need for multiple universes as discussed by, for
example, Everett (1957) and Wheeler (1957)]. In this way, a transient
superposition of slightly differing space-time geometries persists
until an abrupt quantum to classical reduction occurs. If as various
philosophers claim (cf. Chalmers, 1994; 1996) the nature of conscious
experience is somehow embedded in the nature of reality,
self-selections in fundamental space-time geometry may address the
"hard problem" of consciousness.
Unlike the random, "subjective reduction" (SR, or R) of standard
quantum theory caused by observation or environmental entanglement, the
OR we propose in microtubules is a self-collapse and it results in
particular patterns of microtubule-tubulin conformational ("eigen-")
states that regulate neuronal activities including synaptic functions.
Possibilities and probabilities for post-reduction tubulin states are
influenced by factors including attachments of microtubule-associated
proteins (MAPs) acting as "nodes" which tune and "orchestrate" the
quantum oscillations. We thus term the particular self-tuning OR
process in microtubules "orchestrated objective reduction" ("Orch OR"),
and calculate an estimate for the number of tubulins (and neurons)
whose coherence for relevant time periods (e.g. 500 milliseconds) will
elicit Orch OR. We calculate an estimate of 10^9 tubulins, equivalent
to a range of from hundreds to ten thousand neurons, as the number
required for a 500 msec conscious event. A "more intense" conscious
event, for example one which emerges in only 50 msec, would require
10^10 tubulins. Any Orch OR would "bind" varying time scale processes,
so that a particular conscious event can include various contents
emerging over differing time scales (for example responding to an
immediate situation, and recalling an overdue bill).
In providing a connection among 1) pre-conscious to conscious
transition, 2) fundamental space-time notions (thus potentially
addressing the "hard problem"), 3) non-computability, 4) non-
determinism, and 5) binding of various (time scale and spatial)
reductions into an instantaneous event ("conscious now"), we believe
Orch OR in brain microtubules is the most specific and plausible model
for consciousness yet proposed.
Stuart Hameroff
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