Some things you wanted to know about memory but forgot to ask
This essay was written by Tim Bliss and was first published in the 1996 Mill Hill Essays.
How can I dazzle my friends with the extraordinary accuracy and retentiveness of my deplorably inadequate memory? From time to time the front page of the Guardian newspaper carries a curiously dated advertisement, in which a confident male face stares out at the reader with the poignant query ‘IQ of 145 and Can’t Remember?’. The text begins – as it has done for at least twenty years – ‘A famous international publisher reports that there is a simple technique for acquiring a powerful memory’. It must be admitted that this is by no means an unfounded claim. The technique which the famous publisher will reveal upon application was already venerable when a book on the subject appeared in the first century BC. The method works by associating the items to be remembered with specific locations in a familiar building such as your house, school or workplace. And it is remarkably efficient, as the following demonstration should rapidly convince you. Ask someone to read aloud a list of twenty familiar or famous names, with an interval of a few seconds between them. An hour later, most people will be able to recall twelve to fifteen of the names on the list. But using the famous publisher’s technique, more usually credited to the Greek poet Simonides, you will have no difficulty recalling every name on the list an hour or even a day later, and what’s more, you will be able to recite the names in order forwards or backwards. This is how it’s done. As the names are called out, imagine walking around your house, placing the first person in the kitchen, for example, engaged in some faintly ridiculous, and therefore easy to remember activity; then move out of the kitchen into the dining room where the second and third persons, a maiden aunt, perhaps, and Sir Isaac Newton, are sitting at the table engaged in a vigorous debate on who possesses the superior recipe for Tarte Tatin; on into the hall where the fourth person is hanging up a dripping raincoat, and so on. Try it – if you fail to retrieve all twenty names on demand, apply to the publisher for a full refund.
This kind of memory trick or mnemotechnic – the word was coined by Frances Yates in her history of mnemonics, “The Art of Memory”, accords with our common sense understanding of how memory works in practice. Memories are triggered by similarities and parallels. In extreme cases, a single experience can trigger the recovery of the lost memories of a lifetime. A recent case provides a dramatic example. In April 1992, a twenty-four year old Italian salesman, was involved in a high-speed car crash. He walked away from the accident with apparently nothing worse than a badly cut lip. Taken to hospital for treatment, he appeared alert and cooperative. A couple of hours later, however, he became agitated and frightened, complaining that he couldn’t remember anything. In a matter of minutes, the report states, he had turned totally blank about himself. He had no idea who he was, and failed to recognize his parents, or his girlfriend with whom he had been living for two years. Later, in what must be one of the oddest identity parades ever staged, he failed to identify her among a group of three girls, though he professed himself more than satisfied when told which one was ‘his’. Over the next few days, he re-acquired an emotional familiarity with his family, friends, and with his girlfriend although he could not remember any specific episodes about them. His loss of memory for past events, known as ‘retrograde amnesia’, extended to recently introduced words such as ‘Aids’ and ‘perestroika’ and to technical innovations like mobile telephones and fax machines. He showed no signs of inability to form new memories, and was able to absorb from his parents, friends and girlfriend a good deal of information about his past, but without recognizing or re-experiencing these reported events. Then, exactly a month after the accident, while he was playing tennis, he misplayed the ball in a certain way, and was suddenly struck by the awareness that he had made the same mistake in a tournament years before. This single memory triggered the recall of other details of that tournament, and then other memories began to pour back ‘as if I had turned a tap on and let the water run’. In contrast to the passive learning of past events, these newly restored memories were as vivid as the originals.
We can learn a good deal about the way in which memories are stored in the brain from this strange case. New memories it seems, at least for a time, must be stored in a different place from old memories. Moreover, the young Italian’s amnesia for words and skills was less severe than his loss of memory for autobiographical events, suggesting that different sorts of information are stored in different memory systems. So the question arises, how many sorts of memory are there?
The first distinction to be made is between short-term or working memory, and long-term memory. Short-term memory can be thought of as a scratch pad to store information which is only needed for a limited time. It is short-term memory that has failed you when you forget the telephone number you have just been told, or where you left the car in the car park, or where you put your glasses. Short-term memory can be disrupted without affecting long-term memory, as, for example, following concussion or after general anaesthesia, both of which result in loss of memory for events occuring in the hour or so before the event. Until fairly recently it was assumed that information passed to long-term memory through short-term memory. But observations on patients with brains damaged in a way which causes loss of short term memory without affecting the ability to form new long-term memories, suggests that this view is wrong, and that the brain employs separate pathways for short and long-term memory. Information routed into short-term memory stops there, and has no opportunity to proceed into the long-term memory store. Information destined for long-term memory necessarily passes through a short-term phase, and this short-term phase of long-term memory is stored in a part of the brain called the hippocampus, from which, in a process called consolidation which may take several years to complete, it passes to long-term memory stored elsewhere in the brain. Long-term memory can itself be subdivided into two types, procedural and declarative memory. Procedural memory includes habits and skills, for example knowing how to drive a car. Declarative memory is further subdivided into memory for what we know, termed semantic memory, and memory for events, termed episodic memory. So, remembering that the M6 motorway passes west of Manchester is semantic memory, while remembering what happened during your driving test is episodic memory. These different forms of memory reside in different parts of the brain, and show differing susceptibilities to disruption. Procedural memories are the most durable, episodic memories the easiest to lose.
The distinction between these different types of memory is strikingly demonstrated by an American patient known to scientists throughout the world only as HM. In 1953 at the age of twenty-seven he had an operation in which part of his brain, including the hippocampus, was removed in an attempt to cure his severe epilepsy. While his epilepsy improved, the loss of the hippocampus and neighbouring regions resulted in an immediate and devastating impairment of his memory for events. The most dramatic symptom was an inability to form new memories. HM’s memory span is a few minutes at most, so that he lives now, as he has done since 1953, in a perpetual present. In his words “At this moment everything looks clear to me, but what happened just before? That’s what worries me. It’s like waking from a dream; I just don’t remember”. On the other hand, his memory of how to do things is relatively less affected. He can learn new physical tasks. In one such task, HM had to trace the outline of a star-shaped figure without crossing the borders, while only able to view the figure and his hand through a mirror. This is a task which most normal people find difficult at first. To the surprise of his doctors, over three days of training, HM learnt this task as well and as rapidly as normal subjects. On the second and third days, he began at the level he had achieved the previous day, even though he was quite unaware that he had practised the task before. There could hardly be a clearer demonstration of the separation between different types of memory.
Are HM’s problems due to a failure to store memories, or a failure to retrieve memories from store? Tests have shown that he and similar patients can remember some words or pictures previously presented over a three day period, when hints are given about the correct answer. So some information must have been stored over at least this length of time, though, again, at the conscious level amnesic patients have no awareness of doing the tests. Information about emotional episodes is also stored. One psychologist concealed a drawing pin in his palm before shaking hands with HM. Next day the same psychologist returned to see him again. HM was not aware that he had ever met the man before; nevertheless, he refused to shake hands with him.
HM does retain some information about events which took place before his operation, but he can no longer update his autobiography. The study of HM’s memory loss, and that of other patients with damage to the hippocampus, has led, as we have seen, to the conclusion that this part of the brain is required to form new episodic memories, and that in the normal person, these memories are eventually transferred from the hippocampus to other regions of the brain.
The hippocampus is one of the regions of the brain most susceptible to attack in Alzheimer’s disease. A protein known as beta-amyloid forms tiny aggregates or plaques in the spaces between nerve cells, and tangled skeins of a different protein build up within the cells, eventually killing them. We do not yet know why this happens, although a gene has been identified in Alzheimer’s patients that appears to be an indicator for the disease. Research in this area is being vigorously pursued but meanwhile, is there any treatment which can delay the deterioration of memory in Alzheimer’s disease, and in old age generally? It has been observed that a group of nerve cells deep inside the brain which connect with the hippocampus and other areas are destroyed in Alzheimer’s disease. Nerves communicate with each other by using substances called neurotransmitters and for this particular group of nerves the neurotransmitter is called acetylcholine An approach being tried in the United States for treatment of Alzheimer’s patients is to use drugs to boost the amount of acetylcholine in the brain. This strategy is similar to that used with some success in the treatment of Parkinson’s disease, where nerve cells in the brain which supply a different neurotransmitter, known as dopamine, die for unknown reasons. Administering a drug which is converted to dopamine in the brain relieves the symptoms of Parkinson’s disease. In the case of Alzheimer’s disease, the success of the drugs tried so far is modest, and there are unpleasant side effects. If there is currently no successful drug treatment for Alzheimer’s disease, even less can it be claimed that there are drugs which help with the memory problems that occur in normal ageing, or improve normal memory in the young. There has recently been wide coverage in the mass-media of experiments showing that nicotine, which acts in a similar way to acetylcholine may help faltering brain function. Time will tell whether this is good news for smokers, or just a puff-of-the-moment.
What is the most efficient way to learn? Last minute cramming may be the only hope left for passing an exam, but it is not an efficient route into the long-term memory store. Learning little but learning often, is a better method of acquiring information. An experiment carried out by the Medical Research Council’s Applied Psychology Unit in Cambridge some years ago illustrates the point. When postcodes were first introduced, the Post Office had to train a large number of letter sorters in keyboard skills. The Unit was commissioned to find the most efficient schedule for a given number of hours in training. Several different schedules were compared, ranging from two two-hour sessions in a single day, to one hour sessions spread over four days. Learning per training hour was more rapid and memory retention was better in those trained on the one-hour-a-day schedule. However, although the one-hour-a-day sorters were learning more per training hour, they were, in terms of real time, learning more slowly, and felt they were being left behind in the typing stakes by their four-hours-a-day blockbuster colleagues. The scientists’ research was not wholly welcomed by the sorters.
What about stress and learning? Mild stress levels improve while high stress levels inhibit learning. This is true in both humans and animals. Events which are in some way emotionally arousing or stressful are stored more efficiently. In a recent experiment, volunteers were read one of two stories, which had the same beginning – a boy and his mother leaving home to visit the boy’s father, a surgeon at a local hospital – and the same ending – the boy leaves the hospital, with his parents. The middle part of the story was different. In one case the boy’s father shows him around the operating theatre and talks to him about his work. In the other the boy is run over on the way to the hospital, and emergency surgery has to be carried out in his father’s theatre. The volunteers were tested twenty-four hours later. Recall of the beginning and end of the story was the same for both groups, but recall of the middle section was better in the group that had been read the emotionally loaded version. If, however, the volunteers were given a drug which blocks the effect of emotional stress before being read the story the difference between the two groups disappeared; the drug, in other words, had selectively blocked memory of the emotionally arousing episodes.
This summer an American research team published the first brain-scan study of false or illusory memory. Volunteers were read a list of twenty words which were all related to a similar topic (eg thread, pin, eye, sewing, sharp, point, prick, thimble, pain ), and then, during the next hour, a number of other word lists were shown on a screen, and the volunteers were asked to indicate whether or not these words were in the original list. One list contained words selected from the original list (thread, pin, sharp etc); another list contained words which were not on the original list but were related (eg, button, needle, cotton); and a third list contained randomly chosen common words of similar length. In each case, there were twelve words on the list, and the volunteer had to press a key, as the words were presented one by one, to indicate whether the word was recognized from the original list. Brain scans were obtained during the presentation of each of these lists.
When words from the original list were shown, the volunteers recognized on average two out of every three words. When the list of related words, which were not on the original list, was presented the volunteers falsely ‘recognized’ over half the words as being from the original list. The frequency with which words from the random list were ‘recognized’ was much less. Differences in the brain scans were apparent after they were processed to remove signals which occurred in all three cases – for instance in the areas involved in reading and in pressing the response key. Two conclusions emerged from this study: first, both true recognition, as well as false or illusory recognition of previously heard words leads to an increase of blood flow in the region of the brain implicated in memory for events. Second, a distinctly different region which was known from previous research to be involved in understanding speech, was activated during true but not during false recognition. What is going on here? It seems that however vivid the false memory might be, it lacks some of the additional attributes that are attached to genuine memories but stored somewhere else; in this case, words that were listened to during the presentation of the original list set up activity in areas of the brain dealing with sounds and these areas only responded when words from the original list were read.
The kind of memory task used in these experiments is of course highly artificial, and it is not obvious how these findings will contribute to the debate about the validity or otherwise of recovered memories of childhood abuse. All that we can say is that brain-scanning technology may eventually allow genuine memories to be distinguished from illusory memories.
The fact is our memories are distressingly fallible; we remember things that never happened, and forget completely things that did happen. One researcher asked college students about an event in their childhood, which, as their parents confirmed, had never occurred. One example was an overnight stay in hospital for an ear infection when they were five years old. After repeated questioning, one fifth of the students began to evolve a memory of this event, usually based on memories of unrelated actual events which could be appropriated into this false hospital-based history. Another psychologist kept a personal diary for six years, noting the events of each day. At the end of this period, he re-read the events he had recorded in the first year, and found he could remember nothing at all about one third of them. So episodic memories which certainly enter the long-term memory stream can be wholly forgotten, either because they were never consolidated into long-term memory or because they are buried so deep in the long-term memory store that they are effectively irretrievable.
A final thought. There is a difference between what is possible given the brain one has, and what is not possible. You may not possess the mathematical prowess to solve Fermat’s last theorem, or the linguistic flexibility to solve the “Times” crossword in under ten minutes. For me, as probably for you, the thing’s impossible. But other things are at least imaginable. For instance, implantation of memories you would like to possess of events that might well have happened, but didn’t. So some time in the spring of the next millenium, summon up the synapse surgeons, and with scarcely credible skill, and for a wholly incredible fee, they will change the linkages of the appropriate set of synapses, sew you up and creep away. When you come round you will possess the memories you wanted. Your false memory will be just as real to you as a real memory. And why shouldn’t it be; thanks to the surgeons it is an identical brain state. Still, to be on the safe side, stay away from the brain scanners.