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Tallis in Wonderland
A Smile At Waterloo Station
Raymond Tallis on the true mystery of memory.
Regular readers of this column will know that despite my background in neuroscience, I am not persuaded that brain activity is a sufficient explanation of any aspect of human consciousness. Notwithstanding the claims of ‘neurophilosophy’, I do not think my feelings, emotions and thoughts are reducible to nerve impulses. What may be less obvious is that I am very grateful to the errors of the neurophilosophers for inadvertently and indirectly making visible the true mystery of consciousness. I felt this gratitude particularly strongly the other day when I was reading about Eric Kandel’s research on one particularly intriguing aspect of consciousness: memory. This was the exquisite work for which he received the 2000 Nobel Prize for Medicine.
Kandel’s studies were carried out using the giant (almost a foot long) sea snail Aplysia. Aplysia has two features which make it attractive to neuroscientists. First, it has relatively few neurons (20,000, compared with the hundreds of billions in your cerebral cortex alone). Second, its neurons are strapping cables of a millimetre or more in diameter, and uniquely identifiable, so it is easy to see what is happening inside them, and, more importantly, inside their connexions, the synapses. The beast has the additional advantage of being ugly and dim, and so it’s unlikely to attract the protection of the Animal Liberation Front or seek legal advice. This last is relevant because of the unkind nature of the conditioning of the response Kandel used for his investigations, which involved a defensive withdrawal reflex. He sprayed the snail with water and then returned it to its shell while at the same time applying an electric shock to its tail. Soon the snail would withdraw into its shell at the slightest disturbance. This is a form of learned behaviour, which Kandel saw as a model for memory.
Because of those giant neurons, Kandel was able to identify the changes that occurred in the electrical and biochemical properties of the neural synapses as the snail learned to be jittery. This, Kandel said, was ‘memory in a dish’. Subsequent research on a variety of animals, such as young chicks, showed that when an animal is trained on a novel task, there are increases in the size and strength of certain synaptic connexions in particular regions of the brain: the connexions enlarge, and the effectiveness of the neurotransmitters within them is increased. As Kandel points out, since there are no fundamental functional or biochemical differences between the nerve cells and synapses of humans and those of a snail, a worm or a fly, this suggests that similar changes underpin your memories and mine. Exposure to events leaves a biochemical imprint on neurons which alters their excitability. Human memory, like that of Aplysia, is stored in the modification of the connectedness between nerve cells.
It is of course a little more complicated with me than with Aplysia, but the principles are supposed to be the same. I remember the smile on your face when we last met at Waterloo Station. My memory is stored in the form of the altered connectivity of the neurons associated with the smile, which makes them likely to fire in response to present cues, prompting me to recall it. The experiments on Aplysia and other animals supposedly show how this rewiring takes place, and hence how memory works.
You might be resistant to this idea, as I am. For all its altruistic commitment to advancing the science of memory, Aplysia does not, so far as I know, have any of the following: a semantic memory of facts – such as that there is a Waterloo Station; explicit episodic memories of events that it locates in the past – like a meeting at Waterloo Station; or autobiographical memories it locates in its own past – the sense that it was I who saw your smile. Nor does it have any explicit sense of time, of the past; even less of that collective past in which we locate our shared history. Nor can one imagine it actively trying to remember past events, racking its meagre allocation of 20,000 neurons to recall the shocks that now make it jittery, any more than one can think of it feeling nostalgic for the time when it had confidence in a benign world free of electric shocks. In short, the altered behaviour of Aplysia has little, perhaps nothing, in common with memory as I understand it.
Neurophilosophers will not be impressed by my objection. The difference between the shock-chastened sea snail and my feeling sad over a meeting that passed so quickly, is simply the difference between 20,000 neurons or a hundred billion; or, more importantly, between the modest number of connexions within Aplysia’s nervous system, and the unimaginably large number of connexions in your brain (said to be of the order of a 100 trillion). Well, I don’t believe that the difference between Kandel’s ‘memory in a dish’ and my actual memory is just a matter of the size of the nervous system or the number or complexity of the neurons in it. Clarifying this difference will enable us to see what is truly mysterious in memory.
A Matter of Memory
Let us return to that smile. It is supposed to be ‘stored’ in the changed state of excitability of a neural circuit resulting from my exposure to it. The state of the circuit is ‘a present propensity to react’, and when it does react, the memory is ‘activated’. That is to say, the memory is a present state of part of my nervous system: a physical state of a physical entity, namely my brain. This state has somehow to be about, or refer to, the smile. Yet it is difficult to understand how physical activity can be ‘about’ something other than itself. We encounter this difficulty in the case of the perception of something even actually present. The perceptual state is about the thing perceived. Outside of the brains of sentient beings, no other physical event has this ‘aboutness’. But understanding this aboutness presents an even greater challenge in the case of physical states that are supposed to correspond to memories: the states have to be about something that is no longer present – that no longer even exists. What’s more, memory in us humans is explicitly memory: it is not simply past experience acting upon us by reverberating in the present. That remembered smile is located by me in the past – indeed, in a past world, which, as John McCrone has put it, is, “a living network of understanding rather than a dormant warehouse of facts.”
Making present something that is past as something past, that is to say, absent, hardly looks like a job that a piece of matter could perform, even a complex electrochemical process in a piece of matter such as a brain. But we need to specify more clearly why not. Material objects are what they are, not what they have been, any more than they are what they will be. Thus a changed synaptic connexion is its present state; it is not also the causes of its present state. Nor is the connection ‘about’ that which caused its changed state or its increased propensity to fire in response to cues. Even less is it about those causes located at a temporal distance from its present state. A paper published in Science last year by Itzhak Fried claiming to solve the problem of memory actually underlines this point. The author found that the same neurons were active in the same way when an individual remembered a scene (actually from The Simpsons) as when they watched it.
So how did people ever imagine that a ‘cerebral deposit’ (to use Henri Bergson’s sardonic phrase) could be about that which caused its altered state? Isn’t it because they smuggled consciousness into their idea of the relationship between the altered synapse and that which caused the alteration, so that they could then imagine that the one could be ‘about’ the other? Once you allow that, then the present state of anything can be a sign of the past events that brought about its present state, and the past can be present. For example, a broken cup can signify to me (a conscious being when I last checked) the unfortunate event that resulted in its unhappy state.
Of course, smuggling in consciousness like this is inadmissible, because the synapses are supposed to supply the consciousness that reaches back in time to the causes of the synapses’ present states. And there is another, more profound reason why the cerebral deposit does not deliver what some neurophysiologists want it to, which goes right to the heart of the nature of the material world and the physicist’s account of its reality – something that this article has been circling round. I am referring to the mystery of tensed time; the mystery of an explicit past, future and present.
That remembered smile is located in the past, so my memory is aware that it reaches across time. In the mind-independent physical world, no event is intrinsically past, present or future: it becomes so only with reference to a conscious, indeed self-conscious, being, who provides the reference point – the now which makes some events past, others future, and yet others present. The temporal depth created by memories, which hold open the distance between that which is here and now and that which is no longer, is a product of consciousness, and is not to be found in the material world. As Einstein wrote in a moving letter at the end of his life, “People like me who believe in physics know that the distinction between past, present and future is only a stubbornly persistent illusion.” I assume that those who think of memory as a material state of a material object – as a cerebral deposit – also believe in physics – in which case they cannot believe that tensed time exists in the brain, or more specifically, in synapses. A material object such as the brain may have a history that results in its being altered, but the previous state, the fact of alteration, or the time interval between the two states, are not present in the altered state. A synapse, like a broken cup, does not contain its previous state, the event that resulted in its being changed, the fact that it has changed, the elapsed time, or anything else containing the sense of its ‘pastness’ which would be necessary if it were the very material of memory. How could someone ever come to believe it could?
Those who imagine that experiments with Aplysia cast light on memory betray the origin of the erroneous belief that memory is inscribed in matter. The belief is based on a slither from memory as you and I understand it, to learning; from learning to altered behaviour; from altered behaviour to altered properties of the organism; and viola! – the materialisation of memory! However, with Einstein’s help, we can see that sincere materialists must acknowledge that they have no explanation of memory. Instead of thinking that memories can be located in the brain (or even more outrageously, captured in a dish), they ought to hold, along with Bergson, that “memory [cannot] settle within matter” even though (alas), “materiality begets oblivion.” In short, they should take off their dull materialist blinkers and acknowledge the wonderful mystery of memory.
© Prof. Raymond Tallis 2010
Raymond Tallis is a physician, philosopher, poet and novelist. His new book Michelangelo’s Finger: An Exploration of Everyday Transcendence is published by Atlantic.