L4 Neuropsychology

• 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 can make undergeneralisation and overgeneralisation errors

  
• 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 when a semantically-related word facilitates the processing/identification of a target word
• Imageability


  The ease with which a word or concept can be visualized
• Imageability is an important variable implicating semantics, with high imageability words (e.g. CHAIR) processed differently than low imageability words (e.g. JUSTICE)
• Some semantic dementia patients display a "reverse concreteness effect", performing better on abstract words than concrete words

  
• The reverse concreteness effect in semantic dementia is not consistently found across tasks and may be rare

  
• Semantic memory impairment


  Can occur due to damage to semantic/conceptual representations themselves or damage to access to those representations
• Semantic aphasia refers to disorders of semantic memory access, characterized by inconsistency, sensitivity to presentation manipulations, and effects of priming and mis-cueing

  
• Category-specific impairments in semantic dementia are very rare, with only a few cases reported of better performance with living or non-living things

  
• Proposed explanations for category-specific deficits, such as the sensory-functional theory, have limitations and may be too simplistic
• Cree and McRae's multiple feature approach can explain various patterns of category-specific deficits by considering the relative reliance on different types of semantic properties
• Sensory and functional properties

  May be too simplistic a distinction
• Cree and McRae's (2003) Multiple Feature Approach

  • Argued for subdivisions of the two properties
  • Farah and McClelland's sensory property is divided into: Visual, auditory, taste, and tactile sensations
  • Farah and McClelland's functional property is divided into: Entity behaviors (what a thing does), Functional information (what humans use it for)
• Deficit patterns that Cree and McRae's (2003) approach can explain

  • Multiple categories consisting of living creatures (impaired by damage to visual motion, visual parts, color)
  • Multiple categories of nonliving things (impaired by damage to function, visual parts)
  • Fruits and vegetables (impaired by damage to color, function, taste, smell)
  • Fruits and vegetables with living creatures (impaired by damage to color)
  • Fruits and vegetables with nonliving things (impaired by damage to function)
  • Inanimate foods with living things (impaired by damage to function, taste, smell)
  • Musical instruments with living things (impaired by damage to sound, color)
• 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) obtained ratings for objects in each sensory-motor modality and found verbal listing overestimated visual form/functional knowledge and underestimated other sensory channels (particularly motion & colour)
• Hoffman & Lambon-Ralph (2013) found 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

  
• These results suggest man-made artifacts are not a single category
• Current imaging evidence suggests a complex pattern of neuroanatomical specialisation in object representation, which cannot be accounted for by a simple sensory-functional dichotomy
• Processing an object's visual characteristics activates ventral occipitotemporal cortex, auditory properties activate posterior superior temporal gyrus, somatosensory properties activate post-central gyrus, gustatory information activates medial orbitofrontal cortex, visual motion processing associates with middle and superior temporal gyri, and visual form and colour rely on posterior fusiform
• Praxic processing is associated with the parietal lobule, but appears to be neurally distinct from information about the object's function
• Some patients present with a focal impairment of a particular semantic category, but other researchers have argued that semantic representations may be differentiated into different 'kinds' of information or different 'modalities' of information, rather than explicit categories

  
• Not all semantic 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'

  
• Early fMRI studies did not show ATL activation, likely due to signal drop-out, 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 rTMS to ATL disrupted living & non-living, manipulable/non-manipulable items, consistent with a 'hub' principle, while rTMS to inferior temporal lobes induced a nonliving worse than living pattern and greater impairment for highly manipulable items
• Hurley et al. (2012) found semantic dementia cases showed 'blunting' of intra-category boundaries linked to left ATL atrophy, suggesting items are easier to understand "at the generic rather than specific level of representation"