Memory

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Memory

• Gateway to Learning

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Types of Memory

• Declarative memory Explicit memory
• Learning about people, places and things
• Nondeclarative memory Implicit memory
• Memory for skills or behavior
• Working memory temporary information storage
• includes several types of information
• probably from several sites in the brain
• Spatial memory
• memory of location
• Relational memory
• things that happen at same time get stored
  together in a manner that ties them together

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Explicit Memory

• Learning about people, places and things
• Can be verbally reported
• Requires conscious awareness
• Short-term memory
• temporary, limited in capacity
• requires continual rehearsal
• Long-term memory
• more permanent, much greater capacity
• does not require continual rehearsal
• Consolidation
• transfer from short-term memory to long-term
  memory

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Implicit Memory

• Procedural memory
• Memory for skills or behavior
• Perceptual and motor learning
• Does not require conscious awareness

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Major Features of Memory

• Memory has stages
• Long term memory is represented in multiple
  regions of the CNS
• Implicit and explicit memory involve different
  neural circuits
• Explicit requires the temporal lobe
• Implicit involves the cerebellum, amygdala

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Memory is Not Unitary

• Memory is not an all or none phenomenon
• Can be implicit or explicit or a mixture
• Depends on how the information is stored and
  recalled

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Amnesia - Loss of Memory

• Dissociated amnesia
• not associated with any other cognitve deficits
• Retrograde amnesia
• loss of memory for the time period before a
  trauma
• typically is gradational from essentially
  complete loss just before trauma to less and less
  complete loss earlier and earlier before trauma
• Anterograde amnesia
• inability to form new memories
• Transient global amnesia
• sudden, brief onset of anterograde amnesia
• The different types of amnesia suggest that
  several mechanisms for memory work together

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Causes of Amnesia

• Causes
• concussion, chronic alcoholism, encephalitis,
  brain tumor, stroke
• Transient amnesia
• Probably caused by interruptions in cerebral
  blood flow
• Blows to head, physical stress, cold showers,
  sex, drugs
• Clioquinol (anti-diarrheal drug) - no longer on
  market became famous for causing transient global
  amnesia in some people

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Localization of Memory Functions

• Long thought memory was a function of the whole
  cerebral cortex
• We now realize that different types of memory are
  localized in different regions
• Engram
• physical representation or location of a memory

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Cortical Ablation Studies

• Karl Lashley 1920s
• Tried to localize memory engram to association
  areas of the neocortex
• Studied effect of brain lesions on ability to
  learn a maze in rats
• As more and more of the rat's cortex was ablated,
  more and more errors were made
• ability to learn was progressively impaired
• Lashley incorrectly concluded that the whole
  neocortex equally participated in memory
• now know that the problem was that the lesions
  were too large

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Cell Assembly

• Donald Hebb 1949
• The internal representation of an object consists
  of the cortical cells that are activated by the
  stimulus
• Group of simultaneously acting neurons cell
  assembly
• Same neurons are involved in sensation and
  perception
• All of these cells are reciprocally
  interconnected
• Internal representation of an object remains in
  short term memory as long as the cell assembly is
  active
• If assembly active long enough, consolidation
  occurs
• Long term memory
• Neurons that fire together wire together
• Activating any cells in the assembly activates
  the memory
• Led to neural network model                      

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Localization of Declarative Memories

• Studies in macaque monkeys - 
• Lesions in the inferotemporal cortex (IT) cause
  loss of memory about previously learned visual
  discrimination tasks, even though vision itself
  remains normal
• Cells in the IT may respond preferentially to a
  familiar face in a particular orientation
• Cells in IT may change their response during
  repeated exposure to an unfamiliar face
  learning
• The IT is involved both in vision and visual
  memory

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Human Studies

• fMRI shows what part of the brain is activated
  during exposure to various types of objects
• Bird watchers respond more vigorously to pictures
  of birds
• Car buffs respond more vigorously to pictures of
  cars and responses are in different places

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The Temporal Lobe

• Wilder Penfield 1940s
• Electrical stimulation of human temporal lobe
• Studied patients with epilepsy
• During surgery for epilepsy, stimulated temporal
  lobe electrically
• Some patients reported sensations like
  hallucinations or vivid memories
• but the patients had other cortical abnormalities
• Still, temporal lobe stimulation caused different
  effects than stimulation of other parts of the
  neocortex
• Consistant with seizure aura

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Kluver-Bucy Syndrome

• Researchers at U. Chicago in the 1930s
• Studying emotion circuitry
• Bilateral temporal lobectomy in rhesus monkeys
• Produces bizarre behavioral abnormnalities
• One of these is psychic blindness
• Although they could see, could not recognize or
  understand the meaning of common objects
• Thus loss of declarative memory
• More on this phenomenon with emotion

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Building on Penfield

• Brenda Milner 1950s
• Studied patients with surgical interventions to
  treat epilepsy
• Bilateral removal of the hippocampus and
  neighboring regions of the temporal lobe
• Most famous case was H.M.

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Patient H.M. Sheds New Light

• Bilateral removal of mid-temporal lobe
• Stopped seizures
• Short term memory was OK
• Memories formed prior to surgery were OK
• Ability to form new, long term memories was lost
• Couldnt transfer information from short term to
  long term memory!
• Similar findings in all bilateral temporal lobe
  surgical patients

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Impact on Learning

• All of the things these patients could recall
  have an automatic quality
• Do not require conscious recall
• Do not require complex cognitive skills such as
  comparison
• If the patient practices a puzzle, they improve
  their ability to solve it, but they dont
  remember how.

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Medial Temporal Lobe Anatomy

• Hippocampus
• deep in the medial temporal lobe
• Entorhinal cortex, perirhinal cortex,
  parahippocampal cortex
• Three cortical regions ventral to the hippocampus
• all involved in memory functions

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Function of the Medial Temporal Lobe

• Lesions in monkeys impair discrimination memory
• ability to recognize whether object has been seen
  before
• Lesions impair declarative memory, not procedural
  memory
• Lesions impair long-term memory storage, but
  short-term memory seems to be normal
• This region seems to be involved in packaging
  short-term memory for relay to the rest of the
  neocortex for long-term storage

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The Diencephalon Memory

• Outside of the temporal lobe, one of the regions
  most associated with memory
• Axons from the hippocampus project to the
  mammillary bodies which project to the thalamus
• Thalamus also receives input from temporal lobe
  structures including the amygdala IT
• Large midline thalamic lesions in monkeys produce
  severe deficits in ability to learn a matching
  task
• Lesions which impact fewer nuclei produce smaller
  deficits

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The Diencephalon

• Three regions have been implicated in memory
  processing
• Anterior nucleus of thalamus
• Dorsomedial nucleus of thalamus
• Mammillary bodies in hypothalamus
• The thalamus mammillary bodies receive nerve
  fibers from the medial temporal lobe

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The Case of N.A.

• Accidentally stabbed with a fencing foil
• Through his right nostril into his brain
• Produced a lesion in his left dorsomedial
  thalamus
• Cognitive ability normal but memory impaired
• Caused moderate retrograde amnesia (for the 2
  years prior) and profound anterograde amnesia
  similar to the more extensive damage to H. M.
• Short term memory and preservation of old
  memories was intact
• Suggests that both the temporal lobe and parts of
  the thalamus may be involved in the formation of
  long-term declarative memories

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Korsakoff's Syndrome

• Usually due to chronic alcoholism
• Results from alcohol associated thiamin
  deficiency
• Produces confusion, severe memory impairment,
  apathy
• First presents as abnormal eye movements, loss of
  coordination, tremors
• Can be treated in early stages
• Untreated thiamin defiency leads to brain damage
  which produces Korsokoffs
• Lesions in dorsomedial thalamus and mammillary
  bodies
• Anterograde and severe retrograde amnesia
• Further supports role of diencephalon in memory

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Anterograde Retrograde Amnesia

• While often found together, may have different
  causes
• The degree of severity of the two does not
  correlate in Korsokoffs
• Suggests different mechanisms involved
• Damage to the dorsomedial thalamus and mammillary
  bodies probably causes anterograde amnesia
• What causes retrograde amnesia is still unclear

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Working Memory the Hippocampus

• Hippocampus is involved in memory function for a
  diverse range of tasks
• Studies of hippocampal ablation in rats
• Studied working memory
• Used a radial maze containing food
• Normal rats learn to visit each arm only once
• If only some arms are used, they learn only to go
  down those arms, and then only once.

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The Maze Study

• Rats with hippocampal lesions still find the
  food, but they aren't very efficient, going down
  the same arm repeatedly
• Rats with lesions can learn to avoid the arms
  that never have food, but they still explore the
  food containing arms inefficiently and repeatedly
  
• Inability to use changing information

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Place Cells in the Hippocampus

• Neurons in the hippocampus selectively respond
  when rat is in a particular location.
• If vision is used to determine place, (like
  landmarks) cell fires in response to where the
  animal thinks he is
• May be responsible for learning radial arm maze
• More than spatial memory is involved
• Hippocampus may control relational memory

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Relational Memory

• Highly processed sensory information comes into
  the hippocampus cortex
• Processing occurs leading to the storage of
  memories
• All things happening at the same time are stored
  together
• Thus, remembering one thing brings back related
  memories
• It's easier to remember events that you had
  strong feelings about.
• Spatial navigation is based on a spatial map
  relational memories

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Striatum and Procedural Memory

• Procedural memory memory involved in forming
  behavioral habits
• Striatum is the major structure involved in
  procedural memory
• Striatum may be involved in forming 'habits' in
  rats, humans, non-human primates
• Data from humans suggests that the striatum is
  involved in a procedural memory system that is
  separate and distinct from the medial temporal
  system used for declarative memory

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Neocortex and Working Memory

• Humans have much more prefrontal cortex than any
  other animals
• Pathways
• Medial temporal lobe gtgt hypothalamus gtgt anterior
  nucleus of thalamus gtgt cingulate cortex
• Medial temporal lobe gtgt dorsomedial nucleus of
  thalamus gtgt frontal cortex
• Experiments suggest that frontal cortex is
  involved with working memory for problem solving
  and planning of behavior

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Frontal Lobe Memory

• The left frontal lobe (colored regions at left)
  supports our ability to retrieve the meaning of
  words and objects.

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Lateral Intraparietal Cortex (LIP)

• Cortex buried in intraparietal sulcus
• Involved in working memory
• Responses specific to vision
• Example monkey looks at fixation point while a
  stimulus is flashed in periphery after delay,
  monkey moves eyes to where the stimulus was
• Cells in LIP seem to store information about
  where the eyes are to be moved
• they remain active during the delay - specific
  for visual working memory
• There is a different area specific for auditory
  working memory

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Many Structures are Involved in Memory
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Summary Memory Pathway 1

• Visual information is first routed through the
  thalamus to the visual area of the cerebral
  cortex.
• This neural activity is the basis for the sensory
  register

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Summary Memory Pathway 2

• Information is relayed to the frontal lobes where
  it is held in short term memory

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Summary Memory Pathway 3

• Information that is stored in long-term memory is
  held in the hippocampus for weeks or months, and
  then transferred to the area of the cerebral
  cortex near where it was originally process for
  long-term storage

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Summary Memory Pathway 4

• When we recall information from long term memory,
  it is routed again to the frontal lobes, where it
  is held in short-term, or working memory.