Memory, p.35
Memory, page 35
If I offer you the word hammer and ask you to tell me what ‘hammer’ means, you come up with a workable definition of the thing, without any difficulty, in no time at all. One basis for the definition is the rapid deployment of a number of explicit mental patterns concerning some of the varied aspects of hammer itemised above. Although the memory of separate aspects of our interaction with hammers is kept in separate parts of the brain, in dormant fashion, those different parts are co-ordinated in terms of their circuitries such that the dormant and implicit records can be turned into explicit albeit sketchy images, rapidly and in close temporal proximity. The availability of all those images allows us, in turn, to create a verbal description of the entity and that serves as a base for the definition.
I would like to suggest that the memories for the entities and events that constitute our present autobiography are likely to use the same sort of framework used for the memories we form about any entity or event. What makes those memories special is that they refer to established, invariant facts of our personal histories.
I propose that we store records of our personal experiences in the same distributed manner, in as varied brain regions as are needed to match the variety of our live interactions. Those records are closely co-ordinated by neural connections so that the contents of the records can be recalled and made explicit, as ensembles, rapidly and efficiently.
The critical elements of our autobiography that need to be reliably activated in a nearly permanent fashion are those that correspond to our identity, to our recent experiences, and to the experiences that we anticipate in the future. I propose that those critical elements arise from a continually reactivated network based on convergence zones which are located in the temporal and the frontal higher-order cortices, as well as in subcortical nuclei such as those in the amygdala. The co-ordinated activation of this multisite network is paced by thalamic nuclei, while the holding of the reiterated components for extended periods of time requires the support of prefrontal cortices involved in working memory. In brief, the autobiographical self is a process of co-ordinated activation and display of personal memories, based on a neural network with several sites. The images which represent those memories explicitly are generated and exhibited in several early sensory cortices. Finally, they are held over for as long as they are needed by the process of working memory. They are, in and of themselves, treated as any other objects. They become known to us because they can generate their own pulses of core consciousness.
Another hidden gift of memory opens the way for interpreting our own mental states and, no less importantly, the mental states of others. In the early 1990s I proposed that the brain not only represents, on-line, the states of our body that are associated with emotions and with general physiology but also has the possibility of simulating body states that are not actually happening. I outlined this hypothesis, the as-if-body-loop mechanism of feeling, at a time in which the evidence for its existence was merely circumstantial. Today the evidence for this curious brain device is abundant and we can begin to take stock of its functional significance and of its connection to autobiographical memory.
First, personal memories of past events have the power to reconstitute in the brain’s body-sensing regions (such as the insular cortex) the configuration that our body assumed during a particular kind of emotion in the past. Never mind that the body will not be in that exact state at the moment; the body-sensing regions will behave ‘as-if’ it were, in a most convenient sleight of simulation. Thus memory helps us reconstitute integrated past experiences by commanding the manufacture of a simulated body state which signifies past feelings. This is a most effective device because it saves time and energy (it takes less time and less energy to concoct an ‘as-if’ body state than to replicate a real one in the body), and because it saves on storage space as well (provided we memorise the facts of an event we do not need to store records for the accompanying emotions and feelings; we just simulate them).
So far this is all very sensible and very much in keeping with the usual habits of brain evolution, i.e. save and make simple and make quick, whenever possible. What is less expected is the marvellous use to which the as-if-body-loop mechanism can be put. By connecting our body states, as sensed through the somatosensory system, to the visual and auditory counterparts of those body states, we have developed the possibility of connecting the seen or heard presentations of others to our own felt body states. We can achieve this because we can use the as-if-body-loop mechanism to simulate not just our states but also those of others. By means of those simulations we can connect those simulations with the memories which reveal their significance. In brief, we can place ourselves in someone else’s shoes, feel what it is like, and discover the meaning of that feeling.
The effects of mirror neurons recently studied in non-human primates are grounded on this sort of arrangement, which combines body simulations and specific kinds of associated memory. So here are two gifts of memory to the human mind. They are less well known and heralded than the patently obvious ones. But in the end they are of no small importance to the construction of personhood.
From ERIC R. KANDEL, In Search of Memory (2007)
To be useful, a memory has to be recalled. Memory retrieval depends on the presence of appropriate cues that an animal can associate with its learning experiences. The cues can be external, such as a sensory stimulus in habituation, sensitization, and classical conditioning, or internal, sparked by an idea or an urge. In the Aplysia gill-withdrawal reflex, the cue for memory recall is external: namely, the touch to the siphon that elicits the reflex.fn5 The neurons that retrieve the memory of the stimulus are the same sensory and motor neurons that were activated in the first place. But because the strength and number of synaptic connections between these neurons have been altered by learning, the action potential generated by the sensory stimulus to the siphon ‘reads out’ the new state of the synapse when it arrives at the presynaptic terminals and the recall gives rise to a more powerful response.
In long-term memory, as in short-term memory, the number of changed synaptic connections may be great enough to reconfigure a neural circuit, but this time anatomically. For example, prior to training, a stimulus to a sensory neuron in Aplysia might be strong enough to cause motor neurons leading to the gill to fire action potentials, but not strong enough to cause motor neurons leading to the ink gland to fire action potentials. Training strengthens not only the synapses between the sensory neuron and the motor neurons to the gill but also the synapses between the sensory neuron and the motor neurons to the ink gland. When the sensory neuron is stimulated after training, it retrieves the memory of the enhanced response, which causes both gill and ink motor neurons to fire action potentials and causes inking as well as gill withdrawal to take place. Thus, the form of Aplysia’s behavior is altered. The touch to the siphon elicits not just a change in the magnitude of the behavior – the amplitude of gill withdrawal – but also a change in the animal’s behavioral repertory.
Our studies showing that the brain of Aplysia is physically changed by experience led us to wonder: does experience change the primate brain? Does it change the brains of people?
When I was a medical student in the 1950s, we were taught that the map of the somatosensory cortex discovered by Wade Marshall is fixed and immutable throughout life. We now know that idea is not correct. The map is subject to constant modification on the basis of experience. Two studies in the 1990s were particularly informative in this regard.
First, Michael Merzenich at the University of California, San Francisco discovered that the details of cortical maps vary considerably among individual monkeys. For example, some monkeys have a much more extensive representation of the hand than other monkeys. Merzenich’s initial study did not separate the effects of experience from those of genetic endowment, so it was possible that the differences in representation were genetically determined.
Merzenich then carried out additional experiments to determine the relative contributions of genes and experience. He trained monkeys to obtain food pellets by touching a rotating disk with their three middle fingers. After several months, the area of the cortex devoted to the middle fingers – especially the tips of the fingers used for touching the disk – had expanded greatly. At the same time, the tactile sensitivity of the middle fingers increased. Other studies have shown that training in visual discrimination of color or form also leads to changes in brain anatomy and improved perceptual skills.
Second, Thomas Ebert and his colleagues at the University of Konstanz in Germany compared images of violinists’ and cellists’ brains with images of non-musicians’ brains. Players of stringed instruments use the four fingers of the left hand to modulate the sound of the strings. The fingers of the right hand, which move the bow, are not involved in such highly differentiated movements. Ebert found that the area of the cortex devoted to the fingers of the right hand did not differ in string players and non-musicians, whereas representations of the fingers of the left hand were much more extensive – by as much as five times – in the brains of string players than in those of non-musicians. Furthermore, musicians who began playing the instrument before age thirteen had larger representations of the fingers of their left hand than musicians who began playing after that age.
These dramatic changes in cortical maps as a result of learning extended the anatomical insights that our studies in Aplysia had revealed: the extent to which a body part is represented in the cortex depends on the intensity and complexity of its use. In addition, as Ebert’s study showed, such structural changes in the brain are more readily achieved in the early years of life. Thus, a great musician such as Wolfgang Amadeus Mozart is who he is not simply because he has the right genes (although genes help), but also because he began practising the skills for which he became famous at a time when his brain was more pliable.
Moreover, our results in Aplysia showed that the plasticity of the nervous system – the ability of nerve cells to change the strength and even the number of synapses – is the mechanism underlying learning and long-term memory. As a result, because each human being is brought up in a different environment and has different experiences, the architecture of each person’s brain is unique. Even identical twins with identical genes have different brains because of their different life experiences. Thus, a principle of cell biology that first emerged from the study of a simple snail turned out to be a profound contributor to the biological basis of human individuality.
Our finding that short-term memory results from a functional change and long-term memory from an anatomical change raised even more questions: what is the nature of memory consolidation? Why does it require the synthesis of new protein? To find out, we would have to move into the cell and study its molecular makeup. My colleagues and I were ready for that step.
Memory and Imagination
From Beowulf, c.8th century, translated by Harriet Harvey Wood (2007)
Bid the warriors build a barrow after the bright funeral fire, on the promontory above the sea. It shall serve as a reminder to my people, as it towers high on Hronesnesse, so that in future the seafarers shall call it Beowulf’s barrow as they urge their ships far over the dark waters.
From ROBERT HENRYSON, The Testament of Cresseid (c.1470)
Than upon him scho kest up baith hir Ene
And with ane blenk it come into his thocht,
That he sumtime hir face befoir had sene.
But scho was in sic plye he knew hir nocht,
Yit than hir luik into his mynd it brocht
The sweit visage and amorous blenking
Of fair Cresseid sumtyme his awin darling.
Na wonder was, suppois in mynd that he
Tuik hir visage sa sone, and lo now quhy?
The Idole of ane thing, in cace may be
Sa deip Imprentit in the fantasy
That it deludis the wittis outwardly,
And sa appeiris in forme and lyke estait,
Within the mynd as it was figurait.
Ane spark of lufe than till his hart culd spring
And kendlit all his bodie in ane fyre.
With hait Fewir ane sweit and trimbling
Him tuik, quhill he was reddie to expyre.
To beir his Scheild, his Breist began to tyre
Within ane quhyle he changit mony hew,
And nevertheless not ane ane uther knew.
Scho, she; kest, cast; ene, eyes; blenk, blink; thocht, mind; plye, plight; luik, face; brocht, brought; blenking, expression; sumtyme, once; tuik, noticed; visage, face; quhy, why; Idole, image; in cace, sometimes; fantasy, imagination; wittis, brain; sa, so; figurait, imagined; than, then; till, to; culd, did; kendlit, kindled; hait Fewir, hot fever; sweit, sweat; him tuik, seized him; quhill, till; quhyle, instant; mony, many; hew, colour; ane, one; ane uther, each other.
VOLTAIRE (FRANÇOIS-MARIE AROUET), ‘The Adventure of Memory’, in Romans et Contes (1773), translated by Harriet Harvey Wood (2004)
The thinking part of the human race, which is to say about the hundred thousandth part of it, had believed for a long time, or at least had often said it did, that we had no ideas except those which came to us through our senses, and that memory was the only instrument by means of which we could join two ideas and two words together.
This is why Jupiter, representing Nature, was in love with Mnemosyne, goddess of memory, from the first moment he saw her; and of their marriage were born the nine Muses, who invented all the arts.
This belief, on which all our knowledge is founded, was universally accepted, and even the Sorbonne embraced it from the moment of its conception, even although it was true.
Some time later there came a theoristfn1, half a geometrician and half a dreamer, who argued against the five senses and against memory; and he said to the tiny number of thinking human beings: ‘You have been mistaken up to now; your senses are useless to you, since your ideas were innate in you long before any of your senses could act, and you had all the basic knowledge you needed when you came into the world; you knew everything without ever having perceived anything through your senses; all your ideas, which were born with you, were present to your intelligence, which is called your soul, without the help of memory. Memory is no use for anything.’
The Sorbonne condemned this proposition, not because it was absurd, but because it was new: however, when later an Englishmanfn2 started to prove at some length that there were no innate ideas, that nothing was more essential than the five senses, and that memory was extremely useful in remembering what was received through the five senses, it denounced its own former opinions because they had become those of an Englishman. Consequently it ordered the human race to believe henceforward in innate ideas, and to dismiss belief in the five senses and memory. The human race laughed at the Sorbonne instead of obeying, which put it into such a fury that it wanted to burn a philosopher: for this philosopher had said that it is impossible to have any complete conception of a cheese without having seen and tasted one; and the scoundrel had even dared to advance the idea that men and women would never have been able to work a tapestry if they had not had needles and fingers to thread them with.
The Jesuits joined with the Sorbonne for the first time ever, and the Jansenists, mortal foes of the Jesuits, joined with them temporarily; they summoned to their aid the old ministers who were eminent philosophers; and all of them, before they died, proscribed memory and the five senses, and the author who had spoken well of these six things.
There was a horse who happened to be present when this judgement was pronounced by these gentlemen, although he was not of the same species, and although between him and them there were various differences in matters such as the tail, the voice, the hair and ears; this horse, I say, which had sense as well as senses, spoke of the matter one day to Pegasus in my stable; and Pegasus went with his customary energy to retail the story to the Muses.
The Muses who for the past hundred years had shown particular favour to the country where these events took place, which before that had for a long time been barbaric, were deeply shocked; they were tenderly attached to Memory or Mnemosyne, their mother, to whom all of these nine daughters owe everything they know. The ingratitude of the human race annoyed them. They did not satirise the old ministers, the Jesuits, the Jansenists and the Sorbonne, because satires do no good to anyone, they irritate fools and make them behave more badly than ever. They devised a method of educating them by punishing them. Men had blasphemed against Memory: the Muses took from them this divine gift so that they might learn once and for all what life would be like without its assistance.
Thus it happened that in the middle of the night every brain was dulled, so that the following morning everyone woke without the slightest recollection of the past. Some of the ministers who were in bed with their wives desired, by a remnant of instinct unconnected with memory, to make love to them; the wives, who very rarely had any instinct to embrace their husbands, tartly repulsed their disgusting caresses. The husbands were furious, the wives wept and most households came to blows.
Gentlemen, finding a square hat, made use of it for certain needs which neither memory nor good sense could relieve. The ladies employed their cosmetic pots for the same purpose; the servants, oblivious of the contract which they had made with their employers, invaded their rooms without realising where they were; but, since man is born inquisitive, they opened all the drawers; and, since man instinctively and without the need of memory loves the sparkle of money and gold, they helped themselves to whatever they found to hand. The masters would have liked to shout ‘Robber!’; but since the idea of a robber had deserted their brains, they could not think of the word. Everyone, having forgotten his own language, spoke gibberish. It was far worse than Babel where everyone had instantly invented a new language. The innate instincts of the young jackanapes for pretty women operated so powerfully that these insolent fellows threw themselves carelessly on the first women or girls they came across, whether they were bar-maids or the President’s lady; and they, oblivious now of the dictates of modesty, let them have their way freely.
