Transcript Further Cognitive Science

Cognitive Computing
2012
Consciousness and Computation:
human and machine intelligence
2. FRANCIS CRICK’S ‘ASTONISHING
HYPOTHESIS’
Mark Bishop
Crick’s ‘Astonishing Hypothesis’
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Francis Crick (along with Watson and Wilkins) won the
Nobel prize for pioneering work on the structure of DNA.
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Crick’s ‘Astonishing Hypothesis’ about consciousness
simply states that:
“You, your joys and your sorrows, your memories and
your ambitions, your sense of personal identity and
free will, are in fact no more than the behaviour of a
vast assembly of nerve cells and their associated
molecules. As Lewis Carroll's Alice might have
phrased, ‘You're nothing but a pack of neurons’. This
hypothesis is so alien to the ideas of most people
today that it can truly be called astonishing.”
Francis Crick, (1994), The Astonishing Hypothesis, the
scientific search for the soul, pp. 3.
Stained glass window in the dining
hall of Caius College, Cambridge,
commemorating Francis Crick and
representing the structure of DNA.
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Why the astonishment?

For many people Crick’s hypothesis on consciousness is not especially
astonishing, for others the statement verges on heresy.
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Ward, K., (2008), Materialism and its Discontents, “I remember the occasion when
materialism first hit the world of Oxford philosophy … I was sitting in one of Gilbert Ryle's
seminars in 1963 when a visiting Australian scholar, David Armstrong, presented a paper
defending a materialist theory of mind. I still remember the sense of shock as this heretical
Australian laid into Ryle's concept of mind and insisted on the need for a purely materialist
account of consciousness”, (Keith Ward, Oxford Regius Professor. of Divinity [1991-2003]).
But even for strict materialists the hypothesis contains an element of
surprise …

… as on one level since classical times the brain has been perceived as the seat
of thinking;
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… but on another level, contra say the heart and the circulatory system, 100
billion neurons is not the most obvious design for a thinking machine.
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The astonishing hypothesis: the
scientific search for the soul
Overview of the book
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Part 1 – discussion about consciousness,
vision and memory
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Part 2 – Details about the brain and
neuroscience. E.g.
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On neurons;
On the visual system and the visual cortex;
On evidence from brain damage;
On neural networks.
Part 3 – possible experimental approaches to
the problem of visual awareness; oscillations
and processing units.
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The visual pathway
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Light falls onto photoreceptors on the retina …
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… firstly onto the rods/cones; then horizontal cells; bipolar
cells; amacrine cells and finally retinal ganglion cells, (RGC).
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From the RGC signals propagate via the optic nerve across the
chiasm to the Lateral Geniculate Nucleus, (LGN).
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The LGN relays signals on to the visual cortex at back of the
head with feedback from the visual cortex to the LGN.
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The visual cortex: V1, V2 etc.
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A simple overview of the brain
From Crick, The Astonishing
Hypothesis, pp. 83.
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From Searle, The Mystery of
Consciousness, pp. 24.
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The features of a neuron
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A neuron is a cell like any other:
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Neurons differ from other cells in having a long
thread like spindle growing out of one side:
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With a cell membrane, cell body (soma) and cell nucleus.
the axon
And a bunch of spiny, tree like structures attached
to the other:
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dendrites.
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Signal processing at the neuron
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Signals are received at the dendrites, processed in the cell body and a
signal is output for processing by other cells via the axon.
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Cells are not directly connected to each other.
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The point at which the axon of one neuron connects to the dendritic tree of
another there is a small (synaptic) gap.
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Signal cross the gap by chemical neurotransmitters such as acetylcholine;
dopamine; serotonin etc.
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A signal along the axon causes the release of a neurotransmitter into the
synaptic cleft.
These chemicals contact receptors at the post synaptic dendritic side of the
synapse, causing ions to to flow in or out of the dendritic side thus altering
the electrical charge there. Hence synapses can be excitory or inhibitory.
If the sum of such electrical charge exceeds a threshold, then the cell will
fire.
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The structure of neurons
From Searle, The Mystery
of Consciousness, pp. 25.
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From Searle, The Mystery of
Consciousness, pp. 26.
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An astonishing event …
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Although much simplified, this story of signal
processing in populations of brain neurons is
the story of our entire mental life …
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… for example the suggestion is that our
memories are formed by the alteration of
synaptic connections (weights) between
neurons.
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The binding problem
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Crick comments that, “on balance it is
hard to believe that our vivid picture of
the world really depends entirely on the
activities of neurons that are so noisy
and so difficult to observe”, (p. 246) …
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For example how does the brain
associate the multiple parts and
different modalities of a sensory
experience together so we can perceive
them all at the same time?
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e.g. The colour, smell, sounds, feelings
etc triggered by seeing a friend in a
crowd of people?
… Crick suggests that coherent
oscillations – around 40Hz - between
groups of neurons found across the
cortex, are the ‘binding mechanism’
that is used.
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On the location of consciousness
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“While the idea that consciousness may be clustered within either
one or multiple areas of the brain may seem preposterous, this
conjecture is certainly much more efficient than each cell carrying
within itself the instructions for consciousness*.”
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Francis Crick, (1994), The Astonishing Hypothesis, the scientific search for the soul.
* Conversely Steven Sevush and also Jonathan Edwards [at a Whitehead
lecture in January 2006] have both – independently - argued that,
“phenomenal consciousness must be a property of an individual cell”
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“… it is proposed that each neuron in the nervous system is independently conscious, with
conscious content corresponding to the spatial pattern of a portion of that neuron's dendritic
electrical activity”, (Sevush, 2005).
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(i) Crick’s processing postulate
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The thalamus is the driving force in the
realisation of consciousness, (ibid. pp.
249).
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With each level of visual processing
coordinated by a single thalamic region.
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(ii) The reverberatory circuit
postulate
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Consciousness and short term memory need
the activity of ‘reverberatory’ [oscillating]
circuits to maintain them.
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In region V1 of the primary visual cortex Crick
suggests that the large number of
interconnections between the thalamus and layer
six of the cortex provide the neural basis for such
reverberatory circuits.
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Other reverberatory circuits?
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Awareness requires the activity of the various cortical areas as
well as the thalamus, however the major visual area of the
thalamus (the Lateral Geniculate Body/Nucleus) projects almost
solely onto V1 …
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… but if interactions between the thalamus and V1 [layer six ] are
so vital to consciousness, then where do connections between
the higher visual areas, e.g. V4 and V5, [layer six] perform
interactions with the thalamus?
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Crick suggests that the Pulvinar nucleus might be such a site …
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But also notes that, ‘experimental evidence indicates that its
[Pulvinar] projections to the higher visual areas are not strong’.
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Searle on Crick
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Searle argues that Crick makes three fundamental
philosophical mistakes:
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(1) Crick misunderstands the problem of qualia;
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(2) Crick is inconsistent with his account of the reduction of
consciousness to and from neural firings;
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(3) Crick is unclear in the logical structure of the account
he purports to offer.
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(1) The problem of qualia
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Crick suggests that the problem of qualia “… springs
from the fact that the redness I perceive so vividly cannot
be precisely communicated to another human being”.
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Francis Crick, (1994), The Astonishing Hypothesis, the scientific search
for the soul, (pp. 9).
When the real philosophical problem of qualia is, “how
does the brain move from electro-chemistry to feeling?”
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A puzzle that famously leads Chalmers to postulate a fundamental,
‘explanatory gap’
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(2) Two forms of reduction
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Eliminative reduction:
 Eliminate the reduced phenomena by showing that it is really something
else
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Causal reduction:
 In another sense we might explain a phenomena by reduction but not
eliminate it
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The sun does not ‘really set’ over the dusky New Cross skyline …
… it only ‘appears to set’, an effect caused by the earth’s rotation as it orbits
around the sun!
The solidity of an object is explained by the behaviour of its component
molecules.
i.e. ‘Solidity’ causally emerges via behaviour of component molecules.
In the book Crick preaches ‘eliminative reduction’ (of consciousness)
while he practises ‘causal emergentism’ (of consciousness)!
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An eliminative account of
consciousness ??
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Searle objects that even if we had a ‘complete, perfect
neuroscience’ we would not eliminate separate talk of
consciousness, (c.f. Frank Jackson’s ‘epistemological argument’
refuting physicalism).
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NB. It is important to note here ‘physicalism’ refers to the ‘epistemological
doctrine’ that ‘all knowledge is knowledge of physical facts’ and not the
metaphysical doctrine that ‘all things are physical things’; hence Jackson’s
argument is known as ‘Epistemological Argument’…
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Although, confusingly, Jackson’s argument could equally also be deemed an
ontological argument as it is about being: ‘how things are in the world’..
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Thus the subjective feel of pain is ontologically distinct from the neural firings that cause it.
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There’s something about Mary
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Mary is Frank Jackson’s colour blind ‘perfect neuroscientist’.
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I.e. Mary has total and correct knowledge (her perfect knowledge of a complete
neuroscience) of what happens to her brain, in any state, under any stimuli.
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Mary acquired this knowledge whilst living only in a black and white
room.
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When the door to her prison opens and she sees the ineffable red of
a rose.
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Does she then learn/experience something new? If so it is
inescapable that her previous knowledge was incomplete.
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But she had all the physical information. Ergo there is more to have
than that, and Physicalism is FALSE.
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(3) Logical structure of Crick’s argument:
the neural correlates of consciousness
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Thus, although Crick appears to move towards an eliminative
explanation of consciousness he actually offers a correlation;
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But a correlation is a relationship between two different things and
hence is inconsistent with Crick’s eliminativism;
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As Crick prefers to say that he is merely looking for the ‘neural
correlates’ of consciousness.
… as there should only be one thing in the explanation of
consciousness – via neural firings!
And of themselves, correlations do not explain phenomena
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There is a correlation between thunder and lightning, but on its own the
correlation doesn’t explain either phenomena.
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Conclusion
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Searle certainly believes that Francis Crick’s (and Christof Koch)
thesis, “that the synchronised firing in the range 40Hz may be
the ‘brain correlate’ of visual consciousness”, is an interesting
idea …
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… but is it an adequate explanation of consciousness? After all:
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What is special about 40Hz?
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How does it work?
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What laws link such neural firings to phenomenal experience?
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