Medial Temporal Memory System:
- Hippocampus
- Associated subcortical structures (mammillary bodies, fornix)
- Surrounding regions (entorhinal, perirhinal cortex and para-hippocampal gyrus –
- Medial surface of temporal lobes curls inwards
- Hippocampus at tip of curled lip
- Readily visible from a coronal section
encode important info that makes up memory
Damage to MTS: Medial Temporal Amnesia
- H.M:
- Other causes:
surgical removal of hippocampus (loss of 10 years of prior memories)
hypoxia, infection, Alzheimer’s disease, Korsakoff’s disease (damage to mamillary bodies)
Medial Temporal Amnesia: Two Components
Retrograde Amnesia:
Anterograde Amnesia:
loss memories for a least one year before illness
cannot remember new facts/events
Retrograde Component: Features
- Impaired period at least…
- Can be more extensive if…
one year prior to illness
damage extends to prefrontal and/or lateral temporal cortex
Retrograde Component: Tests
- Autobiographical Memory Interview
- The more detailed the description..
(childhood, early adult life, recent events)
the higher the score
Retrograde Component: Features
Pattern may be more continuous than..
Patient PZ:
- recall of events from autobiography
- the earlier the memory, greater…
Memory for semantic facts learned before illness is…
absolute > temporal gradient
% correct recall (graded loss)
spared because different parts of the brain are involved, confound of the period choosing the memories from
Anterograde Component: Features - More severely... - Knowledge and personal history are... - Unable to recall events from... - Ways of measuring: 1. ? 2. ? > Exposure phase: Aid, Young, Sake… > Test phase: which did you see? Aid or lamp? 3. ? (highly sensitive – episode-specific linking) > Exposure phase: doctor-apple > Test phase: what word was paired with doctor?
- Both episodic and semantic memory are affected…
- Usually some semantic learning….
- Issue with measuring amnesia…
impaired
effectively ‘frozen in time’
5mins before
Free recall
Recognition
Paired Associate Learning
(cannot remember events after illness onset or facts learned since illness onset), episodic more affected
can occur
general knowledge is learned in early life, memories of early life are preserved, have to test new knowledge (very limited and restricted)
Function of MTS: fMRI Evidence
- Hippocampus critical for…
- Right hippocampus more…
learning associations that occurred on a single occasion
activated on correct trials
Teasing apart the MTS subregions
- MTS incorporates…
- Do these different structures contribute differently to memory?
- Einchenbaum:
> the outer regions can code memories for individual elements, but the HC is crucial for binding the various elements into a coherent experience
> HC crucial for episode specific pairing. Link together elements that occur together once in a single episode. Surrounding regions involved in memorising some elements. HC binds elements together.
- Davachi:
> participants saw words and either (1) generated an image in their head, (2) imagine the sounds of the word backwards
> 20h later asked (1) which words they had seen and (2) which task they had performed (visual or sound imagery)
> perirhinal cortex responded more…
> HC responded more strongly when…
> perirhinal cortex important for…
> HC remembers binding/linking between elements and surrounding areas remember elements, work together in conjunction to create structure memory of the event
HC and surrounding regions (entorhinal, perirhinal cortex & para-hippocampal gyrus)
strongly for remembered than forgotten words > item memory
the participant correctly remembered the task they had performed > memory for entire event
remembering the words
BUT many types of memory/learning occur outside the MTS:
- In medial temporal amnesia, STM may be…
- Skill learning also spared:
- Patient may not be aware of…
- This type of learning without awareness is called…
entirely spared (digit span task)
learning piano pieces, mirror tracing
having done the task before
IMPLICIT memory!
Other Types of Implicit Learning that don’t require MTS:
Perceptual learning/repetition priming:
- severe amnesiacs learn…
- effect lasts…
Word stem completion:
- study words (e.g., motel cyclone) > delay (2h) > complete: mot_____
- participants will choose…
- effect lasts…
- preservation of…
slightly more slowly than normal months or years motel rather than motorcycle 1 to 2h motor skills and perceptual learning
MTS Conclusions
- vital for establishing…
- key role in retrieval of…
- vital for….
- HC important for developing…
- crucial for awareness…
new LTM
previously acquired memories
acquiring new episodic and semantic memories
episodic associations (binding together the elements of an experience)
of prior learning
Consolidation Theory
- all aspects of an experience cause…
e.g., gondola experience – multi-modal experience, visual scene (high-level visual processing in extrastriate, temporal and parietal), HC integrates and binds the experience together
- …but these one-off changes usually…
- HC/surrounding structures can rapidly form…
> creates a new “network” which…
- then if you activate one component, activation will…
- notes: temporary changes happen in the cortex during an experience, HC binds together the elements of the experience, there is a network established between HC and the experiences, when you think about one element of the experience, it’ll remind you about entire experience
- need HC to retrieve memory > but repeated recollection will…
- notes: reason amnesiacs have memory of early life, the more times an experience is recalled, the less the HC is required. memory recalled repeatedly start to strengthen the cortical connection and association independently of HC. key factor that makes memories resilient to HC damage is recollection!
changes in the relevant cortical areas…
…not enduring
new connections between itself and these cortical areas
binds the components
spread throughout the network, ‘re-revoking’ the entire experience
strength the direct connections between cortical components > eventually able to recall the entire experience without the HC
Multiple Trace Theory
- repeated recall changes…
- each time you recall a memory, you create…
- the memory is becoming less like…
- notes: memories of experience become less accurate each time recall it, natural decay of LTM, activate parts of original memory and recall a new memory for the event of recollection
the character of the memory
a new memory (trace)
an experience and more like a “fact”
What happens to the older ‘semanticised’ memories?
- semantic dementia is the exact opposite of…
> memory for facts more…
> the earlier the memory was learned, the more likely…
> pathology focussed on…
MTA
impaired than for events
it is to be affected
tip of temporal lobe
So…
- medial temporal lobe (HC) is important for…
- lateral temporal lobe seems (temporal pole) important for…
- the two aren’t independent: need MTS (the memory for the specific experience) before able to…
memory of unique episodes
older and/or more semantic memories
start building a more semantic memory
Other systems that support LTM:
- visual imagery (most resistant to decay)
> D.H:
- fMRI study: regions involved in imagining past experiences.
> HC activity is…
- ventral PFC: patients confuse recent with more…
problems with tasks involving visual retention and bilateral damage to occipital lobes
early, activity of visual areas occurs later
remote memories and personal experiences with fictional ones
sense of a stable self throughout time seems important for personal memories
MTS:
Lateral Temporal Lobes:
Posterior Visual Association Areas:
Ventral PFC:
Default Mode Network:
encoding and retrieving single-trial episode-specific associations
knowledge extracted from multiple experiences
reconstruction of visual imagery
connecting sense of self to one’s own memories
network of structures (HC, ventral PFC, anterior temporal lobe, inferior parietal lobe) activated when no activity (close eyes)
