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Memory Systems . Chapter 23 Friday, December 5, 2003. What is Memory?. Memory is a change in the synapses of the neurons comprising a network. Memories are not static “records” but products of a change in the connections among nerve cells.

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Memory systems l.jpg

Memory Systems

Chapter 23

Friday, December 5, 2003

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What is Memory?

  • Memory is a change in the synapses of the neurons comprising a network.

  • Memories are not static “records” but products of a change in the connections among nerve cells.

  • Memories extract and store the meanings of events, not their exact details.

    • Sensory aspects are reexperienced not recalled.

    • Recall is reconstructed, not just retrieved.

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The Search for the Engram

  • Engram – the physical memory trace.

  • Lashley couldn’t find one:

    “This series of experiments…has discovered nothing directly of the real nature of the engram. I sometimes feel in reviewing the evidence on the localization of the memory trace, that the necessary conclusion is that learning just is not possible.” pg 276 of text

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Where is Memory?

  • Both cortical and subcortical structures are involved in creating memories.

  • Although memories appear to be stored in the cortex, they are:

    • Distributed – spread out in a network

    • Redundant – represented in multiple ways

  • Hebbian cell assemblies (loops) permit access at various points in a network.

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Stages of Memory

  • Sensory buffers – one for each sense:

    • Iconic – for visual information

    • Echoic – for auditory information

  • Working memory/short term memory:

    • Limited capacity – 7 plus or minus 2

    • Capacity can be expanded by chunking

    • Executive functions

  • Long term memory – permanent.

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  • Retrograde amnesia – inability to recall previously stored memories of one’s life.

  • Anterograde amnesia – inability to form new memories of events in one’s life.

    • H.M. -- surgery removed parts of the temporal lobe, including hippocampus.

  • With amnesia, some kinds of memory are spared, others impaired, suggesting there are different kinds of memory.

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Working Memory

  • Unimpaired in amnesics.

  • In humans, localized to frontal lobes.

  • Delayed development of frontal lobes in humans and monkeys:

    • “A not B” task

    • Can’t tolerate delays (8-10 sec) until 1-1/2 yrs old

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Infantile Amnesia

  • Inability to remember events of one’s childhood before ages 3-4.

    • 10 year olds couldn’t remember preschool classmates

    • Attributed to immaturity of memory systems in children.

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Long Term Memory

  • Localized to temporal lobes:

    • Hippocampus – memory formation and consolidation

    • Entorhinal cortex (adjacent to hippocampus)

  • During memory storage the temporal region forms a connection between the relevant networked areas of the brain.

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Procedural vs Declarative

  • Procedural memory – memory for how to do things, skills.

  • Declarative memory – memory for abstract knowledge, facts and events of one’s life.

  • Only memory for events of one’s life is affected by amnesia – not procedural memory or memory for facts.

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Explicit vs Implicit

  • Implicit memory – does not involve conscious awareness – unimpaired in amnesics.

    • Procedural memory

    • Priming, recognition tasks

    • Classical conditioning, habituation

    • Semantic memory

  • Explicit memory – involves conscious attention – impaired in amnesics.

    • Episodic and autobiographical memory, recall

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  • A form of learning:

    • Classical (Pavlovian) conditioning – present at birth, unimpaired in amnesics.

    • Operant (instrumental) conditioning.

  • How are learned associations maintained?

    • Amygdala (and emotional arousal) mediates activity of the hippocampus to form neural connections.

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Types of Amnesia

  • Damage to Temporal Lobe – H.M.

  • Damage to Thalamic Structures – N.A.

    • Most impaired with verbal material.

  • Korsakoff’s Syndrome

    • Chronic alcoholics with thiamine deficiency

    • Retrograde and anterograde amnesia.

  • Electroconvulsive Shock Therapy (ECT)

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Amnesia and Forgetting

  • Forget at normal rate – thalamic (diencephalon):

    • N.A.

    • Korsakoff’s patients

    • Monkeys with medial thalamic lesions

  • Forget rapidly – hippocampus:

    • H.M.

    • Bilateral ECT

    • Monkeys with amygdala/hippocampus lesions

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Semantic Memory

  • Unimpaired by anterograde amnesia.

    • Children with amnesia develop normal semantic memory.

    • Intact entorhinal cortex may be used to store semantic memories despite hippocampal damage.

  • Context-rich, episodic memory impaired.

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Cellular Mechanisms

  • Habituation involved suppression of action potentials through decreased release of neurotransmitter – change is at the synapse.

    • Sensitization – increased neurotransmitter release

  • Explicit memory involves long-term potentiation, LTP (a physical change to neurons) in hippocampus and neocortex.

    • See Chapter 24 for details