Download Memory (László Acsády)

Memory (László Acsády)
Part 1 – Features of the memory trace (engram)
 Engram:
 persistent alteration in the brain as a result of a specific event
 its content is linked to the information perceived during encoding and
predicts what can be recalled later
 ecphory – the ability to change behavior when reactivated with the
appropriate keys
 “life cycle”:
 consolidation: the system to store memory traces by spontaneous
activity of the brain
 reconsolidation: significant stimuli, previously consolidated
memories being recalled and new consolidation is needed
 dormant state between encoding and retrieval
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Plasticity and the substrate of engram:
 nuclear ↔ synaptic (LTP = Long-Term Potentiation) → dendritic spines →
neuronal networks → brain regions, memory systems
Replay of the engram during sleep and memory consolidation:
 after encoding the engram reactivates spontaneously during sleep
 the degree of reactivation is correlated with the degree of engram retention
 inhibiting reactivation impairs the memory trace
 sensory stimuli during encoding may recall the engram at a later phase
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Labeling (dormant) engram cells:
 activity dependent neuronal labeling
 immediate early genes: c-fos, Arc
 linked to markers: Channelrhodopsin, GFP, LacZ
 Tet Tag method
→ result – specific markers are expressed in the cell population which was
activated during the learning situation
 optogenetic activation and inactivation of labeled engram cells:
 yellow light, bad condition – inhibited engram neurons → less fear
and freezing
 blue light, good condition – activated engram neurons → more fear
and freezing
→ context specific excitation/inhibition of engram cells alters behavior
 dendritic spines – involved in coding an engram
Allocation: inserting given neurons to the engram
 more than average depolarized neurons are easier to allocate
 step function opsin and DREADD experiments
Semon’s four postulates of engrams and its studying
1. structural basis (long-term Ca-imaging, electron microscopy)
2. content (recording the activity of neuronal networks, fMRI)
3. ecphory, the effect on behavior (opto-, chemogenetic experiments)
4. sleeping engram (opto-, chemogenetic experiments)
Part 2 – Memory types, memory systems
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case of HM (Henry Gustav Molaison)
 lobectomy, due to epilepsy, involved the majority of hippocampus
 old memories remained
 memory of events impaired
Motor/Procedural/Implicit memory:
 requires training, practice
 can be recalled in situations identical to the
learning situation (rigid)
 can be distracted by learning similar things
 can display plasticity ii case of brain damage
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Event/Declarative/Explicit memory:
 requires a single event
 can be recalled in various situations
 more difficult to distract
 no recovery after damage
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Fear, reward memory
 significant events require single association
 can be recalled in situations identical to the
learning situation
 highly pronounced vegetative responses