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Chapter 11

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 11: Learning Memory and Amnesia • Learning and memory are two ways of thinking about the same thingL both are neuroplastic processes; they deal with the ability of the brain to change its functioning in response to experience • Learning deals with how experience changes the brain • Memory deals with how these changes are stored and subsequently reactivated • Our knowledge of these roles has come to a great extent from the study of neuropsychological patients with brain-damage-produced amnesia and from research on animal models of the same memory problems AMNESIC EFFECTS OF BILATERAL MEDIAL TEMPORAL LOBECTOMY • Formal Assessment of H.M.’s Anterograde Amnesia: Discovery of Unconscious Memories • In order to characterize H.M.’s anterograde memory problems, researchers began by measuring his performance on objective tests of various kinds od memory • The results of the first two tests documented H.M.’s severe deficits in long-term memory, whereas the results of the last five indicated that H.M.’s brain was capable of storing long-term memories but that H.M. had no conscious awareness of these memories • Digit Span + 1 Test • H.M.’s inability to form certain long-term memories was objectively illustrated by his performance on the digit span + 1 test • H.M. had not managed to repeat the 8-digit sequence • Block-Tapping Memory-Span Test • H.M. had global amnesia - amnesia for information presented in all sensory modalities • H.M.’s amnesia was not restricted to verbal material by assessing his performance on the + 1 version of the block-tapping memory span test • H.M. had a block-tapping span of 5 blocks, which is in the normal range; but h could not learn to correctly touch a sequence of 6 block • Mirror-Drawing Test • H.M.’s task was to draw a line within the boundaries of a star-shaped target by watching his hand in a mirror • H.M. was asked to trace the star 10 times on each of 3 consecutive days, and the number of times he went outside the boundaries on each trial was recorded • H.M.’s performance improved over the 3 days, which indicates retention of the task • However, despite his improved performance H.M. could not recall ever having seen the task before • Rotary-Pursuit Test • In the rotary-pursuit test, the subject tries to keep the tip of a stylus in contact with a target that rotates on a revolving turntable • H.M.’s performance on the rotary-pursuit test improved significantly over 9 daily practice sessions • Incomplete-Pictures Test Chapter 11: Learning Memory and Amnesia • The discovery that H.M. was capable of forming long-term memories for mirror drawing and rotary pursuit suggested that sensorimotor tasks were the one exception to to his inability to form long-term memories • However, this view was challenged by the demonstration that H.M. could also form new long-term memories for the incomplete-pictures test - a nonsensorimotor test of memory that employs five sets of fragmented drawings • Each set contains drawings of the same 20 objects, but the sets differ in their degree of sketchiness • Despite his improved performance, H.M. could not recall previously performing the task • Pavlovian Conditioning • H.M. learned an eye-blink Pavlovian conditioning task • A tone was sounded just before a puff of air was administered to his eye; these trial were repeated until the tone alone elicited an eye blink; trials were repeated until the tone alone elicited an eye blink • Three Major Scientific Contributions of H.M.’s Case • Three contributions proved to be particularly influential • First, by showing that the medial temporal lobes play an especially important role in memory, H.M.’s case challenged the view that memory functions are diffusely and equivalently distributed throughout the brain • H.M.’s case renewed efforts to relate individual brain structures to specific mnemonic processes • Second, the discovery that bilateral medial temporal lobectomy abolished H.M.’s ability to form certain kinds of long-term memories without disrupting his performance on tests of short-term memory or his remote memory supported the theory that there are different modes of storage for short-term, long-term and remote memory • H.M.’s specific problem appeared to be a difficulty in memory consolidation • Third, H.M.s case was the first to reveal that an amnesic patient might claim no recollection of a previous experience, while demonstrating memory for it by improved performance • This discovery led to the creation of two distinct categories of long-term memories • Conscious long-term memories became known as explicit memories and long-term memories that are demonstrated by improved test performance without conscious awareness became known as implicit memories • Many people with amnesia lose their ability to form explicit memories while maintaining their ability to form implicit memories • Medial Temporal Lobe Amnesia • Neuropsychological patients with a profile of mnemonic deficits with preserved intellectual functioning, and with evidence of medial temporal lobe damage are said to suffer from medial temporal lobe amnesia • Research on medial temporal lobe amnesia has shown that H.M.’s difficulty in forming explicit long-term memories while retaining the ability to form implicit long- term memories of the same experiences is not unique to him Chapter 11: Learning Memory and Amnesia • Tests that have been developed to assess implicit memory are called repetition priming tests • First, the participants are asked to examine a list of words; they are not asked to learn or remember anything • Later, they are shown a series of fragments of words from the original list and are simply asked to complete them • Presumably, the implicit system was the first to evolve because it is more simple so the question is “What advantage is there in having a second, conscious system?” • The answer is flexibility • Semantic and Episodic Memories • Found that explicit memories fall into two categories and that many of these amnesics tend to have far greater difficulties with one category than the other • Explicit long-term memories come in two varieties: • Semantic memories are explicit memories for general facts or information • Episodic memories are explicit memories for the particular events of one’s life • People with medial temporal lobe amnesia have particular difficulty with episodic memories • It is difficult to spot episodic memory deficits, even when the deficits are extreme • This occurs in part because neuropsychologists usually have no way of knowing the true events if a patients life and in part because the patients become very effective at providing semantic answers to episodic questions • Effects of Cerebral Ischemia on the Hippocampus and Memory • Patients who have experienced cerebral ischemia - that is, have experienced an interruption of blood supply to their brains - often suffer from medial temporal lobe amnesia • Suggested that hippocampal damage by itself can produce medial temporal lobe amnesia AMNESIA OF KORSAKOFF’S SYNDROME • Korsakoff’s syndrome is a disorder of memory that is common in people who have consumed large amounts of alcohol; the disorder is largely attributable to the brain damage associated with the thiamine deficiency that often accompanies heavy alcohol consumption • In its advanced stages, it is characterized by a variety of sensory and motor problems, extreme confusion, personality changes, and a risk of death from liver of sensory and motor problems, extreme confusion, personality changes, and a risk of death from liver, gastrointestinal or heart disorders • Examination typically reveals lesions to the medial diencephalon and diffuse damage to several other brain structures, most notably the neocortex, hippocampus, and cerebellum • The amnesia of Korsakoff’s syndrome is similar to medial temporal lobe amnesia in some respects • During the early stages of the disorder, anterograde amnesia for explicit episodic memories is the most prominent symptom • As the disorder progresses severe retrograde amnesia, which can extend back into childhood, also develops Chapter 11: Learning Memory and Amnesia • The gradual, insidious onset and progressive development of Korsakoff’s syndrome complicate the study of the resulting retrograde amnesia • The brain damage associated with Korsakoff’s syndrome is diffuse, it has not been easy to identify which part of it is specifically responsible for the amnesia • Damage to another pair of medial diencephalic nuclei: the mediodorsal nuclei of the thalamus • Unlikely that the memory deficits of Korsakoff patients are attributable to the damage of any single diencephalic structure AMNESIA OF ALZHEIMER’S DISEASE • Alzheimer’s disease is progressive: eventually dementia develops and becomes so severe that the patient is incapable of even simple activities • Efforts to understand the neural basis of Alzheimer’s amnesia have focused on predementia Alzheimer’s patients • The memory deficits of these patients are more general than those associated with medial temporal lobe damage, medial diencephalic damage, or Korsakoff’s syndrome • Predementia Alzheimer’s patients often display deficits in STM and in some types of implicit memoryL their implicit memory for verbal and perceptual material is often deficient, whereas their implicit memory for sensorimotor learning is no • The level of acetylcholine is greatly reduced in the brains of Alzheimer's patients • This reduction results from the degeneration of the basal forebrain which is the brain’s main source of acetylcholine • The brain damage associated with Alzheimer’s disease is extremely diffuse and involves many areas, including the medial temporal lobe and the prefrontal cortex, which play major roles in memory • Damage to some structures of the basal forebrain produces attentional deficits, which can easily be mistaken for memory problems AMNESIAAFTER CONCUSSION: EVIDENCE FOR CONSOLIDATION • Concussion are the most common causes of amnesia • Amnesia following a nonpenetrating blow to the head is called posttraumatic amnesia (PTA) • Posttraumatic Amnesia • The coma following a severe blow to the head usually lasts a few seconds or minutes, but in severe cases it can last weeks • Victims of concussion are typically not tested by a neuropsychologist until after the period of confusion - if they are tested at all • Testing usually reveals that the patient has permanent retrograde amnesia for the events that led up to the blow and permanent anterograde amnesia for many of the events that occurred during the subsequent period of confusion • The patient may seem reasonably lucid at the time, because STM is normal, but later may have no recollection whatsoever of the conversation • Duration of the period of confusion and anterograde amnesia is typically longer than that of the coma, which is typically longer than the period of retrograde amnesia Chapter 11: Learning Memory and Amnesia • Islands of memory - surviving memories for isolated events that occurred during periods for which other memories have been wiped out • Gradients of Retrograde Amnesia and Memory Consolidation • Gradients of retrograde amnesia after concussion seem to provide evidence for memory consolidation • The fact that concussions preferentially disrupt recent memories suggests that the storage of older memories has been strengthended • The most prominent theory of memory consolidation is Hebb’s theory • Argued that memories of experiences are storied in the short term by neural activity reverberating in closed circuits • These reverberating patterns of neural activity are susceptible to disruption • Electroconvulsive shock seemed to provide a controlled method of studying memory consolidation • Electroconvulsive shock (ECS) is an intense, brief, diffuse, seizure inducing current that is administered to the brain through large electrodes attached to the scalp • ECS would erase from storage only those memories that had not yet been converted to structural synaptic changes • Most seemed to suggest a rather brief consolidation time of a few minutes or less • But some researchers observed very long gradients of ECS-produced retrograde amnesia • Long gradients of retrograde amnesia are incompatible with Hebb’s theory of consolidation • Long gradients of retrograde amnesia indicate that memory consolidation can continue for a very long time after learning, perhaps indefinitely • Hippocampus and Consolidation • The hippocampus and related structures play a role in consolidation • To account for the fact that the bilateral medial temporal lobectomy disrupted only those retrograde memories acquired in the period just before H.M.’s surgery, they suggested that memories are temporarily stored in the hippocampus until they can be transferred to a more stable cortical storage system • This theory has become known as the standard consolidation theory • Multiple trace theory proposed that the hippocampus and other structures involved in memory storage store memories for as long as they exist - not just during the period immediately after learning • According to Nadel and Moscovitch, retained memories became progressively more resistant to disruption by hippocampal damage because each time a similar experience occurs or the original memory is recalled, a new engram is established and linked to the original engram, making the memory easier to recall and the original engram more difficult to disrupt • Reconsolidation • Reconsolidation hypothesis is that each time a memory is retrieved from long-term storage, it is temporarily held in labile short-term memory, where it is once again susceptible to posttraumatic amnesia until it is reconsolidated • Researchers infused the protein synthesis inhibitor anisomycin into the amygdalae of rats shortly after the rat had been required to recall a fear conditioning trial Chapter 11: Learning Memory and Amnesia •The infusion produced retrograde amnesia for the fear conditioning, even though the original conditioning trial had occurred days before NEUROANATOMY OF OBJECT RECOGNITION MEMORY • Many important questions about the neural bases of amnesia can be answered only by controlled experiments • Because such experiments are not feasible with humans, there has been a major effort to develop animal models of human brain-damaged-produced amnesia • In retrospect, there are two reasons for the initial difficulty in developing an animal model of medial temporal lobe amnesia •First, it was not initially apparent that H.M.’s anterograde amnesia did not extend to all kinds od long-term memory - that is, that i
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