Memory Systems

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Memory Systems
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Two Views of Episodic and Semantic Memory

• Declarative memory comprises both fact and event
  memory and is a unitary process
• Episodic and semantic memory are independent

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Is there amnesia after childhood brain lesions?

• Until recently few cases reported with lesions
  early in life
• Why??

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Is there an age below which hippocampal injury
does not cause a selective amnesia?

• Plausible explanations
• Very early damage to the memory system would have
  a widespread impact on cognitive development -
  result would be a profound and global mental
  delay
• Plasticity and re-organizational capacity of the
  immature brain would lead to a sparing/rescue of
  the memory system.

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Neither is correct ...

• Cases identified with hippocampal damage in
  childhood and an amnesic syndrome
• Amnesia different from that seen in adults
• Named developmental amnesia
• Researchers Vargha-Khadem, Baddley, Isaacs,
  Mishkin others

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Developmental Amnesia

• Case 1 Beth age 14
• difficult birth, anoxia
• memory deficit noted when she went to school at
  age 5
• Case 2 Jon age 19
• born prematurely (26 weeks)
• age 4 had 2 long seizures
• memory deficit noted by parents about 1.5 years
  later.

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• Case 3 Kate age 22
• age 9 years - a toxic dose of a drug for asthma
• acute episode of seizures, unconscious,
  respiratory arrest
• left profoundly amnesic.
• At age 17 developed temporal-lobe epilepsy.
• MRI scans showed that the hippocampi were
  abnormally small bilaterally in all three cases.

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Neuropathological findings Jon

• Hippocampal volume 50 less than normal mean
  volume
• More subtle pathology in putamen bilaterally,
  ventral part of the thalamus, and the midbrain
• Consistent with effects of hypoxia-ischemia
• Preserved memory might be due in part to
  preservation of subjacent cortex in the
  parahippocampal region

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Nature of memory impairment

• Failed to remember the events of daily life.
• Spatial
• Temporal
• Episodic
• BUT made progress in School

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Episodic Memory
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Semantic Memory
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Developmental Amnesia

• Selective pattern of impairments affecting new
  episodic but not new semantic memories
• Bilateral hippocampal damage
• No obvious damage to cortex surrounding the
  hippocampus
• entorhinal
• perirhinal
• parahippocampal

cortices
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Developmental Amnesia

• Episodic memory impairment -
• due to hippocampal pathology
• Intact semantic memory
• due to integrity of surrounding cortices

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Alternative possibilities for limited form of
memory deficit in developmental amnesia

• Partial sparing of hippocampal formation
• OR
• Functional reorganisation and compensation after
  early damage
• not possible with damage acquired later in life

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Test of the early injury hypothesis
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Test of the early injury hypothesis

• If timing of hippocampal lesion was the
  determining factor, then the late onset group
  should have impairments similar to the amnesia
  seen in adults

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Findings

• Memory impairment in the late-onset group was
  immediately apparent after the injury
• Otherwise, few significant differences
• Equally impaired on episodic memory tests
• Equivalent sparing on semantic memory tests
• Late group worse on immediate memory

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Early vs. Late Injury Episodic Tests
Normal range
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Early vs. Late Injury Semantic Tests
Normal range
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Conclusion re Age of Lesion

• If developmental amnesia is a special syndrome
  related to early lesion of the hippocampus, the
  effective age at injury can extend to
  adolescence.

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Another Feature of Developmental Amnesia

• Intact recognition
• Not usually seen in adult amnesia (we will see
  the exceptions later)

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Delayed Recognition Tests

• Forced choice recognition test (5 min. delay)
• Yes/no recognition test (5 min. delay)
• Jons performance approximated 2 age-matched
  controls and exceeded that of amnesia group

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Normal Recognition / Impaired Recall
Visual
Verbal
Amnesic
controls
Lower limit of normal range
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Normal Recognition / Impaired Recall

• Problem Measures differ in difficulty
• Solution Doors and People Test
• Recall and recognition of visual and verbal
  stimuli designed to be or equal difficulty
• Jon Recall
• Recognition 50 75th percentile
• Controls 50 75th percentile on both recall and
  recognition

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Continuous Recognition Test

• Sequence of words is presented
• Test of running recognition
• Decide when each word is presented
• Decide where it has been shown before
• Repetitions can occur immediately or after
  1,2,4,8, or 16 intervening words
• Results
• Score 97.3, exceeding controls and mean for
  normal subjects
• Also had a higher false alarm rate

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Neuropsychological evidence for a
recall-recognition dissociation

• A number of atypical cases have been reported in
  which recognition is preserved in amnesic
  patients
• Bilateral fornix damage
• Damage to mamillary bodies
• Patients who sustained anoxia
• Alcohol-induced Wernickes encephalopathy
• Anterior communicating artery aneurysm
• Possible preservation of recognition memory in
  amnesics when damage is limited to the
  hippocampus or its diencephalic targets

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Implications of test results

• Despite clear evidence of amnesia, Jon has a well
  preserved recognition memory
• He is atypical from adult-onset amnesia in that
• He has a significant recall-recognition disparity
• He can increment semantic memory despite his
  episodic memory deficit

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• Perhaps its not his ability to recall per se
  thats impaired but his ability to use episodic
  memory to facilitate recall of newly acquired
  information
• Can recall names of historic figures, word
  definitions, discuss current political issues at
  length

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Another Test

• Since Jon has acquired an extensive amount of
  semantic knowledge, learning should occur when
  naturalistic material is presented repeatedly
• Presented with a series of video-based newsreels
• One presented 4 times in 2 days, other only once
• Recall and recognition tested
• Immediately after final presentation in each case
• 18 hours later
• Results
• Immediate recognition comparable to controls
• Delayed recognition slightly below controls
• Immediate recall much lower than controls
• With 4 presentations, immediate recall increases
  substantially

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Remember vs. Know

• Words presented and tested by yes/no recognition
• For yes answers, do you remember or know?
• Ask why they remember on some questions
• Results
• Overall Hit rate .917
• Jon used the remember judgment more than
  controls
• When questioned, he said he looked at the word,
  tried to imagine it printed on the stimulus card,
  and if a clear image came to mind he remembered
  it
• May lack ability to recall contextual detail
  consistent with a remember response

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Discussion

• While recognition performance is in the normal
  range, his performance may still be impaired
• Initially tempting to conclude that dissociation
  between recall and recognition reflects 2 memory
  systems, but shouldnt because
• Single dissociation
• Recall and recognition not direct measures of
  memory systems
• John is able to recall a lot of semantic
  information

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Discussion

• Consider findings in terms of processes
  underlying recall and recognition
• 2 underlying processes of recognition memory
• Recollective (remember)
• Automatic (know)
• Failure to teach Jon the R/K distinction suggests
  he cant use the recollective source of evidence
• Jon shows an electrophysiological index
  associated with stimulus familiarity, but lacks
  one for episodic recollection during recognition
  tasks

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Discussion

• Recall involved in well-established semantic
  memory does not require the recollective process
• Episodic recollection important to recalling new
  material
• Therefore, an impairment in episodic recollection
  could explain his poor performance on standard
  recall tests
• May explain difficulty in initially recalling
  items that improves with practice
• May have acquired large store of semantic
  information through repetition
• His preserved abilities may also be a result of
  the developmental nature of his deficit

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An unusual case of amnesia

• Neil suffered from a pineal tumour at age 13.
• Three impairments
• Alexia,
• Visual agnosia
• Amnesia profound in all sensory modalities
• Able to retrieve postmorbid memories through
  writing without having any awareness of the
  content of his written report.

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Memory Retrieval Through Writing

• Anterograde amnesia is usually attributed to a
  defect in the mechanism responsible for placing
  newly acquired information into a long-term
  store.
• The same interpretation cannot be applied to
  Neils disorder given his ability to retrieve
  memories through writing even though he is unable
  to give an oral account of the contents of his
  writing.

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The dissociation in Neils memory performance
raises two possibilities

• There are normally 2 different long-term stores
  for the same information, or
• The information in any given long-term store can
  be retrieved through two different response
  modalities.