Semantic memory

• Semantic Memory


  Memory for meaning, facts, and general knowledge about the world
• Classifications of Memory (Tulving, 1989)


  • Episodic (events)
  • Semantic (meaning)
  • Procedural (motor)

  
• Semantic dementia


  • Progressive loss of verbal and non-verbal semantic memory
  • Preservation of other cognitive domains (e.g., working memory, visuo-spatial ability, non-verbal problem solving ability, phonology & syntax)
  • Good orientation and recall of recent events (unlike the amnesia patients)
  • Atrophy to the infero-lateral temporal neocortex (anterior temporal lobes – ATL) with relative preservation of the hippocampus early in the disease

  
• Conversation in Semantic Dementia


  • "Oh, I like to do things outside and things in the house"
  • "what's a hobby?"
• Hierarchical Network Model


  Semantic memory is organized into a series of hierarchical networks, with major concepts represented as nodes and properties/features associated with each concept
• Warrington (1975) describes cases of progressive semantic degeneration (Semantic Dementia) which follows similar principles to this hierarchical model (lower properties lost first)

  
• Hierarchical distance is often confounded with familiarity, and controlling for familiarity greatly reduces the hierarchical distance effect
• Typicality effect


  Verification is faster for more representative member categories, independent of hierarchical distance
• Typical items have more commonalities with their category than atypical items, and categories are fuzzier than the Hierarchical Network Model believed
• Typicality Ratings


  • A German Shepherd is a very doggy dog, while a Pekinese is a less doggy dog

  
• SD patients find it hard to match items on conceptual rather than superficial similarities, and may judge a fox is a dog but a chihuahua is not
• Spreading Activation Model


  Semantic memory is organized by semantic relatedness/distance, with activation spreading from one concept to related concepts, decreasing as it gets further away
• Semantic priming occurs, where a semantically-related word facilitates the processing/identification of a target word
• Imageability


  The ease with which a word or concept can be visualized, an important variable implicating semantics
• Imageability effects are apparent on language tasks tapping semantics, like synonym judgement, reading, repetition, and writing
• Some SD patients display "reverse concreteness effects", performing better on abstract than concrete words, but this is not a consistent finding across tasks and patients
• Semantic memory impairment


  Can occur due to damage to semantic/conceptual representations themselves, or damage to access to those representations
• Disorders of semantic "access" are now often described as Semantic Aphasia
• Category-specific impairments in semantic dementia are very rare, with only a few cases reported

  
• Herpes Simplex Encephalitis patients can show selective impairments for living vs non-living items, or vice versa
• Previous studies on category-specificity failed to control for confounding variables like visual complexity and familiarity
• Sensory-Functional Theory


  Suggests concepts for living things rely more on sensory properties, while concepts for non-living things rely more on functional properties
• The Sensory-Functional Theory is limited, as many properties of living things aren't easily defined as purely sensory or functional
• Multiple Feature Approach


  Subdivides sensory and functional properties into more specific features, allowing it to explain various patterns of category-specific deficits
• Sensory and functional properties


  May be too simplistic a distinction for many properties of living things
• Cree and McRae's Multiple Feature Approach


  Argued for subdivisions of the two properties: Sensory property divided into visual, auditory, taste, and tactile sensations; Functional property divided into entity behaviors and functional information
• Patterns of category-specific deficits that Cree and McRae's approach can explain


  • Any category strongly relying on a damaged property will be impaired
  • Multiple categories consisting of living creatures impaired due to visual motion, visual parts, color
  • Multiple categories of nonliving things impaired due to function, visual parts
  • Fruits and vegetables impaired due to color, function, taste, smell
  • Fruits and vegetables with living creatures impaired due to color
  • Fruits and vegetables with nonliving things impaired due to function
  • Inanimate foods with living things impaired due to function, taste, smell
  • Musical instruments with living things impaired due to sound, color
• Hoffman & Lambon-Ralph (2013) noted that 'verbal listing' used to capture features of object categories (e.g. Cree & McRae, 2003) may not always easily capture sensory knowledge
• Hoffman & Lambon-Ralph (2013) found that their ratings system generated a novel pattern - a clear distinction between different types of artifact, with mechanical devices (e.g. vehicles, appliances, instruments) distinct from other non-living objects
• Determining properties can depend on how you test
• Current imaging evidence suggests a complex pattern of neuroanatomical specialisation in object representation, which cannot be accounted for by a simple sensory-functional dichotomy
• Brain regions associated with processing different object properties


  • Ventral occipitotemporal cortex - visual characteristics
  • Posterior superior temporal gyrus - auditory properties
  • Post-central gyrus - somatosensory properties
  • Medial orbitofrontal cortex - gustatory information
  • Middle and superior temporal gyri - visual motion processing
  • Posterior fusiform - visual form and colour
  • Parietal lobule - praxic processing
• Some patients present with a focal impairment of a particular semantic category, which might suggest semantic memory has such explicit categories as part of its functional architecture

  
• Other researchers have argued that semantic representations may be differentiated into different 'kinds' of information (sensory/functional) or different 'modalities' of information (colour, smell, sound etc.), with features being critical rather than category labels per se
• Not all semantically impaired cases have category specific impairments - semantic dementia cases have a more general semantic impairment

  
• Semantic dementia cases have focal atrophy to the anterior temporal lobes (ATL), leading to the suggestion this region is a 'semantic hub' with modality specific semantic information held in 'spoke' regions linked to it

  
• Early fMRI studies did not show ATL activation, likely due to signal drop-out in these regions, but later studies using distortion-corrected fMRI have confirmed ATL activation in semantic tasks
• rTMS studies inhibiting left or right ATL increased picture naming RTs and affected semantic judgement tasks, with all types of concepts (living/nonliving) affected, suggesting a domain general system

  
• tDCS studies have also shown consequences of ATL stimulation/inhibition on semantic processing
• Pobric et al. (2010) found that applying rTMS to inferior temporal lobes (praxis region) induced a nonliving worse than living pattern and greater impairment for highly manipulable items