L15 - Semantic Memory

Question 1

What is semantic memory?

Answer 1

• Stored conceptual knowledge about the world, objects, word meanings, facts and people - generally easier to test if it’s true
• Not connected to any particular time and place (i.e., to an episode)
• Shared across individuals in a culture

Question 2

How would you test semantic memory?

Answer 2

• Picture naming - e.g. what objects are
• Give a word and match with a picture
  
  • E.g. if has semantic deficit - call camel a horse
• Draw from memory
• Ask to describe of define a concept & answer questions about things
  
  • E.g. How many legs do oysters have?
• Match items according to semantic associations
  
  • e.g. pyramids or palm trees test
• Sort pictures according to category

Question 3

What are the different semantic memory impairments?

Answer 3

• Associated with 4 types of pathology:
  
  • Semantic dementia (SD) – one of the syndromes caused by fronto-temporal lobar degeneration
    
    • Progressive neurodegenerative disease
  • Herpes simplex viral encephalitis (HSVE)
  • Stroke (left hemisphere)
  • Alzheimer’s disease

Question 4

What is herpes simplex viral encephalitis?

Answer 4

• 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

Question 5

What are the different deficits in herpes simplex viral encephalitis?

Answer 5

• HSVE patients sometimes have category-specific semantic deficits (Warrington & Shallice, 1984; many others)
  
  • Typically have more difficulty with knowledge of living things
  • Can be altered depending on where the damage is
• E.g., when asked to provide definitions for words - e.g. living things vs non-living things
• Deficits can also sometimes be 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 picture. (McCarthy & Warrington, 1988)

Question 6

How is semantic memory organised via a distributed-only view?

Answer 6

• Distributed-only view
  
  • Semantic memories are stored in areas involved in perception and action
  • Different attributes are distributed around this network in different nodes
  • 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
  • So depending on where damage is, different aspects will be lost
    
    • E.g. may not be able to identify a word but recognise the picture of the same thing

Question 7

What are some limitations of the distributed only view?

Answer 7

• Can’t capture how we can generalise across members of a category (what is it that tells you two people are in the same category?)
• Can’t explain profound semantic deficits that is seen in semantic dementia

Question 8

What is semantic dementia?

Answer 8

• Progressive, selective loss of semantic knowledge when tested in any modality
• No category-specificity
• Profound loss of word meanings: evident in comprehension & production (empty speech)
• Inability to recognise objects (agnosia)
• Other cognitive abilities (e.g. episodic memory) and other aspects of language (syntax, phonology, pragmatics) seem to be much better preserved in activities of daily living (although other deficits become evident on testing)
• Characterised by severe degeneration of anterior and lateral temporal lobe, bilateral but often asymmetric (and left > right)
  
  • 2/3 starts on the left
  • Atrophy extends to the other lobes eventually

Question 9

What is the distributed-plus-hub view of semantic memory?

Answer 9

• All tasks requiring semantic memory are mediated by a representation that is independent of the task (i.e. shared by all tasks).
• Keep other connections and direct traffic through semantic systems
• Easier to see how a category could emerge (through patterns of activation)
• Explains how semantic dementia patients have large deficits with little damage as the hub is critical within semantic system

Question 10

What is the parallel distributed processing (PDP) network?

Answer 10

• Can test ideas about logical semantic architecture
  
  • Associate something with different attributes (outputs)
  • Can adjust weights in middle to direct to correct output
• Category of bird would emerge from the overlap between instances that occur for ‘robin’ and ‘canary’
• Bird category is different to trees and plants structure
  
  • Some categories will be more crowded than others
• If there is any disturbance in the system, there may be more errors in categories that are crowded, leading to semantic deficits. Less overlap in categories such as transport, so less likely to get impacted by disturbance - explains why non-living things are better preserved??
• With degradation of the network, enough of the structure of strongly represented categories is still available, but specific details lost
  
  • e.g. say that all birds fly (such as penguins)

Question 11

How can the PDP model compare deficits in HSVE and semantic dementia?

Answer 11

• PPT can have damage to the same part of the brain (anterior temporal lobe) but have different issues with specificity
• Lambon Ralph, Lowe & Rogers (2007) directly compared a group of 7 HSVE patients and a group of 8 SD patients on the same battery of tests
  
  • Matched for severity of semantic impairment on a word-picture matching
    task
  • SD = No category specificity
  • Specificity within HSVE ppt
  • SD, make more errors of calling something by a general category name
• Used a computation model to simulate activity in semantic network
  
  • Trained network to match words with visual features
  • Lesioned” the model in two different ways that mimic the neural changes in SD vs HSVE
    
    • SD, damage was simulated by removing a proportion of connections in the network
    • HSVE damage was simulated by introducing noise in weights of the connections
  • When representations are distorted (HSVE), items are easily confused if they have overlapping representations (e.g., living things) → category-specific deficits
  • When representations are dimmed (SD), items are only recognised at a coarse level (most robust to damage) → gradual loss of specificity