Topic 6-longterm memory

• Memory involves multiple:

  • processes
  • types
  • systems
  • tasks
• LTM processes:

  • encoding (initial creation of memory traces in brain)
  • consolidation (continued organization of memory traces)
  • storage (retention of memory over time)
  • retrieval (accessing stored information from memory)
  • reconsolidation (possible reorganization after retrieval)
• Short-term and long-term memory:

  1. Short-term
  2. seconds
  3. extremely limited
  4. sustained activation of neurons
  5. Long-term
  6. minutes, hours, days
  7. massive
  8. number and strength of synapses
• Implicit long-term memory:

  • non-declarative
  • independent of conscious awareness
  • procedural, conditioning, nonassociative, or priming
• Explicit long-term memory:

  • declarative
  • available to conscious awareness
  • semantic or episodic
• Non-associative memory:

  • change in response to an unchanging stimulus
  • Habituation
  • reduced response to an unchanging stimulus
  • Sensitization
  • increased response to an unchanging stimulus
  • primarily involved in sensory-motor reflex pathways
• Habituation:

  • Pre-synaptic depression
  • same action potential
  • reduced neurotransmitter release
  • smaller EPSP
• Sensitization:

  • Pre-synaptic facilitation
  • same action potential
  • increased neurotransmitter release
  • larger EPSP
• Habituation and sensitization:

  • Short-term changes
  • last for minutes
  • changes in amount of neurotransmitter released
  • Long-term changes
  • changes in number of synapses
• Classical conditioning: an initially neutral conditioned stimulus (CS) comes to elicit a conditioned response (CR) due to pairing with an appetitive or aversive unconditioned stimulus (US) which reflexively elicits an unconditioned response (UR)
• Fear conditioning:

  • EPSP to tone increases in lateral nucleus of amygdala after repeated pairing with shock
• Motor skill learning:

  • motor adaptation
  • cerebellar loops
  • motor sequence learning
  • cortico-basal ganglia-thalamocortical loops
• Cerebellum:

  • uses forward model to predict results of motor commands
  • uses differences between actual results and predicted results for online error correction and motor learning
• Motor sequence learning:

  • depends on basal ganglia and reinforcement learning
  • Prediction error = actual reward - predicted reward
  • dopamine signal from substantia nigra represents prediction error
  • large response = better than expected
  • small response = worse than expected
• Reinforcement learning:

  • unexpected rewards generate dopamine signals from the substantia nigra
  • this excites the direct pathway and inhibits the indirect pathway
  • this allows modification of behaviour based on reward
• Priming: changes in stimulus processing due to prior exposure to same or related stimulus without conscious awareness
• Perceptual priming:

  • Korsakoff's syndrome patients
  • amnesia due to severe alcoholism
  • show fragmented pictures (Gollin figures)
  • participants improve day to day despite not remembering the previous day's training
• Priming:

  • depends on region of cortex processing relevant representations
  • Perceptual priming
  • sensory cortices
  • Conceptual/semantic priming
  • unimodal and multimodal association cortices
• Semantic memory: memory for concrete word meanings activates areas of cortex involved in relevant processing

  • actions: motor cortex/somatosensory cortex
  • sounds: auditory cortex
  • colours and movements: ventral visual stream
• Sensory/functional theory:

  • organization of semantic representations is based on relevant sensory and motor features
• Domain-specific theory:

  • organization of semantic representations is based on semantic categories
• Sensory/functional theory vs domain-specific theory:

  • fMRI data from silent naming of tools and animals
  • categories often correlate with sensory/functional distinctions
• Encoding of episodic memory:

  • hippocampus and related structures form indices to bind cortical representations

  Retrieval of episodic memory:

  • hippocampus and related structures to reinstantiate cortical representations
• Role of medial temporal lobes:

  • Cognitive map theory
  • memory for spatial relationships in environment
  • place cells: fire when animal is in a particular location
• Role of medial temporal lobes:

  • Relational memory theory
  • memory for associations in general
  • odor association task
  • trained to prefer A over B
  • lesion to fornix: leaves single pairs intact, disrupts relationships across pairs
• Role of medial temporal lobes:

  • different roles for different subregions
  • preirhinal cortex
  • binding features of objects
  • parahippocampal cortex
  • encoding spatial layout
  • hippocampus
  • encoding relationships more generally
• Hebbian learning: neurons that fire together wire together

  • when presynaptic action potential leads to postsynaptic action potential, connection is strengthened
• Long-term potentiation:

  • increase in synaptic strength
  • exhibits necessary properties for Hebbian learning
  • found in hippocampus
• Early LTP:

  • increased presynaptic release of neurotransmiter
  • increased number of postsynaptic receptors

  Late LTP:

  • increased number of dendritic spines and synapses
• Long-term depression (LTD):

  • when presynaptic action potential does not lead to postsynaptic action potential, connection is weakened
  • reduction in neurotransmitter released, number of receptors, and number of synapses
• Episodic retrieval = reconstruction

  • retrieval of episodic memories is reconstruction
  • a best guess based on memory trace, genes, past experience, internal state, environmental context
• Episodic retrieval can be inaccurate for many reasons, including:

  • semantic relatedness
  • cultural experience
  • source misattributions
  • pragmatic inferences
  • misleading post-event information
• Amnesia:

  • severe anterograde amnesia
  • unable to form new LTMs after surgery
  • temporally-graded retrograde
  • unable to recall existing LTMs from just before surgery
• Consolidation:

  • new memories depend on representations in cortex and links from hippocampus
  • reactivation due to retrieval and replay during sleep reduce dependence on hippocampus
  • old memories depend on representations and links in cortex
• Why have two memory systems?

  1. Hippocampus
  2. learns rapidly
  3. more important for episodic memory
  4. Cortex
  5. learns slowly
  6. more important for semantic memory
• Reconsolidation: when a memory is retrieved, it is reformed, and once again subject to interference

  • Nader (2000) fear conditioning in rats
• Fear conditioning in rats conclusions:

  • recall/activation leads to reconsolidation
  • if memory formation is blocked during reconsolidation, then the memory is forgotten
• Which type of memory is primarly used to recall the particular items on the DRM paradigm?

  Episodic
• Semantic priming is best able to describe the false memory lure for items in the DRM paradigm
• False memory effects that occur in the DRM padadigm suggest that retrieving episodic memories is reconstructuve, involving making a best hypothesis based on the evidence from memory plus prior experience