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Chapter 11 Learning and memory -

Both are Neuroplastic processes: deal with the ability of the brain to change its functioning in response to experience Learning = how experience changes the brain Memory = how these changes are stored and subsequently reactivated Knowledge of the roles played by various brain structures in the processes of learning and memory comes from the study of neuropsychological patients with brain damage produced amnesia (any pathological loss of memory) and from research on animal models of the same memory problems

Amnesic effects of bilateral medial temporal lobectomy -

HM: medial portions of his temporal lobes were removed for the treatment of a severe case of epilepsy o before his surgery: he suffered from one generalized seizure each week and many focal seizures each day even though he was taking high doses of anticonvulsive medication o electroencephalography suggested that his seizures arose from foci in the medial portions of his left and right temporal lobes  removal of one medial temporal lobe had proved to be an effective treatment for patients with unilateral temporal lobe focus – the decision was made to perform bilateral medial temporal lobectomy – removal of the medial portions of both temporal lobes  including most of the hippocampus, amygdala and adjacent cortex  lobectomy = removal of a lobe or major part of one removed from the brain  lobotomy = lobe or major part of lobe is separated from rest of brain by large cut but not removed (incision into)  surgery was unqualified success: his generalized seizures were almost completely eliminated and incidence of partial seizures was reduced to one or two per day –even tho his medication dose was substantially reduced  he entered surgery with normal perceptual and motor abilities + normal intelligence and left surgery the same with an increase in IQ probably ebcuase of the decline in the incidence of his seizures  last person to receive a bilateral medial temporal lobectomy because of its devastating amnesic effects  assessing amnesic effects of brain surgery: administer tests of patients ability to remember things learned before the surgery and tests of the patients ability to remember things learned after the surgery  anterograde amnesia = deficits in the patients ability to remember things learned after surgery lead to this diagnosis

short term memory: storage of new info for brief periods of time while a person attends to it o long term memory: storage of new information once the person stops attending to it  retrograde amnesia = deficits in the patients ability to remember things learned before the surgery leads to this diagnosis HM’s memory for events predating his surgery remained intact – mild retrograde amnesia for events that occurred 2 years prior to surgery, his memory for remote events (of childhood for ex) was normal His short term anterograde memory = normal  Digit span = classic test of short term memory o His digit span = 6 digits – normal – had difficulty repeating longer lists Almost total inability to form new long term memories – once he stopped thinking about something it was lost forever o

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formal assessment of HM’s anterograde amnesia: discovery of unconscious memories -

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Results of the firsts two tests documented severe deficits in long term memory and results of the last 3 indicated HM’s brain was capable of storing long term memories but he had no conscious awareness of those memories Digit span + 1 test o Verbal long term memory test o Asked to repeat 5 digits that were read to him at 1 second intervals – he repeated the sequence correctly o On next trial: same 5 digits were presented in same sequence + 1 new digit added = this 6 digit sequence was presented a few times until he got it right and then another digit was added o After 25 trials, HM had not managed to repeat the 8 digit sequence o Most ppl can correctly repeat about 15 digits after 25 trials of this Block tapping memory span test o Hm had global amnesia = amnesia for information presented in all sensory modalities o his amnesia was not restricted to verbal material o 9 blocks – watch psychologist touch a sequence of them and then to repeat the same sequence of touches o He had block tapping span of 5 blocks – normal range o But he could not learn to correctly touch 6 blocks even when the same sequence was repeated 12 times Mirror drawing test o First indication that his anterograde amnesia did not involve all long term memories o Asked to draw a line within the boundaries of a star shaped target by watching hand in mirror o He was asked to trace the star 10 times for 3 days in a row

His performance improved over the 3 day= retention of the task BUT he could not recall ever having completed the task before o Sensorimotor tasks = exception to his inability to form long term memories Incomplete pictures test o A nonsensorimotor test of memory that employs 5 sets of fragmented drawings o HM’s performance improved 1 hour later but despite his improved performance he could not recall previously performing the task Pavlovian conditioning o Learned an eye pavlovian conditioning task although at a retarded rate o Tone sounded – before puff of air administered to his eye – trials repeated until tone alone elicited an eye blink o 2 years later HM retained this conditioned response almost perfectly although he had no conscious recollection of the training o

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3 major scientific contributions of HM’s Case 1. By showing the medial temporal lobes play an especially important role in memory – it challenged the then prevalent view that memory functions are diffusely and equivalently distributed throughout the brain – renewed efforts to relate individual brain structures to specific mnemonic (memory-related) processes; it spawned a massive research effort aimed at clarifying the mnemonic functions of the hippocampus and other medial temporal lobe structures 2. Discovery that bilateral medial temporal lobectomy abolished HM’s ability to form certain kinds of long-term memories without disrupting his performance on tests of short term memory or his remote memory (memory for experiences in the distant past) supported the theory that there are different modes of storage for short term, long term and remote memory a. His specific problem appeared to be a difficulty in memory consolidation (the translation of shortterm memories into long term memories) 3. Case was first to reveal that an amnesic patient might claim no recollection of a previous experience while demonstrating memory for it by improved performance (mirror drawing and incomplete picture tests)  led to creation of two distinct categories of long term memories a. Explicit memories (declarative memories): conscious long term memories b. Implicit memories: long term memories demonstrated by improved test performance without conscious awareness  Many ppl with amnesia lose their ability to form explicit memories while maintaining their ability to form implicit memories Medial temporal lobe amnesia – pts with mnemonic deficits similar to HM, preserved intellectual functioning and evidence of medial temporal lobe damage said to suffer from MTLA -

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HM’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 – symptom of medial temporal lobe amnesia as well as many other amnesic disorders – so the assessment of implicit long term memories now plays important role in study of human memory Tests that assess implicit memory called repetition priming tests – incomplete pictures test = example but ones that involve memory for words are more common

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Participants asked to examine a list of words – not asked to learn or remember anything – later they are shown series of fragments _ob_ _ER (LOBSTER) of words from original list and asked to complete them  Controls who have seen the original words perform well surprisingly participants with amnesia often perform equally well even though they have no explicit memory of seeing the original list Why do we have two parallel memory systems one conscious (explicit) and one unconscious (implicit)?  Presumably, the implicit system was the first to evolve because it is more simple (It does not involve consciousness) What is the advantage in having a second conscious system?  Two experiments (one with amnesic pts and one with amnesic monkeys) with medial temporal lobe lesions suggests that answer is “flexibility”  In both experiments: amnesic subjects learned an implicit learning task as well as control subjects did; however, if they were asked to use their implicit knowledge in a different way or in a different context, they failed!  Presumably, the evolution of explicit memory systems provided for the flexible use of information

Semantic and episodic memories -

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Explicit memories fall into two categories and that many of these amnesics tend to have far greater difficulties with one category than the other Semantic memories = explicit memories for general facts or information Episodic memories = explicit memories for particular events of ones life Ppl with mtla have difficulty with episodic memories  remembering specific events from their lives even though their memory for general info is often normal Episodic memory also called autobiographical memory – likened to travelling back in time mentally and experiencing one’s past Followed 3 pts with mtla who experienced bilateral medial temporal damage early in life – they could remember few of the experiences they had during their daily lives (episodic memory), they progressed through mainstream schools and acquired reasonable levels of language ability and factual knowledge (semantic memory) - however, despite their academic success, their episodic memory did not improve Its difficult to spot episodic memory deficits even when deficits are extreme – because we have no way of knowing the true events of a pts life and because pts become very effective at providing semantic answers to episodic questions

Effects of global cerebral ischemia on the hippocampus and memory -

pts who have experienced global cerebral ischemia – experienced an interruption of blood supply to their entire brains – often suffer from medial temporal lobe amnesia RB: underwent cardiac bypass surgery – surgery was messed up so RB suffered brain damage – pump that was circulating RB’s blood to body while heart was disconnected broke down – left him amnesic

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post mortem of brain: obvious brain damage was restricted largely to the pyramidal cell layer of just one part of the hippocampus – the CA1 subfield  major components of the hippocampus: CA1, CA2, CA3 and CA4 subfields and dentate gyrus suggested the hippocampal damage by itself can produce mtla but in such cases of cerebral amnesia it is difficult to rule out the possibility of subtle damage to other areas of the brain strongest evidence that selective hippocampal damage can cause mtla comes from cases of transient global amnesia = sudden onset in the absence of any obvious cause in otherwise normal adults  severe anterograde amnesia and moderate retrograde amnesia for explicit episodic memories – but in the case of transient global amnesia, the amnesia is transient typically lasting only 4 to 6 hours  sudden onset of transient global amnesia suggested it was caused by stroke but until recently no brain pathology could be linked to the disorder  but in recent years, abnormalities have been identified to the CA1 subfield of the hippocampus – time course of these abnormalities are not usually apparent for several hours after the beginning of the attack and have usually cleared up 10 days later –suggestive of stroke induced damage

Amnesias of korsakoff’s syndrome and Alzheimer’s disease Amnesia of Korsakoff’s syndrome -

korsakoff’s syndrome: disorder of memory, common in ppl who drink lots of alcohol, largely attributable to brain damage associated with thiamine deficiency that comes with alcohol consumption o advanced stages: sensory and moto problems, extreme confusion, personality changes and risk of death from liver, gastrointestinal or heart disorders o post-mortem: reveals lesions to medial diencephalon (medial thalamus and medial hypothalamus) + diffuse damage to other brain sturcutres such as neocortex, hippocampus and cerebellum o amnesia is similar to medital temporal lobe amnesia – anterograde amnesia for explicit episodic memories = most prominent symptom in early stages but as disease progresses, retrograde amnesia which can extend back into childhood also develops o deficits in implicit memory less severe than explicit memory o gradual onset and progressive development of disorder complicates the study of retrograde amnesia – not clear what extent K amnesia for recent events reflects retrograde disruption of existing memories or the gradually increasing anterograde blockage of the formation of new ones o brain damage is diffuse and not easy to identify which part is responsible for amnesia o first hypothesis = damage to mammillary bodies of hypothalamus responsible but then there were cases of K amnesia with no mammillary body damage

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in all of these cases, there was damage to mediodorsal nuclei of the thalamus but unlikely that memory deficits of K pts can be attributed to damage of any single structure medial diencephalic amnesia: amnesia associated with damage to medial diencephalon

Amnesia of Alzheimer’s disease -

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AD = another major cause of amnesia First sign of AD = mild deterioriation of memory Disorder is progressive and terminal Eventually dementia develops and becomes so severe that pt is not able to do simple activities (eating, speaking, recognizing spouse or bladder control) efforts to understand neural basis of Alzheimer’s amnesia has focused on predementia Alzheimer’s pts (AD pts who have not developed dementia yet) o memory deficits of these pts are more general than those associated with mtl damage, medial diencephalic damage or Korsakoff’s syndrome o in addition to major anterograde and retrograde deficits in tests of explicit memory, predementia A pts often display dificits in short term memory and in some types of implicit memory: verbal and perceptual material = deficient, for sensorimotor learning its not level of acetylcholine is greatly reduced in the brains of AD pts. – reduction results from degereation of basal forebrain (midline area located just above the hypothalamus – it’s the main source of acetylcholine o this finding + strokes in the basal forebrain can cause amnesia  view that acetylcholine depletion = cause of Alzheimer’s amnesia o acetylcholine depletion is not he only factor contributing to Alzheimer’s amnesia – brain damage associated with AD is diffuse involving many areas including medial temporal lobes and prefrontal cortex which play major roles in memory

Amnesia after concussion: evidence for consolidation -

blows to head that don’t penetrate skull are severe enough to produce concussion (temporary disturbance of consciousness produced by a nonpenetrating head injury) are most common causes of amnesia

Posttraumatic amnesia (amnesia following a non penetrating blow to head (PTA) ) -

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coma = pathological state of unconsciousness following a severe blow to head usually lasts a few seconds or minutes but in severe cases it can last weeks once pt regains consciousness – they experience period of confusion – they aren’t tested by neuropsychologist till after period of confusion when period of confusion ends, victim has retrograde amnesia for events that occurred during the period just below the blow and anterograde amnesia for events that occurred during the period of confusion testing reveals –pt 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

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duration of the period of confusion and anterograde amnesia is typically longer than that of the coma, which is typically longer than retrograde amnesia more severe blows to head tend to produce longer comas, longer periods of confusion and longer periods of 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 -

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gradients of retrograde amnesia after concussion seem to provide evidence for memory consolidation fact that concussions preferentially disrupt recent memories suggests that storage of older memories has been strengthened (Consolidated) Hebb’s theory of consolidation o Argued that memories of experiences are stored in the short term by neural activity reverberating (circulating) in closed circuits. These reverberating patterns of neural activity are susceptible to disruption (eg blow to the head) – but eventally they induce structural changes in the involved synapses which provide stable long-term storage Controlled method of studying memory consolidation = electroconvulsive shock Electroconvulsive shock (ECS) = intense, brief, diffuse, seizure-inducing current that Is administered ot the brain through large electrodes attached to the scalp o Rationale = by disrupting neural activity, ECS would erase from storage only those memories that had not been converted to structural synaptic changes; the length of period of retrograde amnesia produced by an ECS would thus provide an estimate of the amount of time needed for memory consolidation o Used as treatment for depression and studies conducted with lab animals Hebb’s theory implies that memory consolidation is relatively brief, a few seconds or minutes about as long as specific patterns of reverbratory neural activity could conceivably maintain a memory – but many studies found evidence for much longer gradients Classic study example of long gradient of ECS produced retrograde amnesia o Measured memory of group of ECS treated pts for tv shows that had played for only one season in diff years prior to their Electroconvulsive therapy o Tested each pt twice on different forms of the test: once before they receveid a series of 5 ECS and once after o difference between before and after scores served as an estimate of memory loss for the events of each year o 5 electroconvulsive shocks disrupted the retention of tv shows that had played in the 3 years prior to treatment but not those that had played earlier Current view of memory consolidation = it continues for a very long time if not indefinitely o evidence indicates that lasting memories become more and more resilient to disruption throughout a persons life o each time a memory is activated, it is updated and linked to additional memories o these additional links increase the memory’s resistance to disruption by cerebral trauma such as concussion or ECS

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hippocampus and consolidation: HM’s case provided evidence of memory consolidation and seemed to suggest that the hippocampus played a special role in it o to account for the fact that medial temporal lobectomy disrupted only those retrograde memories acquired in the few years before HM’s surgery, S and M suggested that memories are temporarily stored in the hippocampus until they can be transferred to a more stable corticol storage system = theory = standard consolidation theory or dual trace theory o temporally graded retrograde amnesia = feature of many forms of human amnesia and it seems unlikely that the hippocampus plays a special role in consolidation o it appears that when a conscious experience occurs, it is rapidly and sparsely encoded in a distributed fashion throughout the hi...