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Semantic Dementia Notes

Psychology Notes > Neuropsychology of Memory Notes

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Semantic Dementia (1) Semantic Memory ? general knowledge about the world - memory for facts, ideas, and concepts - associated with a feeling of "knowing" ? gives meaning to our sensory experience - it is culturally shared , not temporally specific + is to a large extent acquired early in life Tulving's taxonomy of memory -> LTM divided into declarative + non-declarative
? declarative - divided between - episodic + semantic MODELS OF SEMANTIC MEMORY ? how are concepts organised in semantic memory ?

Spreading Activation Model ? Collins & Loftus (1975) Notion of logically organised hierarchies is too inflexible Preferable to assume that semantic memory is organised on the basis of semantic relatedness or semantic distance???The length of the link matters - less related concepts are further apart Search involves the spreading of activation This activation spreads to related concepts Strength of activation depends on length of link Activated concepts are easier + quicker to retrieve from memory

Semantic relatedness can be measured by asking people to decide how closely related pairs of words are/list as many members of a particular category
- Those which are produced most often are regarded as most closely related to that category Whenever a person sees, hears or thinks about a concept ? the appropriate node in semantic memory is activated
- This activation spreads most strongly to other concepts that are closely related semantically

Parallel Distributed Processing Model ? McClelland & Rumelhart (1985) Model assumes that info processing takes place through interactions of large numbers of simple processing elements (units) - neurons, which send excitatory or inhibitory signals to other unitsProcessing takes place by the spread of activation among simple, neuron like processing units


Semantic info is not stored as such, but is reconstructed in response to probes , in a process called pattern completion
- Info not input in step by step manner ? info is distributed to all parts of the memory system at once Filling in occurs through the spread of activation among units + their connections
- The outcome depends on the strengths (weights) of the connections
- These are shaped by experience
- E.g. "A canary is..." - this is filling in The model has a feed-forward structure ? activation flows only in one direction
- From units which represent items (e.g. canary) + relations (e.g. is a) through 'hidden' layers to an output layer containing units corresponding to possible completions to 3 constituent propositions e.g. fill ? a canary is a...
- Hidden units combine concept + relation info

Schemas & Scripts Semantic memory doesn't just store info about objects or animals - but also about more complex knowledge bases Schema ? well integrated chunk of knowledge about the world, events, people or actions Scripts ? type of schema relating to typical sequences of event - e.g going to a restaurant Frames ? knowledge structures referring to aspects of the world containing fixed structural info Schemata is useful for 4 reasons: 1) Forms expectations ? e.g. in restaurant expect to be shown table + given menu 2) Enables us to fill in the gaps ? when reading or listening schema allows us to fill in the gaps in what we see/hear - enhances understanding 3) Assists in the perception of visual scenes ? activation of schematic knowledge facilitates visual perception - make inferences about what we expect to see if a given scene 4) Efficiency ? Anderson & Schooler (1991) ? don't need to remember all specific details of all experiences - cognitive efficiency Can amnesics learn new semantic information?
Kitchner et al (1998) o o o o o

Case study of RS - severely amnesic He can recognise some recent famous faces for the time e.g. John Major He can define the meaning of some recently acquired vocan e.g. interent Has some knowledge of recent famous events BUT


He didn't acquire semantic information normally - was slow + laborious
- involving many repetitions HM also used to use 1950's vocab - he could not define new words - e.g. biodegradable
- Attempts to teach him 8 new words failed (Gabrielli et al, 1983)

? Patient R.S's ability suggests that some semantic knowledge can be acquired in the absence of EM
? Seems to support the dichotomy between semantic + EM
? However many amnesics - HM have impairment in acquiring both new episodic info + semantic info
? Semantic problems are not v. noticeable in adult amnesics - little semantic info is learned in adulthood
? Amnesia is seen as a syndrome - not a pure deficit - some have problems in recall + familiarity + therefore both in EM + SM Developmental AmnesiaVargha-Khadem et al (1997)
- 3 patients with amnesia resulting from early events
- Profound amnesia for episodic materials
- Still progressing through school - normal IQ + academic achievement
- Able to comprehend passages + could acquire knowledge about the world
? Seems semantic memory can develop in the absence of EM
? SM may develop normally in developmental amnesia - may be an adaptive thing a- but adults have more difficulties in SM due to EM impairments - more reliant on EM to form SM whereas children adopt coping strategies

Visual Object Agnosia?Def => failure in visual object recognition which is not due to general intellectual impairment, sensory impairment or language disorder
- Word "recognition" here is used to mean 'comprehending the meaning' of the object (e.g. its name or function) Rare neurological disorder Lissauer (1890) ? divided agnosics into 2 subcategories
- Apperceptive agnosics => fail to recognise visually presented objects
+ cannot copy and match - no adequate percept - visual problem (not related to neuropsychology)
- Associative agnosics => fail to recognise visually presented objects yet can copy + match adequately - percept is adequate but no association with stored info ? this could look like a disorder of semantic memory

Warrington (1975) ? first report of an impairment interpreted as an impairment of SM Described 3 patients with cerebral atrophy presenting with progressive anomia
+ impaired word comprehension

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