HSM330 article 3 summary

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Department
Health Services Management
Course
HSM 330
Professor
Daolun Chen
Semester
Fall

Description
Squire: Memory and Brain Systems -during past 2 centuries, study of L&M has been central to: philosophy, psycholo gy, biology -Society of Neuroscience (1969) was founded before discovery of place cells, LTP , animal model of human memory impairment, evidence of multiple memory systems, Aplysia and Drosophila b ecame productive simple systems for studying memory, development of neuroimaging technology -most of what is now understood about L&M became known after the Society was est ablished -Brenda Miller (1957) described profound effects on memory of bilateral medial t emporal lobe resection performed to relieve epilepsy on HM -HM showed profound forgetfulness, although his intellectual and perceptual func tions were largely intact -Findings from HM = 3 principles 1) memory is a distinct cerebral function, separable from other cognitive abilit ies 2) medial temporal lobe is not needed for immediate memory (HM did as well as ot hers his age at retaining a number or a visual image for a short time) 3) structures damaged in HM are not the ultimate repository of memory, b/c he re tained his remote childhood memories -only declarative memory was impaired in HM and other similar patients -thus, memory is not a unitary faculty of the mind but is composed of multiple s ystems that have diff operating principles and diff neuroanatomy -major distinction is btwn the capacity for conscious, declarative memory about facts and events and a collection of unconscious, nondeclarative memory abilities, such as skill learni ng and habit learning -nondeclarative memory: experience modifies behaviour but w/o requiring any cons cious memory content or even the experience that memory is being used, nondeclarative memory is expressed thr ough performance -declarative memory is expressed through recollection, as a way of modeling the external world -the diff memory systems operate in parallel to support behaviour - ex: an avers ive childhood event such as being knocked down by a large dog might lead to stable declarative memory for th e event itself as well as long-lasting fear of dogs (a nondeclarative memory) that is experienced as a per sonality trait rather than as a memory -animal model of human memory impairment succeeded initially in the monkey -anatomical components of medial temporal lobe memory system that support declar ative memory: the hippocampus (incl CA fields, dentate gyrus, subicular complex), entorhinal, perirhinal, and parahippocampal corticies that make up much of parahippocampal gyrus -behavioural work in monkey reproduced impt features of human memory impairment, emphasizing the key idea that only tasks of declarative memory should be expected to reveal an impairment -neuroanatomical studies identified the boundaries and the connectivity of impt araes, initially in the monkey and subsequently in the rat -analyzing function of specific connections w/i medial temporal lobe has been im proved by use of new genetic and physiological techniques, and neuroimaging techniques (which take advantage of detailed neuroanatomical info now available about connectivity of these regions) -ex: studies have begun to assess the separate contributions of: the direct (temporoammonic) pathway from entorhinal cortex to CA1 subfield and the indirect (trisynaptic) pathway from the entorhinal cortex to CA1 via dentate gyrus and CA3 -hippocampus and related structures are essential for formation of memory and it s reorganization and consolidation during a lengthy period after learning -role of these structures in memory retrieval has been rejected 2 lines of work underlie the idea that medial temporal lobe structures have a te mp. role in memory storage: 1) damage to these structures usu spares remote memory and impairs more recent m emory in a temporally graded manner (so, in exp animals, damage limited to hippocampus, entorhinal cortex, or fornix impairs memory for material learned up to 30d before damage is introduced - in humans, damage limit ed to hippocampus impairs memory for material learned up to a few years before damage occurred - discussio n continues about special status of spatial memory and autobiographical memory, but in each of these cases the temporally graded pattern has already been described) 2) second line of work involves studies that track neural activity or structural changes in medial temporal lobe structures after learning (ex: expression patterns of activity-related gene s such as c-Fos describe gradually dec activity in hippocampus after learning (such as in context fear co nditioning) and parallel inc in activity in a number of cortical regions) -these findings show inc importance of distributed cortical regions for represen tation and expression of memory as time passes after learning (similar results obtained in neuroimaging s tudies when volunteers recall news events that occurred 1-30 yrs ago) -gradual changes in neocortex establish stable LTM by inc connectivity among dis tributed cortical regions, as these changes occur, role of hippocampus (which initially works w/ neocortex to support LTM storage) gradually declines -direct evidence for gradual, spontaneous changes in neocortex across 6 weeks af ter training seen in case of trace eyeblink conditioning in rats -subsequent to initial acquistion, neural activity selective to the acquired ass ociation inc in medial prefrontal cortex, even in absence of continued training -peak inc in activity observed after learning when medial prefrontal cortex beca me necessary for memory retrieval -formation & maintenance of of hippocampus-dependent long-term visual memory in inferotemporal cortex of monkey can also be studied in single-cell recordings -successful retrieval from memory (at least in case of recently acquired info) o ccurs when brain activity comes to resemble (reinstate) the brain state that was present during original l earning, and cortical association areas as well as hippocampus and entorhinal cortex participate in th is process -retrieval of a memory provides an opportunity for updating/modulating what was originally learned and even the possibility of disrupting it -process by which a long-term memory transiently returns to a labile state (and then gradually stabilizes) is called reconsolidation -clear that memory can be modified as the result of a retrieval event, but its t oo simple to suppose that an established memory can be permanently abolished after retrieving it (by admin of protein synthesis inhibitor) and facts are still being developed -studies report that a reactivated memory can be disrupted but the disruption is transient and fully reversible -only a memory formed relatively recently (1-7d ago, but not 14-28d ago) can be disrupted after reactivating it -other factors (strength of memory & strength of reactivation) are also impt -learning initiates a consolidation process that eventually stabilizes the resul ting memory; early in this process, memory can be disrupted by treatments such as inhibition of protein syn thesis; later in process, treatments are effective only if they are given after reactivation of memory; st ill later, memory is stabilized, or consolidated, and cannot be disrupted -since discovery of place cells in rat hippocampus, hippocampus has been a focus for studying how spatial info is represented in the NS and how spatial knowledge is used for navigation -question of how ideas about spatial cognition relate to traditional view that h ippocampus supports memory -emphasize that place cells and grid cells provide computational equipment to su pport navigation independently of memory, or emphasize that activity in these cells serves to rep resent the significant features of a task/event, incl spatial features, as an early step in establishin g a memory -Relevant findings supporting second view are: 1) intact path integration in memory-impaired patients w/ large medial temporal lobe lesions, so long as task can be managed w/i STM (working memory) 2) evidence that, when a task is introduced, hippocampal 'place cells' come to b e activated in relation to all significant task features (odor, space, reward) 3) findings that hippocampal place cells can signal future choice , past events, and motivational state, not just the current spatial position of the animal -sleep might provide off-line periods favourable to memory consolidation -recordings of neural activity in rodents showed that firing sequences recorded in assemblies of hippocampal place cells during waking behaviour are replayed during SWS -finding of similar, coordinated activity in neocortex suggests a dialogue occur s btwn hippocampus&neocortex -this coordination could be part of the process by which recent memories become consolidated remote memories -replay phenomenon has been observed mainly in well trained animals running repe ated paths along fixed tracks -need to establish clear link btwn these observations and memory consolidation a nd determine how replay that occurs during sleep relates to replay that can occur during wakefulness -in humans, SWS can modulate declarative memory -ex: forgetting of declarative memory (word-pair memory) was dec by a night of s leep and dec further when duration of SWS was inc by transcranial application of slow oscillations early i n night (0,75Hz not 5Hz) -unsure whether these effects are related specifically to memory consoldiation o r to nonspecific benefits of reduced interference during an early time after learning when memory is vulnerab le -performance on tasks of nondeclarative memory are also improved by sleep -overnight improvement in texture discrimination was linked to REM sleep, but su bsequently, performance on this task was linked to combination of SWS and REM sleep -performance on other nondeclarative tasks (mirror tracing, motor sequence learn ing, and the serial reaction- time task) can also be improved more by sleep than by an equivalent period of wa kefulness -studies have linked nondeclarative memory to REM sleep, but findings are variab le and task-dependent -after brain-based distinction btwn declarative and nondeclarative (procedural) memory was introduced, brain systems that support various kinds of nondeclarative memory came under study -nondeclarative memory: refers to a heterogeneous collection of skills, habits, and dispositions that are inaccessible to conscious recollection, yet are shaped by experience, influence our behaviour and mental life, are a fundamental part of who we are -best understood example of nondeclarative memory in vertebrates is classical co ndition
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