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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 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 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 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 Event/Declarative/Explicit memory: requires a single event can be recalled in various situations more difficult to distract no recovery after damage Fear, reward memory significant events require single association can be recalled in situations identical to the learning situation highly pronounced vegetative responses