[Neuroscience] Semantic and Episodic Memory #1

The human hippocampus

• The hippocampus("seahorse") was given its name by Bolognese anatomist Giulio Cesare Aranzi (circa 1564)

++we talked about this in the vision lecture, "Jennifer Aniston Cell" -respond to Sydney opera house…

 

Hippocampus & Memory

• Modern studies of the hippocampal role in memory began with studies of the famous neurological patient H.M.
• Large portions of H.M.'s medial temporal lobe were resected(절제수술) to treat his severe intractable epilepsy
• Scoville and Milner (1957) studied H.M.'s resulting memory loss

++ to treat the seizure - remove the focus where the seizure starts, and remove the brain tissue, -- operate normally talk, normal IQ --> memory loss

 

Henry Molaison (1926 ~ 2008)

 

H.M.'s Memory Impairment

• H.M. exhibited severe anterograde amnesia for almost all “declarative memories” of facts and episodes that he encountered after his operation

++ cant acquire new life experience as a memory and store them

• H.M. also exhibited temporally-graded retrograde amnesia; he showed better memory for “declarative” information acquired long before surgery than recently before surgery

++ loss of memory before the operation (he can remember distant memory, not recent memory)

• Procedural learning (e.g., motor skill learning) was found to be normal in H.M.

++ could learn new motor skills

 

Mirror Tracing

• Subject must trace a shape on a piece of paper while watching their hand in a mirror (so their view is backwards)
• This is a motor skill learning task, and subjects learn to get better at it with practice
• H.M. and other hippocampal amnesics learn and retain this motor skill normally

++ H.M. could learn this task, but could not remember having this practice before

 

Declarative vs. non-declarative memory

• Two kinds of intermediate and long-term memory:

• Nondeclarative (procedural) memory deals with how—shown by performance rather than conscious recollection (things we may not be aware we have learned).

++ how to ride a bike, mirror tracing, playing the piano

• Declarative memory deals with what—memory for facts and past events acquired through learning that can be stated or described in words (things we are consciously aware of knowing and remembering).

 

Taxonomy of memory

• Two subtypes of declarative memory:
• Three subtypes of nondeclarative memory:

 

Animals models

• Do animals have “declarative” memory? Without language, they cannot “declare” anything! And yet, all mammals have a clearly identifiable hippocampus with very similar anatomical structure to humans. So, what do animals use their hippocampus for?
• Primates: Early studies showed that primates were impaired in a delayed-nonmatch-to-sample (DNMS) memory task (Mishkin, 1978)
• Rodents: Early attempts at finding hippocampal memory impairments in rodents were not successful…

 

Delayed Non-Matching-to-Sample (DNMS) Task

• Monkey is presented with two objects; food is hidden under one of them
• After picking up the object to take the hidden food, the monkey has to wait for a delay period lasting anywhere from a few seconds up to several minutes
• After the delay period, the monkey is shown the same two objects again; food is hidden under the object that was NOT rewarded on the prior trial (the non-matching object)
• In these experiments it is observed that:

 

DNMS Performance after Medial Temporal Lobe Lesions

• Lesions of hippocampus cause impairment in DNMS task
• Impairment is more severe after larger lesions of hippocampus + entorhinal cortex

 

(RODENT) Hippocampal Lesions Impair Context but not Tone Fear Conditioning

• A: Rats with hippocampal lesions do not freeze to the training context (contextual fear - 환경적 공포)
• B: Rats with hippocampal lesions still freeze to the auditory tones CS

Environmental cues vs aversive experiences

++ you put the rodent in a specific location - the rodent explore for few min, figuring out the enviornment (smell) - > food truck  associates the context with the shock --> the rodent will freeze because of the context (shock is associated with the context)

++ different result -> tone fear (shock is associated witht the auditory tone) -> rat will learn that whenver the auditory tone comes out, the shock will come -->

 

Systems consolidation: Hippocampal lesions cause temporally graded retrograde amnesia for context fear

• Just as humans with hippocampal damage show temporally graded retrograde amnesia (left), rats show less impairment of context fear if hippocampal lesions are given long after training versus immediately after training. Hence, lesions impair recent but not remote memories.
• HIPPOCAMPUS -> CORTEX : One possible explanation for temporally graded retrograde amnesia is that declarative and episodic memories may initially be stored in the hippocampus, and then transferred to the cortex, a process called systems consolidation

++ more recent memory is stored in the hippocampus, and older memory is progressively transferred to other parts of the brain (prefrontal cortex)