Amnesia Introductory article Arthur Wingfield, Brandeis University, Waltham, Massachusetts, USA Article Contents Alice Cronin-Golomb, Boston University, Boston, Massachusetts, USA . Introduction . Features of the Amnesic Syndrome Amnesia is a general term that refers to a serious memory deficit typically associated with . Retrograde and Anterograde Amnesia medial temporal lobe or diencephalic lesions. Reference to amnesia implies a memory . Other Forms of Amnesia impairment in the absence of, or out of proportion to, deficits in other intellectual domains, . Summary such as problem solving or general cognitive function. Introduction tion from the past minutes, hours, days or years. The second lesson from case H.M. was that the areas important Amnesia can be defined as a memory loss in the absence of, for consolidation of a short-term memory trace into a more or out of proportion to, other cognitive problems that the enduring long-term memory can be anatomically localized. affected individual might suffer. Much of what we now know about memory and the loss of memory came not Brain lesions that produce amnesia from intentional experiment but from the unexpected consequence of a surgical treatment for epilepsy conducted We now know that the amnesia expressed by H.M. is on a particular patient in 1953. It had been known for many characteristic in type, if not in severity, of damage to a years that the serious seizures associated with psychomo- number of related areas of the brain. These areas include tor epilepsy are most often associated with abnormal the medial temporal lobe structures comprising the neural activity beginning in the temporal lobes of the brain. hippocampus, the amygdala, the entorhinal cortex and An operation that had been performed for some years to the perirhinal cortex. Besides surgical resection of these treat this disorder had involved surgical removal of parts of medial temporal structures that caused the amnesia in the temporal lobe of one hemisphere. In the case in H.M., other causes of damage to these areas that result in question, the patient had especially frequent and severe amnesia include stroke, closed head injury and herpes seizures and it was decided to remove both sides of a simplex encephalitis. Patients with Alzheimer disease have structure called the hippocampus, a band of fibres that is memory loss associated with damage to these same medial embedded deep within the temporal lobes. (The term temporal structures, although their neuropathology is not ‘hippocampus’ literally means ‘seahorse’. The name was restricted to medial temporal areas and accordingly their derived from its irregular curved appearance.) cognitive deficits are not restricted to memory loss alone. After the operation the frequency and severity of the These patients suffer from dementia, which is memory loss patient’s seizures were greatly reduced. It was soon plus impaired functioning in at least one other psycholo- discovered, however, that the patient, whose initials were gical domain, such as language, visuospatial function, H.M., had as a result of the operation an intriguing and judgment, abstract thought or personality. tragic kind of memory deficit. His memory for events years A second brain region associated with amnesia is the prior to the surgery was quite well maintained. There was, midline diencephalic region, which includes the mamillary however, an almost complete absence of memory for any bodies of the hypothalamus and the dorsomedial nucleus events that occurred after his surgery. This included events of the thalamus. Causes of damage to these diencephalic in his personal life as well as broader events in the world. structures include stroke, brain tumours in this region and He could hold new information very briefly in memory for a disorder called Korsakoff syndrome. a matter of minutes (and less if distracted), but he showed an almost complete inability to transfer these short-term Korsakoff syndrome memories into more permanent long-term memory. From ‘case H.M.’ students of memory learned two The most commonly studied single group of amnesics are important lessons. The first was that it is possible to those with Korsakoff syndrome. These patients have dissociate behaviourally short-term from long-term mem- damage to the above-mentioned brain tissue in and around ory. We can define the term short-term memory (also called the diencephalic brain structures as a consequence of long- immediate memory or primary memory) as referring to term alcoholism which often results in poor diet and knowledge of the ‘just past’. By this we mean events that vitamin deficiency. In the case of Korsakoff syndrome the occurred or facts that were learned a few seconds ago and vitamin implicated is thiamin (vitamin B1). that are recalled immediately. By contrast, long-term Korsakoff syndrome is a permanent condition in which memory (also called secondary memory) refers to informa- the individual has difficulty acquiring new information ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Amnesia (anterograde amnesia), combined with a memory loss for table, train, basket, cloud’, even after many presentations. information acquired before the onset of the brain damage This would illustrate a failure in explicit memory. At this (retrograde amnesia). In the case of the retrograde point one could demonstrate that the patient had spared amnesia, the memories acquired long before the onset of implicit memory using a technique referred to as stem- the damage are far better recalled than memories that had completion. In this case the person would be given word been acquired closer in time to the onset of the damage. beginnings, such as ‘cam...’, ‘ta...’, ‘tr...’ and so forth, and These features of the amnesic syndrome will be described in be asked to produce the first words that come to mind that more detail. The consequence of this damage, however, is begin with those letters. For example, the patient might that the Korsakoff patient is unable to learn new produce ‘camel’ for the stem ‘cam...’, ‘table’ for the stem information even when it has been repeated many times. ‘ta...’ and ‘train’ for the stem ‘tr...’, rather than other words When interrogated, these patients often confabulate, of equal or greater frequency in the language such as giving what to them might be a reasonable answer in place ‘camp’, ‘take’, or ‘tree’. of the actual information that is not available. Studies of amnesic patients show that they will very often It is not uncommon for Korsakoff patients to show complete the stems with words from the studied list, thus source amnesia, in which they may remember an item of showing that some trace of the word was in memory even information but not recognize that it had just been told to though it could not be produced in attempted recall. them by the examiner. Source amnesia can also occur in Indeed, being told that the stems were completed with normal elderly individuals who may correctly recall or words that had been studied, the patient may deny any recognize material that was learned previously but may memory of having seen them before or having studied misidentify where and when the material was learned. them. Stem completion and other forms of ‘priming’ are thought to rely on cortical areas that are not usually affected in amnesia. Amnesics can also demonstrate normal or near-normal Features of the Amnesic Syndrome skill learning on a variety of tasks. Consider, for example, attempting to trace a geometric shape with a pencil with The general features of the amnesic syndrome have been direct sight of your hand obscured such that you are able characterized in some detail in the literature. One major only to see your hand and the form you are trying to trace feature of amnesic patients is that while some forms of reflected in a mirror. This task, known as mirror tracing, is memory are devastated by the brain damage, other forms at first quite difficult to perform because of the reversed of memory may be relatively spared. image of your hand and the form produced by the mirror. With sufficient practice, however, speed and accuracy of Explicit memory tracing the reflected image will improve. Amnesics show a similar ability to learn this task, even as they may have no The typical amnesic patient is unable to recall recent memory of ever having seen the apparatus or of using it information, whether it is what they had for breakfast that before. Mirror tracing is an example of ‘procedural’ morning, a new address or a new telephone number. These learning that is preserved in amnesic patients. A real-world memories can be referred to as ‘explicit’ memory, in the example of procedural learning is learning to ride a bicycle. sense that they are memories for facts and events that are This is a skill that is acquired with substantial amounts of capable of being consciously recalled or recognized. These practice, and one that may remain intact even in patients kinds of memories have sometimes been called ‘declara- with severe deficits in explicit memory. tive’ memories because one can declare this knowledge, or Besides motor tasks, procedural learning occurs in the understanding that we have this knowledge, to others. perceptual tasks, such as mirror reading. For example, if one holds a page of text to a mirror and attempts to read it, Implicit memory the task is difficult because of the reversals of the printed letters. Like its production counterpart, mirror tracing, By contrast to the deficit in explicit memory, some forms of performance at mirror reading improves with practice. memory can be retained in amnesic patients, even in Amnesic patients will also show a learning curve for this severely impaired individuals such as H.M. These are task (good implicit memory), even though they may not forms of memory called ‘implicit’ memory.