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BIO SCI N156: Lecture 19: Hippocampus: Mechanisms of Episodic Memory…
BIO SCI N156: Lecture 19: Hippocampus: Mechanisms of Episodic Memory Formation and Retrieval
H.M.
Two key insights
Over time, the brain systems support declarative memory retrieval change
There are multiple memory systems that can function independently of one another
Damage to hippocampal formation and MTL dramatically impacted new declarative formation, caused retrograde amnesia
Damage to hippocampal formation and other elements of MTL impaired some kinds of memories, but not others
Forms of learning and memory that were unaffected
Short-term memory was intact
Could acquire new skills, e.g., mirror tracing
E.P.
Extensive MTL damage due to viral encephalitis
Intact:
Motor function
Reasoning skills
Mood & affect
Working memory
Retrieval of remote semantic and autobiographical memories
Impaired:
Short and long-term memory
Formation of any new declarative memories
Formation of new semantic information requiring episodic memory
Amnesia
Anterograde
Inability to form new declarative memories following brain injury
Retrograde
Loss of recent memories from time point of brain injury
For H.M., retrograde amnesia was approximately 2 years prior to his surgery
Taxonomy of LTM
Long-term memory
Declarative: things you know that you can tell others
Episodic
Biographical events
Semantic
Words, ideas, concepts
Non-declarative: things you know that you can show by doing
Skill learning
Skills and habits
Priming
Perceptual learning
Conditioning
Classical conditioning
Non-associative learning
Caudate nucleus (striatum) vs. Hippocampus
Striatum
Habitual memory
Hippocampus
Declarative memory
Packard & McGaugh, 1996
Training: two correct solutions of a task:
Response (turn left)
Place (go to the "west" location)
Probe trial reveals which strategy the rat is using
Early in training, hippocampus-dependent place strategy is dominant (more cognitive strategy)
With additional training, caudate nucleus response takes over (more habitual strategy)
Place memory still persists
Episodic memory
Components
What, where, who, when
Places, people, actions - all in unique combination
Can be captured incidentally, without conscious effort
Hippocampus thought to be critical for episodic memory, by providing a link to different regions of neocortex
The indexing theory of episodic memory
Hippocampal neurons form a compressed index of neocortical activity patterns
Does this by associating and storing pattern of neocortical activity during learning via LTP-like processes
Are able to activate the entire memory trace with partial inputs during memory retrieval
Storage of spatial location of objects
Rats with damage to hippocampus explored two objects equally
Control rats explored moved object much more than unmoved object
Automatic capturing information about location of placed objects depended on hippocampus
Remembering context
Control animals have memory of the object and contex in which it occurred, but rats with hippocampus damage do not
Hippocampal-neocortical interactions
Hippocampus can integrate "what, where, and when" information from other regions of neocortex
Sensory info:
Unimodal associative area
Polymodal associative area
First level of integration:
Parahippocampal cortex
Perirhinal cortex
Second level of integration:
Entorhinal cortex
Highest level of integration:
Hippocampus
Info flows from highest level of integration and then loops back to neocortical areas
Pathways through the hippocampus
Multiple input paths into hippocampus from entorhinal cortex
Info generally flows from higher order neocortical areas through perirhinal and postrhinal and entorhinal cortices into hippocampus and back out
Only one output path
CA1
Pattern separation
During memory formation:
Hippocampus performs pattern separation, takes noisy and overlapping inputs from neocortex and store them in neuron populations in hippocampus
Pattern separation can reduce interference in episodic memory
Dentate gyrus plays role in pattern separation
During memory retrieval:
Sparse hippocampus index code can reactivate entire population of neocortical neurons of the primary memory trace via pattern completion
CA3 region is critical site for pattern completion
Hippocampal index
Advantages
The associative connectivity problem
The inference problem