Brain plasticity 2: Learning, memory and amnesia

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The brains response to experience

  • Communication of cells through synapses
  • Learning - experience changes cells
  • Memory - how changes strengthen connections between cells
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Bilateral Medical Temporal Lobectomy

Patient HM

  • H.M. was an epileptic who, in 1953, has his medial temporal lobes removed.
  • His seizures were dramatically reduced - but so was his memory.
  • Mild retrograde amnesia and severe anterograde amnesia. 
  • Types of memory problems were show by which bits of brain were retained.

Amnesia

  • Retrograde: Unable to remember the past (backwards acting)
  • Anterograde: Unable to form new memories.
  • While HM was unable to form most types of new, long-term memories, his STM was intact.
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Assessing HM

  • Digit span - HM could repeat digits as long as the time between learning and recall was within the duration of STM (Add numbers and repeat).
  • Block-tapping memory-span test: This test demonstrated that H.M's amnesia was global - not limited to one sensory modality. (Squarelights come and go and you have to tap them and remember order and location).
  • Mirror drawing task - H.M exhibited improvement with practice. He was able to show memory for skills - demonstrating that he can learn also demonstrated by performance of the rotary-pursuit and a drawing task, although he was not aware of learning.
  • HM readily 'learned' responses. 
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Contributions of HM's case

  • Medial temporal lobes are involved in memory
  • STM and LTM are distinctly separate - HM was unable to move memories from STM to LTM (memory consolidation problem)
  • Memory may exist but not be recalled - HM exhibited a skill he does not know he has learned. Explicit and implicit memory.
  • Explicit - conscious memories, knowing-what
  • Implicit - unconscious memories - knowing how (i.e. HM showed intact motor learning).
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Medial Temporal Lobe Amnesia

  • Not everyone was this form of amnesia are unable to form new explicit long-term memories  - as with the case with HM.
  • Semantic memory (general information) may function normally.
  • Episodic memory ( events that one has experienced) may not function normally. Able to learn facts, but not able to remember doing so (the episode that had been learned).
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The hippocampus and object-recognition memory

The hippocampus and consolidation

  • HM has retrograde amnesia.
  • Perhaps the hippocampus stores memories temporarily in order to consolidate.

Object recognition memory

  • Early animal models of amnesia involved implict memory and assumed the hippocampus was key. 
  • 1970's - monkeys with bilateral medial temporal lobectomies show LTM deficits.
  • Like HM, performance was normal when memory needed to be held for only a few seconds.

Object-recognition and Medial Temporal Lobectomy

  • Bilateral removal of the rhinal cortex results in object-recognition deficits (if damaged).
  • Bilateral removal of the hippocampus produces moderate effects on object recognition
  • Bilateral removal of the amygdala has no effect on object-recognition.
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The hippocampus - theories of function

The Hippocampus

  • Rhinal cortex plays an important role in object recognition.
  • Hippocampus plays a key role in memory for spatial location.

Theories of Hippocampal Function

  • Cognitive map theory - constructs and stores allocentric maps of the world.
  • Configural association theory - involved in retaining the behavioural significance of combinations of stimuli.
  • Involved in recognizing spatial arrangements of objects.
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Where are memories stored?

  • Each memory is stored diffusely throughout the brain structures that were involved in its formation.
  • Hippocampus - spatial location.
  • Rhinal cortex - object recognition.
  • Medidorsal nucleus - Korsakoff's 
  • Basal forebrain - Alzheimer's disease.
  • Damage to a variety of structures results in memory deficits.
  • Inferotemporal cortex - visual perception of objects.
  • Amygdala - emotional learning - leads to lack of learned fear.
  • Prefrontal cortex - temporal order of events and working memory.
  • Cerebellum - sensorimotor tasks - conditioned eyeblink.
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