Retrograde Amnesia

Retrograde Amnesia

1. Definitions

   • Retrograde amnesia - Loss of memory that was acquired before onset of memory impairment
   • Recollection- process of comparison of information with memory
   • Recall - depends on recollection. search of memory then a comparison process when something is found

2. Hippocampus

   • Importance found due to H.M when this part was removed and impairment of memory was apparent
   • important for declarative especially episodic memory, important for spacial memory.
   • Part of temporal lobe
   • Hippocampus is connected to all cortical areas, either directly or via parahippocampal, entorhinal or perirhinal cortex (parts of medial temporal lobe)

3. Consolidation - Consolidation means “to make firm” – refers to processes that happen to stabilize short term memory into long term memory

   • system consolidation - where hippocampus-dependent memories become independent of the hippocampus over a period of weeks to years
   • synaptic consolidation - occurs within the first few hours after learning or encoding
   • consolidation theory
     • Consolidation / structural reallocation such that (early) memories eventually become independent of the hippocampi/med. temporal lobes
       • implication - Older memories (semantic or episodic) spared from the effects of focal medial temporal damage.
     • (presumably through rehearsal and simultaneous co-activation), cortical areas acquire the ability to interact with each other such that the hippocampus is no longer needed to retrieve the memory
     • problem - Nadel & Moscovitch (1997)* pointed out that in some patients with medial temporal lobe amnesia

4. Multiple Trace Theory

   • The hippocampus always involved in the storage / retrieval / reactivation of episodic (but not semantic) memories.
     • Implication - Large medial temporal lesions will severely impact on episodic (but not semantic) memory retrieval, irrespective of the time of initial encoding.
   • Each recall creates a new different but related ‘trace’ in the hippocampus > remote memories (which have been recalled many times) will have multiple replications of traces, distributed over hippocampus, and thus become resistant to partial hippocampal damage
   • Complete hippocampal lesions will equally affect remote and recent memories (flat gradient) – thus some patients with medial temporal lesions lose even 40 year old memories