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Is Quantum Mechanics relevant to understanding consciousness?:A review of "Shadows of the Mind" by Roger Penrose
Roger Penrose's book
Shadows of the Mind
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Is Quantum Mechanics Relevant To Understanding
Consciousness?
A Review of
Shadows of the Mind
by Roger Penrose
Stanley A. Klein
Department of Vision Sciences
University of California
Berkeley, CA 94720
USA
klein@adage.berkeley.edu
Copyright (c) Stanley Klein 1995
PSYCHE, 2(3), April 1995
KEYWORDS: consciousness, duality, Libet, metaphysics, Penrose, quantum mechanics, reductionism.
REVIEW OF: Roger Penrose (1994)
Shadows of the Mind
. New York: Oxford University Press. 457 pp.
Price: $US 25 hbk. ISBN 0-19-853978- 9.
1. Introduction
1.1 The present essay explores three issues raised by Penrose in
Shadows of the Mind
(abbreviated
Shadows
from here on): (1) is classical (non-quantum) science incapable of understanding brain
operation?; (2) are long-range quantum effects able to produce measurable changes in neural activity?;
(3) why have so many researchers proposed a strong connection between quantum mechanics and
consciousness? In connection with this third topic, I will argue that although Penrose is probably wrong
about the physics of quantum mechanics being relevant to the (third person) neural correlates of
awareness, the metaphysics of quantum mechanics may be essential to understanding the (first person)
subjective nature of consciousness. In Penrose's approach these two aspects become inseparably
intertwined, adding confusion to an already murky area.
2. The Presumed Deficiencies Of Classical Mechanics
2.1 The first half (Part I) of
Shadows
, titled "Why We Need New Physics to Understand the Mind"
clarifies Penrose's belief that classical, algorithmic neural dynamics is insufficient for understanding the
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Is Quantum Mechanics relevant to understanding consciousness?:A review of "Shadows of the Mind" by Roger Penrose
operation of the brain. I will not examine his main argument, based on Gödel's theorem, here, as it has
been widely discussed elsewhere. Instead I will examine several non-Gödel arguments regarding the
purported inadequacy of classical mechanics for understanding brain operation.
2.2 In his review of
Shadows
, Wilczek (1994) gave Penrose an important challenge. Rather than
searching for a Gödelian feat that humans, but not robots, can do, Wilczek suggests merely looking for
perceptual feats that humans can do more efficiently than robots. As Penrose points out in Section 7.3,
quantum computing can be much faster than classical computing. Perception is a good area to examine
since evolution has been perfecting perception for quite a while, making it an area in which humans
should excel. In my own area of research, visual perception, I am often confronted with counterintuitive
illusions and surprisingly rapid visual effects. However, I have always been able to account for the data
using standard, classical neural models. I have never come across data from any sort of mental
processing (other than non-verifiable ESP effects) that could not, in priciple, be accounted for by clever,
non-quantum, physiologically plausible neural models. While it is true that at the present time robots do
poorly at high level classifications such as face recognition, few researchers would say that future
classical robots will be incapable of doing such tasks well and rapidly.
2.3 At the end of Chapter 7, Penrose cites two studies of Libet and colleagues (see Libet, 1993 for an
excellent summary) as providing evidence for the non-classical nature of consciousness. The studies
were concerned with the timing relationships between brain activity, conscious awareness, and physical
action (sensory stimulation in one set of experiments and motor response in a different set of
experiments). Of these experiments, Penrose says:
we seem to be driven to the conclusion that in any action in which an external stimulus
leads to a consciously controlled response, a time delay of some one and one-half seconds
would seem to be needed before that response can occur. For awareness would not even
take place until half a second has passed; and if that awareness is to be put to use, then the
apparently sluggish machinery of free will would then have to be brought into play, with
perhaps another second's delay.
2.4 Penrose believes (correctly) that a 1.5 second delay like this would contradict our experience. He
interprets this contradiction as providing evidence for quantum processing (p. 388):
...if, in some manifestation of consciousness, classical reasoning about the temporal
ordering of events leads us to a contradictory conclusion, then this is a strong indication
that quantum actions are indeed at work!
2.5 As far as I can tell, however, nothing in Libet's experiments comes close to requiring quantum
explanations. Before invoking exotic explanations it is useful to reconsider Libet's two experiments to
see whether Penrose's estimate of a 1.5 sec response time is valid. (The following analysis has been
developed in collaboration with my colleague Marcus Baldo.)
2.6 Consider first Libet's experiments with skin and cortical stimulation that Penrose takes as evidence
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Is Quantum Mechanics relevant to understanding consciousness?:A review of "Shadows of the Mind" by Roger Penrose
for a 500 msec delay. Libet (1993) is clear that this long delay is only appropriate for stimuli that are
near threshold. He points out (p. 131) that a cortical stimulus of about 100 msec would likely elicit
awareness. The time of the actual awareness event is very difficult to assess (as was pointed out by Ian
Glynn (1990) in his very useful commentary on Penrose's (1989) earlier treatment of Libet), but
psychophysical experiments such as that by Nijhawan (1994) lead me to believe that the time delay
between the sufficient stimulus and conscious awareness is less than 100 msec. I know of no evidence
that would imply the delay between a suprathreshold stimulus and conscious awareness is greater than
100 msec.
2.7 Consider now the evidence that there is a 1-second delay from conscious awareness to motor output.
Penrose cites Libet's experiments with volitional finger flexing, in which the moment of decision was
determined from a subject's report of the position of a rapidly rotating clock hand. According to Libet
(1993, p. 128), however, the subjective awareness of the decision to act occurred only 200 msec before
the motor act with substantial unconscious processing taking place before that. No surprises there.
Furthermore, Libet's use of the rotating clock can overestimate the timing of awareness, as Nijhawan's
(1994) experiments, also using a rotating clock, showed. In any case, Libet's experiments are not
relevant to Penrose's calculation of a 1.5 sec delay: the latter is related to a stimulus-response situation,
whereas Libet's experiments involved volitional motions.
2.8 It would be useful for Penrose, in his response, to clarify his calculation of the 1.5 sec delay, since a
300-400 msec delay seems fully compatible with Libet's data.
2.9 One last comment on Penrose's discussion of the presumed deficiencies of classical mechanics. On
pages 372-373 of
Shadows
, Penrose discusses the global nature of consciousness:
The unity of a single mind can arise, in such a description, only if there is some form of
quantum coherence extending across at least an appreciable part of the entire brain.
2.10 In Penrose's approach the unified feeling of consciousness is associated with long range quantum
coherence, but why can't the required coherence be achieved classically? Classical neural networks with
feedback can produce surprisingly rich, coherent activity. With signals going from one neuron to the
next in a couple of milliseconds (and even faster dendritic processing) it is possible for coherent,
classical activity to spread across the brain on a 10 msec time scale.
3. Penrose's New Physics, Microtubules, And Mind
3.1 Even though we are not driven to quantum mechanics because of an inability of classical mechanics
to deal with neural activity, it is still worth exploring Penrose's quantum ideas. For one thing, there is the
intriguing link between quantum mechanics and the role of the observer. I will deal with Penrose's
quantum ideas in two parts. This section will examine Penrose's ideas on the connection of quantum
physics and brain operation. The next section examines quantum metaphysics and subjective awareness.
3.2 For a brief, elegant introduction to the laws of quantum mechanics one cannot beat Feynman (1985).
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Is Quantum Mechanics relevant to understanding consciousness?:A review of "Shadows of the Mind" by Roger Penrose
There are two steps in quantum calculations. First, one calculates the amplitude for the outcome of an
experiment. This calculation is achieved using the Feynman rules governing the interaction of "particles"
such as electron and quarks with "forces" such as photons and gluons (Feynman, 1985). These rules are
similar to rules governing waves. Second, the square of the wave amplitude gives the probability of
finding a particular outcome of particle locations. A major question is: at what point does the wave
amplitude get reduced (Penrose's R process) to particle probabilities? That is, when does one apply the
second step? Penrose devotes Chapter 6 to a discussion of whether the R process is real and to several
alternative interpretations of R. Penrose believes that the reduction of the wave function is governed by a
quantum gravity effect that occurs at intermediate scales of size between microscopic atoms and
macroscopic brains. I have no hesitation in recommending Chapter 6 for anyone interested in gaining a
deep understanding of the quantum mysteries and the possibility of novel solutions. My problem comes
when he attempts to apply these ideas to the brain in Chapter 7.
3.3 In order to clarify the problems facing Penrose, an example will help. Suppose a neuron is in a
quantum superposition of states A (the neuron fires) and B (the neuron doesn't fire). The superposition
of states can be written as: a|A> + b|B>, where |a|^2 and |b|^2 are the probabilities that the neuron fires
or doesn't fire. For convenience we are using Dirac's notation, |A> to represent the state A. A critical
difference between classical mechanics and quantum mechanics is that in the latter the relative sign (or
phase) of the amplitudes a and b can lead to measurably different outcomes if the overlap between states |
A> and |B> becomes substantial as they evolve in time. The measurable difference is referred to as
quantum interference. In classical mechanics the relative sign (phase) of a and b is not measurable since
only the probabilities |a|^2 and |b|^2 ever enter classical calculations. Thus the question of whether there
are measurable quantum effects that differ from classical predictions is directly connected with the
question of overlap of states. For our example with a neuron's firing I suspect everyone will agree that
the biochemical difference produced by firing is too large to ever allow overlap between the fired and
unfired state. Thus measurable quantum effects would be expected to take place before a neuron fires.
3.4 It will be difficult to find quantum effects in pre-firing neural activity. The big problem facing
quantum superpositions in the brain is that the brain operates at high temperatures (there are constant
thermal vibrations of the proteins and other molecules involved with neural activity) and is made of
floppy material (the neural proteins can undergo an enormously large number of different types of
vibration). Furthermore, it is highly likely that states |A> and |B> will get entangled with different
modes of oscillation of the environment. When that happens it is exceedingly unlikely that states |A>
and |B> will ever develop substantial overlap, so the intrinsically quantum effects (the deviations from
classical predictions) become negligible. The challenge facing Penrose is to convince the reader that
there is much less "floppiness" than appears to be the case.
3.5 To meet this objection, Penrose makes wishful conjectures about properties of microtubules. These
proteins, found in all cells, have properties that could conceivably be useful for computation within
individual neurons (Hameroff & Watt, 1982). But while it may be conceivable that microtubules
maintain quantum coherence within a neuron, it is difficult to imagine how this coherence can be
maintained across neurons. Penrose is aware of this problem when he says (p. 372): "... the quantum
coherence must leap the synaptic barrier between neuron and neuron. It is not much of a globality if it
involves only individual cells!"
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Is Quantum Mechanics relevant to understanding consciousness?:A review of "Shadows of the Mind" by Roger Penrose
3.6 What Penrose's story lacks is an account of how quantum coherence can leap the synaptic barrier.
All he says is that evolution is clever, and maybe it has found a way to achieve this impossible sounding
feat. He discusses how microtubules can alter synaptic strengths in an interesting way, but nowhere is
there any discussion of the nature of synaptic modulations that can be achieved quantum-mechanically
but not classically. The quantum nature of neural activity across the brain must be severely restricted,
since Penrose concedes that neural firing is occurring classically. I kept rereading Sections 7.6
(Microtubules and consciousness) and 7.7 (A model for a mind) in hopes of finding a plausible argument
for how a coherent quantum state could be preserved across the brain. I thought that Penrose might
invoke the .5 cm microwaves that have been associated with microtubules, but he was too smart for that.
He must have felt that invoking microwaves to achieve quantum coherence across neurons would be too
easily disproved. For his hypothesis to have any chance of success, Penrose needs new mechanisms of
neural communications other than the presently known electrochemical mechanisms, but given the
explanatory power of presently-known mechanisms, it is unlikely that neuroscientists will mount a
search for these new and exotic mechanisms soon.
4. Physics vs. Metaphysics
4.1 Given the skepticism I expressed in the preceding section about Penrose's Chapter 7 "Quantum
Theory and the Brain', it might seem that I am skeptical about all aspects of Penrose's attempt to connect
quantum mechanics and consciousness. Not so. I believe that the metaphysical underpinnings of
quantum mechanics are crucial for a deep understanding of consciousness and the connection between
mind and brain. The distinction being made is between science (asking testable questions) and
metaphysics (asking nontestable questions). The metaphysics of quantum mechanics, with its insight
into the role of the observer, may well be relevant to the subjective aspects of how studies of mind can
fit into the scientific worldview.
4.2 Chapters 5 and 6 of
Shadows
provide a wonderful presentation of the paradoxical nature of quantum
mechanics. The framework that Penrose develops in those chapters has direct applications to the
metaphysics of the mind-brain connection. The challenge is to find a satisfactory way to associate the
"observer" of subjective awareness with the "observer" of quantum mechanics (Penrose's R process).
Stapp (1993) and Penrose would like this association to be in the realm of science; Klein (1992, 1993)
has argued that it could be in the realm of metaphysics.
4.3 The big problem in the metaphysics of quantum mechanics is the question of where to place the split
between the observer and the observed. The astonishing finding of von Neumann (1955) is that its
placement is irrelevant to any measured event. The Feynman rules for the world below the split and the
classical rules for the world above the split are so clever that the split is moveable. This is the brilliant
manner in which the quantum duality avoids the difficulties encountered by the previous dualities of
Plato, Descartes and Kant. Previous dualities contained inconsistencies when the two sides were
compared. There are no inconsistencies between the two halves of the quantum duality. Present quantum
theory, with its flexible split placement, allows the neural correlates of awareness to be above the split
(the neural correlates of awareness become the observer) and the remaining (unconscious) neural
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