21 - Semantic Memory Flashcards
What is semantic memory?
- Stored conceptual knowledge about the world, objects, word meanings, facts and people.
- Not connected to any particular time and place (i.e. to an episode)
- Shared across individuals in a culture
What are some ways you can test for semantic memory?
- Picture naming
- Word – picture naming
- Drawing from memory
- Describe/define a concept
- Answer questions about things
- Sort pictures according to category
- Match items according to semantic associations
What are the four types of pathology semantic memory impairments are associated with?
- Semantic dementia (SD); one of the syndromes caused by fronto-temporal lobar degeration
- Herpes simplex viral encephalitis (HSVE)
- Stroke (left hemisphere)
- Alzheimer’s disease
What is herpes simplex viral encephalitis?
- Viral infection (herpes simplex) that affects the brain, particularly the temporal and frontal lobes, resulting in widespread necrosis.
- Differs in extent from patient to patient and can be bilateral or unilateral
What kind of deficits do people with herpes simplex viral encephalitis have?
Can be;
Category-specific semantic deficits
- E.g. when asked to provide definitions for words
- Have more deficits in living things than non-living things
Modality-specific
- E.g. patient TOB could not produce information about living things in response to their spoken names, but could in response to pictures. He could produce information about non-living objects to both spoken name and pictures.
State the two views of the organisation of semantic memory?
- Distributed-only view
- Distributed-plus-hub view
Describe the distributed only view of the organisation of semantic memory
- Semantic memories are stored in areas involved in perception and action
- Different attributes are distributed around this network
- If you damage certain parts of the network, you might lose information about certain parts of the object.
The task determines which part of the representation is accessed. Category and modality-specific impairments are caused by loss of part of the network for a concept, but not all the network.
Describe the distributed-plus-hub view of the organisation of semantic memory
- All tasks requiring semantic memory are mediated by a representation that is independent of the task (i.e. shared by all tasks)
- Allows for generalisation among members of a category
What are the symptoms of semantic dementia?
- Progressive, selective loss of semantic knowledge when tested in any modality
- No category-specificity
- Profound loss of word meanings; evidence in comprehension and production (empty speech)
- Inability to recognise objects (agnosia)
- Other cognitive abilities and other aspects of language seem to be much better preserved in activities of daily living
How is semantic dementia characterised in terms of brain neuropathology?
Severe degeneration of anterior and lateral temporal lobe, bilateral but often asymmetric (and left > right)
What did the experiment which compared HSVE and semantic dementia patients (who were matched on semantic impairment) find in terms of the differences in their deficits?
- Naming pictures; SD patients have lower performance when they have to produce the name of an object. HSVE have deficit with animals
- Patterns of errors; making semantic confusion (dog is cat), herpes patients make more of these errors. Other errors are general labels (everything is an animal), semantic dementia patients made more of those errors
In a model produced to represent HSVE and semantic dementia symptoms, what was lesioned in the model to produce similar results to previous experiments?
“Lesioned” the model in two different ways that mimic the neural changes in SD vs HSVE
- For SD, removing a proportion of connections in the network
- For HSVE, introducing noise in the weights of the connections
What are the main differences in symptoms with HSVE and semantic dementia?
HSVE; Category-specific deficits
- Representations are distorted and items are easily confused if they have overlapping representations (e.g. living things)
Semantic dementia; gradual loss of specificity
- When representations dimmed, items are only recognised at a coarse level
