Memory and Memory Disorders

Memory and memory disorders

Topics covered:

  • Introduction
  • Ways of understanding memory
    • Length of storage
    • Type of information to be remembered
    • The stages involved in remembering
    • Recall and recognition
    • Explicit and implicit memory
    • Retrograde and anterograde memory
  • Conditions that give rise to memory problems, and typical presentations
  • Less common manifestations of memory disorders
  • Management and remediation of memory difficulties
    • Environmental adaptations Improving learning in memory-impaired people
    • Compensating for memory loss
    • Managing anxiety and other emotional sequelae resulting from impaired cognition
  • Conclusions 
  • References

Introduction

A young man with head injuries described his memory functioning in the following words: ‘My memory is like a tape on a tape recorder with large chunks erased or of poor quality'. Like this young man, people often talk about memory as though it were one particular skill or single function. We might hear someone say, ‘He has a photographic memory' or ‘My memory is dreadful'. In fact, memory comprises a number of subskills, subsystems, or subfunctions working together. The number of these subdivisions, and their roles, depend on the model or classification system used to interpret or explain memory functioning.

One influential model proposed by Baddeley and Hitch (1) subdivides memory into three main categories depending on the length of time information is stored. We can also divide memory into a number of systems for remembering different types of information that can be labelled as semantic, episodic, or procedural. Yet another way to conceptualize memory is by considering the different stages involved in remembering: namely encoding, storing, and retrieving. Other ways of regarding memory include subdividing the kinds of remembering required into recall or recognition; or by demonstrating that something has been remembered in terms of whether it is explicit or implicit; or, in the case of lost memories, whether they date from before or after a neuropsychological insult, that is whether there is retrograde or anterograde amnesia. We shall consider all these subsystems and categories in more detail in this article. 

Although dementia is probably the biggest cause of organic memory impairment, memory problems are common after many other neurological insults including traumatic head injury, encephalitis, vascular disorders, chronic alcohol abuse, temporal lobe epilepsy, cerebral tumour, and anoxia.

Whatever the cause, memory-impaired people tend to share certain characteristics in that they do not lose personal identity, their immediate memory functioning is normal or nearly normal, they have problems remembering after a delay or distraction, they have difficulty learning new information, they usually recall things that happened some time before the insult better than things that happened a short time before, and they typically do not forget how to do things they learned well before the insult such as reading, swimming, or riding a bicycle. Of course, there are exceptions to this general pattern particularly with certain syndromes such as semantic dementia. We shall discuss typical amnesic patients together with some of the less common memory disorders as this article progresses.

Although few people working in memory rehabilitation would claim to be able to restore memory functioning in someone whose problems result from an organic cause, there is nevertheless a considerable amount that can be done to help memory-impaired people and their families or carers. We can organize the environment to make it easier for people to cope without adequate memory functioning; we can help memory-impaired people to learn more efficiently; we can teach them to compensate for their impairments; and we can reduce the anxiety or other emotional sequelae resulting from impaired cognition. Again, these rehabilitative approaches will be discussed more fully later in this chapter.

Ways of understanding memory

Length of storage

The working memory model of Baddeley and Hitch (1) subdivides memory into three main types depending both on time-based and conceptual differences.

Sensory memory

The first system, sensory memory, is a brief and rather literal trace that results from a visual, auditory, or other sensory event, probably lasting no longer than a quarter of a second. This is the system we use to make sense of moving pictures (visual sensory memory) or language (auditory sensory memory). Most people with damage to this system would present with perceptual or language disorders and we would not normally think of them as having memory problems.

Working memory

The second system, working memory, is considered to have two main components or functions. The first of these is short-term or immediate memory, which lasts for several seconds. This period of time can be extended to several minutes if the person is rehearsing or concentrating on the particular information. Unlike sensory memory, information in working memory has already undergone substantial cognitive analysis, so it is typically represented in meaningful chunks such as words or numbers. We use this system when looking up a new telephone number and holding on to it long enough to dial.

The second component of working memory is a central executive that can be conceived of as an organizer, controller, or allocator of resources. This component enables us to both drive a car and talk to our passenger at the same time. Sufficient resources are allocated to each of these tasks, and if a demanding or unusual situation occurs on the road we stop talking while all our resources are required to deal with the unexpected situation.

Long-term memory

The third system in the Baddeley and Hitch model (1) is long-term memory, which encodes information in a reasonably robust form and can last for decades. Although there are differences in memory for things that happened 10 minutes ago and things that happened 10 years ago, the differences are less clear-cut than those between sensory (quarter of a second) and immediate (a few seconds) memory systems. Nevertheless, because long-term memory means different things to different people, the following terms can be used to reduce ambiguity:

  1. Delayed memory refers to memory for information presented in the last few minutes.
  2. Recent memory refers to knowledge accumulated in the last few days or weeks.
  3. Remote memory refers to knowledge accumulated over several years.

All the systems described so far are connected with retrospective memory, that is remembering information or events that have already occurred. Frequently, however, we want to remember to do something in the future, such as take our medicine, water the plants, or make a telephone call. The system activated for remembering to do something is known as prospective memory. It is significant that many of the complaints of memory-impaired people refer to failures in prospective memory.

Type of information to be remembered

In 1972 Tulving (2) produced an influential paper distinguishing two types of memory: semantic and episodic. Semantic memory is memory for our knowledge about the world, for example remembering that Dublin is the capital of Eire, or that a fox has a bushy tail. Semantic memory is also concerned with our knowledge of social customs, the meanings of words, the colours and textures of objects, and how things smell. Most memory-impaired patients do not forget this kind of information, although they may have difficulty adding to their store of semantic knowledge. Amnesic patients are often unable, for example, to learn new words that enter the vocabulary after their neurological insult. Thus CW, a patient of a psychiatrist, cannot understand the terms ‘AIDS' or ‘e-mail', or ‘mad cow disease', all of which came into widespread usage after 1985 when CW developed herpes simplex encephalitis.

Episodic memory, on the other hand, represents what most of us would think of as memory, in that it refers to a specific episode that has been experienced and can be recalled. Thus remembering what you ate for dinner last night, when you last phoned your mother, or what you read a few minutes ago are all examples of episodic memory. This system is frequently damaged in people with organic memory impairment and episodic memory deficits are perhaps the most noticeable characteristic of the amnesic syndrome.

A third system that operates differently from either semantic or episodic memory is procedural memory, the system used for learning skills such as riding a bicycle or learning to type. People get better with practice and can demonstrate the skill even though they may not remember how they learned to ride a bicycle or type. For example a young amnesic patient, successfully learned to type even though he had no conscious recollection of learning. In his words, ‘Practical skills developed without me being aware of how this came about. I could do things without being able to explain how'. Procedural memory is typically normal or nearly normal in amnesic patients.

The stages involved in remembering

Typically there are three stages involved in remembering: encoding, storage, and retrieval. Encoding refers to the registration stage or getting the information into memory. Storage refers to the maintenance of information in the memory store, and retrieval refers to the stage of extracting or recalling the information when it is required. After a neurological insult to the brain each of these stages can be affected. The following are some suggestions for improving encoding, storage, and retrieval: 

  1. Simplify the information you give to a memory-impaired person.
  2. Reduce the amount of information supplied at any one time.
  3. Ensure that there is minimal distraction.
  4. Make sure the information is understood—by asking the person to repeat it in his/her own words.
  5. Encourage the person to link or associate information with material that is already known.
  6. Try to ensure processing at a deeper level—by encouraging the person to ask questions.
  7. Use the ‘little and often' rule.
  8. Make sure learning occurs in different contexts to avoid context specificity and enhance generalization.

Recall and recognition

Recall and recognition are two of the main ways we remember information. Recall involves actively finding the information to be remembered. If I asked someone to summarize what they had read in this chapter so far they would demonstrate this by recalling the information. In some situations, however, we do not need to recall the information but to recognize it. Most of us at some time have been unable to tell someone how to find a particular street but can nevertheless take ourselves there with no trouble. We recognize which turns to make and when but cannot actively recall the route. Most memory-impaired people find recall harder than recognition, although both systems are usually affected. Some people might have difficulty with both verbal and visual information, while others might have problems in only one of these modalities.

Explicit and implicit memory

In many situations we need to consciously recall information we have received. For example, if I asked someone where they went for their summer holidays last year, and they could tell me, they would be using explicit memory as they could consciously recall the information wanted. If, on the other hand, asked someone when and where they learned to ride a bicycle, and the steps by which they gained expertise, they would probably find this difficult. They might demonstrate how to ride a bicycle without much trouble, that is they would have implicit memory of this skill, they would remember how to do it even if they were unable to explain it with any great ease or remember when and how they learned the skill. Like procedural memory, implicit memory is usually intact or relatively intact in people with organic memory impairment.

Retrograde and anterograde memory

One of the questions frequently asked by relatives of memory-impaired people is, ‘Why can she/he remember what happened several years ago but not what happened yesterday?' The short answer is that old memories are stored differently in the brain from new memories. Although information acquired before a neurological insult may be forgotten, this is usually for a specific time period—ranging from a few minutes for some head-injured people to several decades for some people with Korsakoff' syndrome or herpes simplex viral encephalitis. Memory loss dating from before the insult is known as retrograde amnesia. This form of amnesia is usually less of a problem and less handicapping for the memory-impaired person than anterograde amnesia, which refers to memory difficulties dating from the time of the neurological insult (although see Kapur (3) for a review of retrograde amnesia).

Conditions that give rise to memory problems, and typical presentations

A variety of brain pathologies can give rise to severe memory impairment, the most common being the following: 

  1. degenerative disorders (particularly Alzheimer' disease and Huntington' disease);
  2. chronic alcohol abuse giving rise to Korsakoff' syndrome;
  3. traumatic head injury;
  4. temporal lobe surgery;
  5. encephalitis;
  6. cerebral vascular disorders (including subarachnoid haemorrhage resulting from ruptured aneurysms);
  7. anoxic brain damage (following, for example, myocardial infarction, carbon monoxide poisoning, or respiratory arrest); 
  8. cerebral tumours.

Clients referred to rehabilitation centres for memory-therapy rehabilitation are most likely to have sustained a severe traumatic head injury, a cerebral vascular accident (CVA), herpes simplex encephalitis, or anoxic brain damage. It should also be remembered that these conditions are not mutually exclusive. For example a psychiatrist once saw a man who had a CVA while driving, thus sustaining some brain damage from the stroke; he then crashed his car because of the CVA and sustained further brain damage from a head injury caused by the crash; following this he stopped breathing for a while and appeared to sustain further damage from the anoxia; then on top of everything else a haematoma developed and the man required surgery to remove the blood clot. Thus he sustained damage from four separate causes. He went on to respond reasonably well to rehabilitation and, although never able to return to work as a university lecturer, he became a secretary of his local Headway Group (The National Head Injuries Association).

Whatever the cause of the organic memory impairment, certain characteristics tend to be seen in survivors. People with a classic amnesic syndrome show an anterograde amnesia, that is they have great difficulty learning and remembering most kinds of new information. Immediate memory, however, is normal when this is assessed by forward digit span or the recency effect in free recall. There is usually a period of retrograde amnesia. This gap or period of retrograde amnesia is very variable in length and may range from a few minutes to decades. Previously acquired semantic knowledge and implicit memory (remembering without awareness or conscious recollection) are typically intact in amnesic subjects. Other cognitive skills, apart from memory, are normal or nearly normal. As the majority of patients with severe memory disorders present with additional cognitive problems such as attention deficits, word-finding problems, or slowed information processing, those with aclassic amnesic syndrome are relatively rare.

Nevertheless, people with a ‘pure' amnesic syndrome and people with more widespread cognitive deficits tend to share certain characteristics. In both cases immediate memory is reasonably normal; there is difficulty remembering after a delay or distraction; new learning is difficult, and there is a tendency to remember things that happened a long time before the accident or illness better than things that happened a short time before. People with organic amnesia never seem to lose memory for personal identity, unlike those with a functional amnesia following, say, an emotional trauma. Despite the rather exaggerated interest in functional amnesia by the media, organic amnesia is far more commonly encountered in clinical practice. In some cases it is not easy to distinguish between the two, and indeed some people have memory problems resulting from both brain injury and from an overlying or functional memory disorder, possibly caused by a need for sympathy or some other secondary gain. Kopelman (6) believes there is a continuum between totally organic amnesia and totally functional problems rather than two orthogonal dimensions.

Less common manifestations of memory disorders

Despite the typical picture of organic memory impairment described above, other manifestations are possible and are encountered every now and again. Returning for a moment to the working memory model of Baddeley and Hitch (1) referred to earlier, one can find patients with deficits in the short-term or immediate (that is to say a few seconds) memory system. Baddeley and Hitch subdivide this system into several ‘slave' systems that aid the central executive in its role as co-ordinator of temporary storage systems. Two of these slave systems have been studied in detail. One is the phonological loop that utilizes subvocal speech and is involved in many short-term verbal memory tasks. The second system is the visuospatial sketchpad—a temporary system used in creating and manipulating visual images. Patients with phonological loop or verbal short-term memory difficulties have been reported, (7) and visuospatial sketchpad or visual short-term memory deficits have also been reported. (8) Wilson et al. (9) describe a sculptress with an autoimmune disorder, systemic lupus erythematosus, that caused her to have an impaired visual short-term memory (VSTM) together with image generation problems. This dramatically affected her sculpting style; prior to the episode causing the VSTM difficulties she produced sculptures that were full of detail, while afterwards the sculptures were abstract and completely lacking in detail. Her style changed because she could not hold images in her mind to see where the details should go. If she tried looking in a mirror or using a model she ‘lost' the visual memory in the brief period between looking at the model and looking at the material with which she was working.

Patients with semantic memory impairments are another group of people who show yet another different pattern of characteristics. Semantic memory is the system we use to store knowledge about the world. Not only knowledge such as the meaning of the word ‘happy' or the name of the world' largest ocean, but knowledge about what things look like, sound like, smell like, or feel like and knowledge about social customs such as when to shake hands. Damage to the semantic memory store (or impaired access to this store) may be caused by brain injury. Warrington, (10) for example, suggests that visual object agnosia (the failure to recognize objects despite adequate eyesight, language, and naming) is due to a deficit of the visual semantic memory system. Furthermore, Warrington and Shallice (11) demonstrated that there are category-specific deficits so that some patients lose the ability to recognize living things but are still able to recognize non-living things. Hillis and Caramazza (12) describe the reverse, that is subjects who show greater knowledge of living things than of manufactured objects. Patients with semantic memory deficits are likely to have problems recognizing objects in the real world, problems expressing themselves, and may be considered intellectually disabled because of the errors they make.

In recent years there has been considerable interest in the syndrome known as semantic dementia, a term coined by Snowden et al., (13) and studied in some detail by Hodges and Patterson.(14) The essential characteristics of semantic dementia are as follows:

  1. selective impairment of semantic memory causing severe anomia, impaired single-word comprehension (both spoken and written), reduced generation of exemplars on category-fluency tests, and an impoverished fund of general knowledge;
  2. relative sparing of other components of speech production, notably syntax and phonology; 
  3. unimpaired perceptual skills and non-verbal problem-solving abilities;
  4. relatively well-preserved episodic memory.

Patients with semantic dementia show a progressive deterioration of the semantic memory store associated with damage to the temporal neocortex. However, semantic memory impairments may also be seen in patients with non-progressive conditions. Wilson (15) describes four patients, two of whom had sustained a severe head injury and two with herpes simplex viral encephalitis ( HSVE). In many ways Wilson' patients were similar to those reported by Hodges and Patterson, (14) although younger and with more serious episodic memory deficits (particularly the two patients with HSVE).

In this section we have looked at a few of the more atypical memory disorders. For those readers interested in retrograde without anterograde amnesia see Kapur, and for those wanting to know how post-traumatic amnesia differs from the amnesic syndrome and from chronic memory impairment see Wilson et al. (16) (3)

Management and remediation of memory difficulties

Although there is no known cure for memory impairment there are a number of ways we can help memory-impaired people and their families or carers. The main methods are: environmental adaptations, improving learning, compensating for memory problems, and managing anxiety or other emotional sequelae resulting from impaired cognition.

Environmental adaptations

One of the simplest ways to help people with memory impairment is to arrange the environment so that they rely less on memory. Examples include using written labels or drawings for cupboards in the kitchen or bedroom as reminders of where things are kept; positioning objects so that they cannot be missed or forgotten (for instance tying a front-door key to a belt); or painting the toilet door a distinctive colour so that it is easier to find. Sometimes changing the wording of our questions or comments can reduce problems. For example, CW, a former musician, became very densely amnesic following encephalitis. He frequently thinks he has just woken up and says, ‘This is the first time I've been awake. I don't remember you coming into this room but now I'm awake' (or words to that effect). Sympathizing with him, or offering explanations, seems to increase his agitation and causes escalation of the number of repetitions he makes about awakening. One partial solution is to distract him by introducing another topic of conversation or asking him a question about music. Such a ploy can also be viewed as an environmental adaptation, although in this case it is the verbal rather than the physical environment that is being modified.

For people with severe intellectual deficits, or progressive deterioration, or extremely dense amnesia, environmental adaptations may be the best we can offer to enable them and their families or carers to cope, and to reduce some of the frustration and confusion associated with their conditions.

Improving learning in memory-impaired people

One of the greatest handicaps for memory-impaired people is their inability to learn new information. In recent years a number of studies have been carried out to investigate errorless learning in memory rehabilitation. Errorless learning is a teaching strategy whereby people are prevented, as far as possible, from making mistakes while they are learning a new skill or acquiring new information. Instead of teaching by demonstration, which may involve the learner in trial and error, the experimenter or teacher presents the correct information or procedure in ways that minimize the possibility of erroneous responses.

There are two theoretical backgrounds influencing errorless learning work with cognitively impaired people. The first is errorless discrimination learning from behavioural psychology, first developed by Terrace (17) in his work with pigeons, and later used with mentally retarded (learning disabled) people. (18) The second influence is from studies of implicit learning in amnesic subjects, (19) showing that people with amnesia can learn some information normally although they may have no conscious recollection (explicit recall) that they have previously engaged in the task.

Building on these two strands of research we posed the question, ‘Do amnesic subjects learn better when prevented from making errors during the learning process?' In one group study and several single-case studies (20) it was demonstrated that people with severe memory disorders learn more successfully with an errorless learning strategy. Others have adapted this strategy with non-progressive amnesic patients, (21) and recently we have used errorless learning procedures with patients who have Alzheimer' disease. (22) All patients benefited to a greater or lesser degree and were able to learn some useful everyday information.

Compensating for memory loss

Much of the work in memory rehabilitation involves teaching people to compensate for their impairments by employing aids such as diaries, tape recorders, organizers, computers, and other similar items. These external memory aids are probably the most useful devices for helping memory-impaired people and they are likely to be used more by them in the long run. (23) Despite their value, it is not always easy to persuade patients to use compensatory strategies. Some feel it is cheating and believe they should not rely on such aids, others feel such devices will reduce their chances of natural recovery occurring, and others simply forget to use them or may use them in a disorganized manner. After all, remembering to use a compensation is in itself a memory task. Despite these difficulties, some memory-impaired people use compensatory aids and strategies very efficiently. Kime et al. (24) describe a young amnesic woman who was able to get back to paid employment once she had been taught to use a comprehensive system of external aids. Wilson et al.(25) describe the 10-year natural history of a compensatory memory system devised by a young man who became amnesic following the rupture of a posterior cerebral artery aneurysm. Using results from a long-term follow-up study, Wilson and Watson (23) made some predictions as to which people are more likely to use compensatory aids effectively and which are not. Age, absence of additional cognitive deficits, and scoring above floor level on a test of everyday memory were all predictors of independence and the use of aids.

Other work looking at how to help memory-impaired people compensate involves the use or modification of new technology. (26) One fairly recent development that appears to help people with a wide range of conditions and degrees of memory impairment is Neuro-Page®, (27) which is a simple and portable paging system with a screen that can be attached to a belt. It utilizes an arrangement of microcomputers linked to a conventional computer memory and, by telephone, to a paging company to produce a programmable messaging system. The scheduling of reminders or cues for each individual is entered into the computer and from then on no further human interfacing is required. On the appropriate date and time NeuroPage® accesses the user' data files and transmits the appropriate information by modem to a terminal where the reminder is converted and transmitted to the receiver corresponding to the particular user. The reminder is graphically displayed on the screen of the receiver. NeuroPage® is easy to use and avoids many of the problems inherent in other external aids. It is highly portable, has an audible alarm that can be adapted to vibrate if required, together with an accompanying explanatory message, and, rather than being embarrassing to use, it conveys a certain amount of prestige. In a pilot study, (28) the average number of target behaviours achieved by 15 people during a 2- to 6-week baseline was 37 per cent. Once the pager was provided (for a 12-week period) the average number of targets achieved rose to over 85 per cent. Preliminary analysis of the first 38 clients in a larger study of 200 people, many of whom have very severe impairments, suggests similar (if less dramatic) results, ranging from 50 per cent of targets achieved in the baseline to 77 per cent in the treatment stage.

Managing anxiety and other emotional sequelae resulting from impaired cognition

Anxiety and depression are frequently seen in memory-impaired people. Kopelman and Crawford (29) found depression in over 40 per cent of 200 consecutive referrals to a memory clinic. Evans and Wilson ( 30) found anxiety to be common in attenders of a weekly memory group. Dealing with these emotional problems should be an integral part of memory rehabilitation. Obviously, listening, trying to understand, and providing information are key factors in encouraging families to cope with their difficulties. Wearing (31) provides a helpful reference on the problems faced by families of memory-impaired people, and makes suggestions as to what can be done to help. Providing information or explanations is one very simple and therapeutic strategy that can help reduce the fear and anxiety accompanying memory impairment. Written information is best, as most people, whether memory impaired or not, are unlikely to have good recall of information at times of stress. Memory Problems After Head Injury,(32) written for the National Head Injuries Association; Managing Your Memory,(33) and Coping With Memory Problem (34) are all useful publications to have available for patients and their relatives.

In addition to providing information, it may be necessary to offer therapy for anxiety and depression. Relaxation therapy can be helpful in reducing anxiety, even if memory problems are severe enough to prevent a participant from remembering the actual therapy sessions. Depression, too, may exacerbate difficulties in people with organic memory impairment. It is possible that cognitive behavioural therapy approaches such as those employed by Beck (35) might be appropriate for those with depression associated with organic memory impairment, although no studies appear to have been reported. Psycho-therapy, on the other hand, is a well-established intervention for patients with neurological damage. Prigatano et al.(36) firmly believe in group and individual psychotherapy with brain-injured clients. Jackson and Gouvier (37) provide descriptions and guidelines for group psychotherapy with brain-injured adults and their families.

Other groups for memory-impaired people can be useful in reducing social isolation, which is also common in people with memory problems. (31) Wilson and Moffat (38) describe several kinds of groups for patients. Moffat (39) reports on a relative' memory group for people with dementia, and Wearing (31) offers suggestions for setting up self -help groups. Evans and Wilson (30) point out the social value of memory groups as well as their effect in reducing anxiety.

Conclusions

This article has emphasized that memory should be regarded as a multifunctional cognitive system that can be understood in a number of ways. We can consider the length of time information is stored, the type of information being stored, the stages involved in remembering, whether information is recalled or recognized, or whether memories date from before or after neurological insult.

The article has described a number of conditions that can give rise to organic memory impairment, the most common of which are degenerative conditions, Korsakoff' syndrome, traumatic head injury, temporal lobe surgery, encephalitis, anoxic brain damage, and cerebral tumours.

Most memory-impaired people have difficulty in learning and remembering new information; they have a normal or nearly normal immediate memory span but have problems remembering after a delay or distraction, and they usually have a period of retrograde amnesia that may range from minutes to decades. Less common memory disorders include semantic memory impairment and immediate verbal or visuospatial deficits.

Although restoration of memory functioning is unlikely to occur in the majority of people whose memory impairments follow neurological insult, there is, nevertheless, much that can be done to reduce the impact of disabling and handicapping memory problems and foster understanding of the issues involved. These include environmental modifications that can enable very severely impaired people to cope in their daily lives despite the lack of adequate memory functioning, the employment of errorless learning principles to improve the learning ability of memory-impaired people, teaching how to use external memory aids to help compensate for memory difficulties, and dealing with emotional sequelae such as anxiety and depression, which are often associated with organic memory impairment.

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