Major Sub Classifications of Memory and How They Are Relatively Damaged, or Spared, by Different Neurological Diseases - Term Paper Example

MAJOR SUB IFICATIONS OF MEMORY AND HOW THEY ARE RELATIVELY DAMAGED, OR SPARED, BY DIFFERENT NEUROLOGICAL DISEASES Table of Contents Tableof Contents 2 3.1 Episodic Memory 8 3.2 SEMANTIC MEMORY 11 3.3 PROCEDURAL MEMORY 14 3.4 Working Memory 15 List of Tables and Figures Figure:1 The model of human memory. 6 Figure 2 Episodic Memory 9 Figure 3 . Semantic, Procedural, and Working Memories 11 Table 1. Selected Memory Systems 8 Table 2 Four Memory Systems and Common Clinical Disorders That Disrupt Them 12 GLOSSARY Alzheimer's disease: A disease marked by the loss of cognitive ability, generally over a period of 10 to 15 years, and associated with the development of abnormal tissues and protein deposits in the cerebral cortex. Amygdala: An almond-shaped mass of gray matter in the anterior portion of the temporal lobe. Anterograde amnesia: Loss of memory for events immediately following a trauma; sometimes in effect for events during and for a long time following the trauma Cerebellum The trilobed structure of the brain, lying posterior to the pons and medulla oblongata and inferior to the occipital lobes of the cerebral hemispheres, that is responsible for the regulation and coordination of complex voluntary muscular movement as well as the maintenance of posture and balance. Concussion: An injury to an organ, especially the brain, produced by a violent blow and followed by a temporary or prolonged loss of function Encephalitis: Inflammation of the brain. Episodic Memory: Memory for episodes in your own life Hippocampus: A ridge in the floor of each lateral ventricle of the brain that consists mainly of gray matter and has a central role in memory processes. LTM: Long-Term Memory Semantic memory: memory for meanings and general (impersonal) facts STM: Short-term Memory Introduction The brain is an intricate set of interacting, heterogeneous structures, the functions of which are mediated by an equally intricate set of physiological processes. The process of memory too, which was once thought to be a simple and single unitary faculty has become an interesting and even more complicated subject area of study. The study of patients with memory deficits is the most significant cause to explore the structure and organization of memory system. Memory refers to the acquisition, storage and retrieval of information. The brain achieves these three aspects of memory by associating neural representations with one another. In this way, organisms learn the causal relationships of the world by exposure to events, continually adjusting and augmenting their internal representations when learned associations do not match reality. Virtually all views of learning share this common thread, from simple conditioning paradigms to cognitive theories.1 A common mechanism does not imply a single, monolithic memory system. Such a view was dispelled long ago by observations of spared and impaired learning abilities in patients with hippocampal damage.2 Modern theories of learning typically divide memory into multiple psychological systems supported by different neural sub-states. From a psychological perspective, different systems learn different kinds of information, performing their functions for different lengths of time. From a biological perspective, memory systems are interconnected neural structures suited to particular information processing demand, either storing information or participating in its storage in other structures. Similarities between these perspectives are presumed to reflect the same phenomena at different levels of analysis, where observations from one perspective serve to validate those of the other.3 Virtually all experimental paradigms for the study of memory reveal specialized components serving different functions. Even basic, associative conditioning involves multiple memory systems, as the cerebellum, hippocampus and amygdala are known to contribute differentially to classically conditioned responses. A multiple memory systems framework, however, does not imply interactions are limited to those between clearly defined components. To begin with, the anatomical and systemic boundaries between memory systems are blurry. For example, the hippocampus and MTL are often considered together as a system supporting declarative memory, but interactions between structures within this system are extremely complex and include cooperative and competitive relationships.4 In cortex, bottom-up, top-down and lateral interactions occur at every processing stage, providing the foundation for information processing in this memory system. 1. Memory Classification Some perceptions about memory such as the concepts of "short-term memory" and "long-term memory" are now being replaced with more elaborated and systematic classifications systems as the result of neuropsychological studies of patients with focal brain lesions, neuro-anatomical studies in animals and humans. Though memory can be classified according to duration, information type and the temporal direction, major classification in this study is presented in accordance with the clinical relevance. This classification includes episodic memory, semantic memory, procedural memory, and working memory (see table 1). However, an overview of the basic classification of "Sensory memory", "short-term memory" and "Long-term memory" is also presented for elaborating the subject. 1.1 Sensory Memory Sensory memory lasts around 200-500 milliseconds and it is the most basic type of memory which lets us remember what we saw over the matter of seconds. This is photographic like memory which fades instantly and the viewer is able only to describe what was displayed before them but hardly would be able to describe what actually they saw in detail. This type of memory cannot be improved or prolonged through the means of rehearsal as compared to the short-term memory. The figure 1 below illustrates that the sensory memory comes first to view objects from the environment input and then the information either has to decay instantly or goes further to the STM or LTM.5 1.2 Short-term Memory As shown in fig.1, some of the information received by the sensory memory is transferred to short-term memory which allows someone to recall the information from several seconds up to a minute without any rehearsal.6 However, the capacity to store information for longer period can be increased by a process called chunking. If the information is received in systematic and small chunks it is easier to retain in memory and increase the retention period. Remembering phone numbers in small chunks of three digits is an illustration as how people store information by dividing a long number into two or three groups. 1.3 Long-term Memory As compared with the sensory and short-term memory, the retention time of information is for unlimited time and it stores larger quantity of information. Any information received in short-term memory can be transferred to long-term memory by repetition and rehearsal. Another characteristic of long-term memory is that it stores information semantically as compared with the short-term memory storing the information acoustically. Long-term memory can further be divided into declarative and procedural memory. 2. Classification by Clinical Relevance 3.1 Episodic Memory Episodic memory is based on the explicit and declarative memory system which is used to recall personal experiences framed in our own context. Episodic memory is largely defined according to the inability of people with amnesia due to lesions of the medial temporal lobe to remember experiences that healthy people can remember. Thus, this memory system depends on the medial temporal lobes including the hippocampus and the entorhinal and perirhinal cortexes. Other critical structures in the episodic memory system include the basal forebrain with the medial septum and diagonal band of Broca's area, the retrosplenial cortex, the presubiculum, the fornix, mammillary bodies, the mammillothalamic tract, and the anterior nucleus of the thalamus.7 The damage in any one of these structures may cause the impairment that is characteristic of dysfunction of the episodic memory system (table 1). Table 1. Selected Memory Systems Memory System Major Anatomical Structure Involved Length of Storage of Memory Type of Awareness Episodic memory Medial temporal lobes, anterior thalamic nucleus, mammillary body, fornix, prefrontal cortex Minutes to years Explicit, declarative Semantic memory Inferolateral temporal lobes Minutes to years Explicit or implicit, nondeclarative Procedural memory Basal Ganglia, cerebellum, supplementary motor area Minutes to years Explicit, declarative Working memory Phonologic: prefrontal cortex, Broca's area, Wernicke's area Spatial: prefrontal cortex, visual-association area Seconds to minutes; information actively rehearsed or manipulated Recent research shows that the episodic memory system includes the frontal lobes.8, 9 Instead of retaining the information, the frontal lobes are involved in the registration, acquisition, or encoding of information10 and the recollection of the source of information.11 It is also involved in the assessment of the temporal sequence and recency of events.12 The left medial temporal and left frontal lobes are most active when a person is learning words whereas the right medial temporal and right frontal lobes are most active when learning visual scenes.13 The figure 2 illustrates the anatomical structure of episodic memory.14 The dysfunction of the episodic memory is explained by the Ribot's law according to which events just before an ictus are most vulnerable to dissolution, whereas remote memories are most resistant. Thus, in cases of dysfunction of the episodic memory system, the ability to learn new information is impaired (anterograde amnesia) and recently learned information cannot be retrieved (retrograde amnesia). Also, remotely learned information is usually spared.15 Disorders of episodic memory may be transient, such as those attributable to a concussion, a seizure, or transient global amnesia. Static disorders, such as traumatic brain injury, hypoxic or ischemic injury, single strokes, surgical lesions, and encephalitis, typically are maximal at onset, improve, and then are stable. Degenerative diseases, including Alzheimer's disease,16 dementia with Lewy bodies, and frontotemporal dementia, begin insidiously and progress gradually. Disorders affecting multiple brain regions, such as vascular dementia and multiple sclerosis, progress in a stepwise manner. Other disorders of memory, such as those due to medications, hypoglycemia, tumors, and Korsakoff's syndrome, can have a more complicated and variable time course. 3.2 SEMANTIC MEMORY Semantic memory refers to our general store of conceptual and factual knowledge, i.e. it is not related to any specific memory. Like episodic memory, semantic memory is a declarative and explicit memory system. Evidence that this memory system is different from episodic memory emerges from neuroimaging studies17 and the fact that previously acquired semantic memory is spared in patients who have severe impairment of the episodic memory system, such as with disruption of the Papez circuit or surgical removal of the medial temporal lobes.18 Since in its broadest sense semantic memory includes all our knowledge of the world not related to specific episodic memories, one could argue that it resides in multiple cortical areas. There is evidence, for example, that visual images are stored in nearby visual-association areas.19 However, a more restrictive view of semantic memory, one that is justified in light of the naming and categorization tasks by which it is usually measured, localizes semantic memory to the inferolateral temporal lobes20, 21 (Fig. 2). Alzheimer's disease is the most common clinical disorder disrupting semantic memory. This disruption may be attributable to pathology in the inferolateral temporal lobes22 or to pathology in frontal cortexes,23 leading to poor activation and retrieval of semantic information.24 In Alzheimer's disease, episodic and semantic memory decline independently of each other, supporting the idea that two separate memory systems are impaired in this disorder.25 Other causes of the impairment of semantic memory include almost any disorder that may disrupt the inferolateral temporal lobes, such as traumatic brain injury, stroke, surgical lesions, encephalitis, and tumors (Table 2). Patients with the temporal variant of frontotemporal dementia, known as semantic dementia, also exhibit deficits in all functions of semantic memory, including naming and single-word comprehension and impoverished general knowledge. They show relative preservation of other components of speech, perceptual and nonverbal problem-solving skills, and episodic memory.26 Table 2. Four Memory Systems and Common Clinical Disorders That Disrupt Them Episodic memory Alzheimer's disease Mild cognitive impairment, amnesictic type Dementia with Lewy bodies Encephalitis (most commonly, herpes simplex encephalitis) Frontal variant of frontotemporal dementia Korsakoff's syndrome Transient global amnesia Concussion Traumatic brain injury Seizure Hypoxic-ischemic injury Cardiopulmonary bypass Deficiency of vitamin B12 Hypoglycemia Anxiety Temporal-lobe surgery Vascular dementia Multiple sclerosis Semantic memory Alzheimer's disease Semantic dementia (temporal variant of frontotemporal dementia) Traumatic brain injury Encephalitis (most commonly, herpes simplex encephalitis) Procedural memory Parkinson's disease Huntington's disease Progressive supranuclear palsy Olivopontocerebellar degeneration Depression Obsessive-compulsive disorder Working memory Normal aging Vascular dementia Frontal variant of frontotemporal dementia Alzheimer's disease Dementia with Lewy bodies Multiple sclerosis Traumatic brain injury Side effects of medication Attention deficit-hyperactivity disorder Obsessive -compulsive disorder Schizophrenia Parkinson's disease Progressive supranuclear palsy Cardiopulmonary bypass Deficiency of vitamin B12 3.3 PROCEDURAL MEMORY Procedural memory refers to the ability to learn behavioral and cognitive skills and algorithms that are used at an automatic, unconscious level. Procedural memory is nondeclarative but during acquisition may be either explicit or implicit. That procedural memory can be spared in patients who have severe deficits of the episodic-memory system, such as patients with Korsakoff's syndrome or Alzheimer's disease or who have undergone surgical removal of the medial temporal lobes,27 demonstrates that procedural memory depends on a memory system that is separate and distinct from the episodic memory and semantic memory systems. Parkinson's disease is the most common disorder affecting procedural memory. Other neurodegenerative diseases that disrupt procedural memory include Huntington's disease and olivopontocerebellar degeneration. Patients in the early stages of these disorders perform nearly normally on episodic memory tests but show an impaired ability to learn skills.28 Tumors, strokes, hemorrhages, and other causes of damage to the basal ganglia or cerebellum may also disrupt procedural memory. Patients with major depression have also been shown to have impairment in procedural memory, perhaps because depression may involve dysfunction of the basal ganglia (Table 2).29 3.4 Working Memory Working memory is a combination of the traditional fields of attention, concentration, and short-term memory. It refers to the ability to temporarily maintain and manipulate information that one needs to keep in mind. Because it requires active and conscious participation, working memory is an explicit and declarative memory system. Working memory has traditionally been divided into components that process phonologic information or spatial information and an executive system that allocates attentional resources.30 Because working memory depends on a network of activity that includes subcortical structures as well as frontal and parietal cortical regions, many neurodegenerative diseases impair working-memory tasks. Studies have shown that patients with Alzheimer's, Parkinson's, or Huntingdon's disease or dementia with Lewy bodies, as well as less common disorders such as progressive supranuclear palsy, may show impaired working memory (Table 2).31.32 In addition to neurodegenerative diseases, almost any disease process that disrupts the frontal lobes or their connections to posterior cortical regions and subcortical structures can interfere with working memory. Such processes include strokes, tumors, head injury, and multiple sclerosis, among others.33,34 Because phonologic working memory involves the silent rehearsal of verbal information, almost any kind of aphasia can also impair it. Although the pathophysiology is not well understood, disorders that diminish attentional resources, such as attention deficit-hyperactivity disorder, obsessive-compulsive disorder, schizophrenia, and depression, can also impair working memory.35,36 Conclusion The psychology of memory has developed significantly since the days when memory was regarded as a single unitary faculty. The study of patients with memory deficits has played a major role in this development and it is hoped that with the passage of time continuous researches would open up new vista of useful knowledge about the functionality of memory and the ways to improve it. 5. References Read More