Cognitive Computing 2012

Cognitive Computing 2012 Consciousness and Computation:human and machine intelligence 2. FRANCIS CRICK’S ‘ASTONISHING HYPOTHESIS’ Mark Bishop

Crick’s ‘Astonishing Hypothesis’ • Francis Crick (along with Watson and Wilkins) won the Nobel prize for pioneering work on the structure of DNA. • 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. (c) Bishop: Consciousness and Cognition

Why the astonishment? • For many people Crick’s hypothesis on consciousness is not especially astonishing, for others the statement verges on heresy. • 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; • … 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. (c) Bishop: Consciousness and Cognition

The astonishing hypothesis: the scientific search for the soul • Overview of the book • Part 1 – discussion about consciousness, vision and memory • Part 2 – Details about the brain and neuroscience. E.g. • 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. (c) Bishop: Consciousness and Cognition

The visual pathway • Light falls onto photoreceptors on the retina … • … firstly onto the rods/cones; then horizontal cells; bipolar cells; amacrine cells and finally retinal ganglion cells, (RGC). • From the RGC signals propagate via the optic nerve across the chiasm to the Lateral Geniculate Nucleus, (LGN). • The LGN relays signals on to the visual cortex at back of the head with feedback from the visual cortex to the LGN. • The visual cortex: V1, V2 etc. (c) Bishop: Consciousness and Cognition

A simple overview of the brain From Crick, The Astonishing Hypothesis, pp. 83. From Searle, The Mystery of Consciousness, pp. 24. (c) Bishop: Consciousness and Cognition

The features of a neuron • A neuron is a cell like any other: • With a cell membrane, cell body (soma) and cell nucleus. • Neurons differ from other cells in having a long thread like spindle growing out of one side: • the axon • And a bunch of spiny, tree like structures attached to the other: • dendrites. (c) Bishop: Consciousness and Cognition

Signal processing at the neuron • Signals are received at the dendrites, processed in the cell body and a signal is output for processing by other cells via the axon. • Cells are not directly connected to each other. • The point at which the axon of one neuron connects to the dendritic tree of another there is a small (synaptic) gap. • Signal cross the gap by chemical neurotransmitters such as acetylcholine; dopamine; serotonin etc. • 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. (c) Bishop: Consciousness and Cognition

The structure of neurons From Searle, The Mystery of Consciousness, pp. 26. From Searle, The Mystery of Consciousness, pp. 25. (c) Bishop: Consciousness and Cognition

An astonishing event … • Although much simplified, this story of signal processing in populations of brain neurons is the story of our entire mental life … • … for example the suggestion is that our memories are formed by the alteration of synaptic connections (weights) between neurons. (c) Bishop: Consciousness and Cognition

The binding problem • 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) … • 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? • 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. (c) Bishop: Consciousness and Cognition

On the location of consciousness • “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*.” • 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” • “… 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). (c) Bishop: Consciousness and Cognition

(i) Crick’s processing postulate • The thalamus is the driving force in the realisation of consciousness, (ibid. pp. 249). • With each level of visual processing coordinated by a single thalamic region. (c) Bishop: Consciousness and Cognition

(ii) The reverberatory circuit postulate • Consciousness and short term memory need the activity of ‘reverberatory’ [oscillating] circuits to maintain them. • 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. (c) Bishop: Consciousness and Cognition

Other reverberatory circuits? • 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 … • … 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? • Crick suggests that the Pulvinar nucleus might be such a site … • But also notes that, ‘experimental evidence indicates that its [Pulvinar] projections to the higher visual areas are not strong’. (c) Bishop: Consciousness and Cognition

Searle on Crick • Searle argues that Crick makes three fundamental philosophical mistakes: • (1) Crick misunderstands the problem of qualia; • (2) Crick is inconsistent with his account of the reduction of consciousness to and from neural firings; • (3) Crick is unclear in the logical structure of the account he purports to offer. (c) Bishop: Consciousness and Cognition

(1) The problem of qualia • 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”. • 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?” • A puzzle that famously leads Chalmers to postulate a fundamental, ‘explanatory gap’ (c) Bishop: Consciousness and Cognition

(2) Two forms of reduction • Eliminative reduction: • Eliminate the reduced phenomena by showing that it is really something else • 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! • Causal reduction: • In another sense we might explain a phenomena by reduction but not eliminate it • 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)! (c) Bishop: Consciousness and Cognition

An eliminative account of consciousness ?? • 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). • 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’… • Although, confusingly, Jackson’s argument could equally also be deemed an ontological argument as it is about being: ‘how things are in the world’.. • Thus the subjective feel of pain is ontologically distinct from the neural firings that cause it. (c) Bishop: Consciousness and Cognition

There’s something about Mary • Mary is Frank Jackson’s colour blind ‘perfect neuroscientist’. • 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. • Mary acquired this knowledge whilst living only in a black and white room. • When the door to her prison opens and she sees the ineffable red of a rose. • Does she then learn/experience something new? If so it is inescapable that her previous knowledge was incomplete. • But she had all the physical information. Ergo there is more to have than that, and Physicalism is FALSE. (c) Bishop: Consciousness and Cognition

(3) Logical structure of Crick’s argument: the neural correlates of consciousness • Thus, although Crick appears to move towards an eliminative explanation of consciousness he actually offers a correlation; • As Crick prefers to say that he is merely looking for the ‘neural correlates’ of consciousness. • But a correlation is a relationship between two different things and hence is inconsistent with Crick’s eliminativism; • … as there should only be one thing in the explanation of consciousness – via neural firings! • And of themselves, correlations do not explain phenomena • There is a correlation between thunder and lightning, but on its own the correlation doesn’t explain either phenomena. (c) Bishop: Consciousness and Cognition

Conclusion • 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 … • … but is it an adequate explanation of consciousness? After all: • What is special about 40Hz? • How does it work? • What laws link such neural firings to phenomenal experience? (c) Bishop: Consciousness and Cognition

Cognitive Computing 2012