Hippocampus & Memory Formation

Question 1

Clive Wearing

Answer 1

• Famous pianist who suffered from hippocampal damage later in his life. He can still play the piano, emotional reactions to his wife, language unaffected, but unable to form any new memories.
• Damage was caused by a herpes virus

Question 2

Henry Molaison (patient H.M.) - 1953

Answer 2

• Severe case of epilepsy so underwent surgery that removed a large part of his brain – medial temporal lobe. Included hippocampus and region around it, including rhinal cortex.
• He remained of normal intelligence and had no psychological illness. However the surgery resulted in anterograde amnesia = events taking place after surgery are never remembered for more than 60 sec.
• Affected:
  • New declarative. E.g. H.M cannot remember new facts, events, people
• Not affected:
  • Old declarative: e.g. remembers events/people from before surgery
  • Short-term (‘Working’) memory: e.g. remembers names for as long as not distracted
  • Old procedural: e.g. language normal
  • New Procedural: e.g. can learn new sports and skills
  • Tells us that: Hippocampus and surrounding regions are critical for forming new memories
  • Other types of memory unaffected – tells us that there are separate memory systems in the brain
• H.M.’s case inspired intensive animal research into the neural basis of the different memory systems

Question 3

Memory systems

Answer 3

Question 4

Rhinal cortex

Answer 4

• Main input pathway to the hippocampus
• Highly processed sensory data from cortical association areas pass through the rhinal cortex to the hippocampus

⇒ Gateway for convergence of unimodal and multi-modal sensory input to the hippocampus

Question 5

Hippocampus

Answer 5

• High level brain-region: receives highly processed sensory data across all sensory modalities
• It’s the convergence point for highly processed sensory data
• Other sense are similarly complex processing upstream of the hippocampus, except the olfactory inputs that reach the hippocampus much more directly
• Such high-level brain areas are expected to be notorioulsy difficult to understand: presumably responses must be extremely complex?

Question 6

Place cells in animals

Answer 6

• Discovered in the hippocampus by O’Keefe and Dostrovsky (1971)
  
  • Place cells fire when the animal is in a specific region of space (place ‘field’)
  • Multi-electrode recording shows that the place fields of these cells ‘tile’ the environment, providing a spatial ‘map’
  • Hippocampal lesions impair spatial navigation E.g. poor performance on the water maze

Question 7

Place cells in humans

Answer 7

• Maguire et al., 2000
• Compared to experience-matched bus drivers, London taxi drivers have larger posterior hippocampus (equivalent to rat dorsal hippocampus)
• The more experience the drivers have the larger their posterior hippocampus (not so for bus drivers)

Question 8

Hippocampus and spatial context for memories

Answer 8

• The role of the hippocampus goes beyond spatial memory (HM could form no new declarative memories, not just routes/locations)
• More general role in memory formation?
  
  • Evidence suggests its involved in linking memories to where (and maybe when) they occurred
  • E.g. Miller et al. (2013) Recorded neuronal activity from epilepsy patients with electrodes implanted in hippocampus
  • Place-responsive cells were active during virtual navigation (delivery game), but were also activated during subsequent verbal recall
  • This occurred without actual navigation/visual cues

⇒ Hippocampus provides spatial context for memories

Question 9

What does hippocampal damage tell us about memory?

Answer 9

• There are dissociable systems (not affected: short term memory, implicit memory)
• It paved the way for animal research into neural basis of these systems
• Hippocampus (and surrounding regions) is crucial for forming new memories BUT NOT for recall of older memories (anterograde not retrograde amnesia)

⇒ Memories must become consolidated and hippocampal-independent over time

Question 10

Systems consolidation of memories

Answer 10

Why are older memories not affected by hippocampal damage?

1. Sensory information is processed in hippocampal regions and then integrated as a memory “episode”
2. Recall involves activation of cortex by hippocampus ⇒ Hippocampus “teaches” the cortex the memory trace
3. Over time, memories are ‘consolidated’ and become independent of hippocampal regions (Squire, 2006)