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Where is the semantic network?. What is the semantic network?. Knowledge of objects, people, concepts and word meanings Spreading-activation theory of semantic processing (Quillian, 1962; Collins & Loftus, 1975)

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slide2

What is the semantic network?

  • Knowledge of objects, people, concepts and word meanings
  • Spreading-activation theory of semantic processing (Quillian, 1962; Collins & Loftus, 1975)
  • Other models also theorise how the knowledge is represented within the mental lexicon; along with phonological and syntactic information (Roelofs, 1992; Bock & Levelt, 1994)
slide4

What is the semantic network?

  • Knowledge of objects, people, concepts and word meanings
  • Spreading-activation theory of semantic processing (Quillian, 1962; Collins & Loftus, 1975)
  • Other models also theorise how the knowledge is represented within the mental lexicon; along with phonological and syntactic information (Roelofs, 1992; Bock & Levelt, 1994)
slide8

Where is the semantic network?

  • Impairments of semantic processing (frontotemporal dementia, aphasia, Alzheimer's disease, autism and schizophrenia)
  • Broadly distributed neural representation, with marked reliance on inferotemporal as well as posterior inferior parietal regions (e.g., Damasio et al., 2004)
  • All these brain regions play a role in high-level interactive processes; they receive extensively processed, multimodal and supramodal input
slide9

Neuroimaging findings

  • 120 fMRI studies (Binder et al., 2009)
    • Left-lateralised seven-region network:
      • Posterior inferior parietal lobe (AG, SMG)
      • Lateral temporal cortex (MTG, ITG)
      • Ventral temporal cortex (PH, FFG)
      • Ventromedial prefrontal cortex
      • Dorsomedial prefrontal cortex
      • Posterior cingulate gyrus
      • Inferior frontal gyrus
  • PET and fMRI studies (Price, 2012)
slide17

Research area

  • Within-system specialisation: distinct tasks, contrasts, processes; some subsystems may specialise in specific object categories, attributes, or type of knowledge
  • Patients with profound object recognition disorders have intact word comprehension; no/minimal overlap between the systems underlying word and object recognition (Warrington, 1985; Farah, 1990)
  • fMRI studies support the view that comprehension of a word does not activate a perceptual representation of the object to which it refers (e.g., Moore & Price, 1999)
slide18

Neuropsychological evidence

  • Semantic dementia
    • Symptomatology:
      • Progressive loss of conceptual knowledge that leads to anomia, impaired comprehension and semantically invalid speech (Davies et al., 2005)
    • Neuropathology:
      • Typically left-sided atrophy in the temporal lobe with a marked anterior gradient
      • Marked loci are: temporal pole, middle and inferior temporal gyri, and fusiform gyrus (Chan et al., 2000; Mummery et al. 2000)
slide23

Neuropsychological evidence

  • Semantic aphasia
    • Symptomatology:
      • Impaired semantic retrieval and control but intact knowledge of meanings when there are no executive control demands (e.g., Robinson et al., 2005; Novick et al., 2009)
    • Neuropathology:
      • Post-stroke lesions to the frontal (IFG) and/or temporoparietal cortices (e.g., Wagner et al., 2001; Badre, 2008)
slide25

Double dissociation

  • Understanding word meanings relies on two processes:
    • Activation of word meanings
    • Retrieval and manipulation of the information in a given situation/task
  • These two processes of semantic cognition seem to be computed by distinct brain areas:
    • Representation – aITG
    • Control/retrieval – pMTG, IFG
slide29

Semantic representation versus control

  • Whitney, Jefferies, & Kircher (2011)
  • Semantic relatedness judgement task
slide30

Semantic representation versus control

  • Whitney, Jefferies, & Kircher (2011)
  • Semantic relatedness judgement task

LION

slide31

Semantic representation versus control

  • Whitney, Jefferies, & Kircher (2011)
  • Semantic relatedness judgement task

LION

STRIPE

slide32

Semantic representation versus control

  • Whitney, Jefferies, & Kircher (2011)
  • Semantic relatedness judgement task

LION

STRIPE

TIGER

slide33

Semantic representation versus control

  • Whitney, Jefferies, & Kircher (2011)
  • Semantic relatedness judgement task

LION

STRIPE

TIGER

Is this word related to any of the last two words?

YES NO

slide34

Semantic representation versus control

  • Prime-target relationships:
    • Ambiguous double-related (game-dance-ball)
    • Unambiguous double-related (lion-stripe-tiger)
    • Single-related superordinate (game-pillow-ball)
    • Single-related subordinate (dance-clock-ball)
slide36

Ambiguous > unambiguous trials

High > low semantic control demands

slide37

Proposed research

  • To identify the brain region (within the temporal lobe) that stores semantic representations
  • Three-phase research project:
    • Acquisition of novel word meanings
    • Mapping semantic representations (fMRI)
    • Validation (TMS)
slide38

Acquisition of novel words

  • 40 novel words (e.g., freckton)
  • Word forms and meanings learnt from context within 100-word paragraphs
  • Word production task
  • 5 online worksheets
  • Lexical decision task:
    • Have the new word forms been integrated?
      • Novel words ~ low-frequency long words
      • Novel words > pseudo-words
slide39

Acquisition of novel words

  • Masked semantic priming task:
    • Have the new meanings been fully consolidated?
      • Presence of the priming effect

#######

slide40

Acquisition of novel words

  • Masked semantic priming task:
    • Have the new meanings been fully consolidated?
      • Presence of the priming effect

#######

FRECKTON

slide41

Acquisition of novel words

  • Masked semantic priming task:
    • Have the new meanings been fully consolidated?
      • Presence of the priming effect

#######

FRECKTON

MOUSE

slide42

Acquisition of novel words

  • Masked semantic priming task:
    • Have the new meanings been fully consolidated?
      • Presence of the priming effect

#######

FRECKTON

MOUSE

DOG

WHISKERS

slide43

Mapping semantic representations (fMRI)

  • Following successful consolidation of both word forms and meanings (indexed by ad hoc latency data cut-off)
  • A number of different tasks that do not necessitate semantic control demands or executive processes (e.g., silent word reading paradigm)
  • Subjects’ attention maintained with inter-trial reward
  • Multiple target word presentation
slide44

Mapping semantic representations (fMRI)

  • Mixed/blocked design
  • Comparison of the activation loci pre and post the acquisition of novel words
  • ROI analysis (temporal lobe)
slide45

Validation of the findings (TMS)

  • Stimulation over the region of interest (aITG)
  • Behavioural data:
    • Performance on semantic access tasks with no/minimal semantic control demands e.g.,
      • Word naming task
      • Attribute recall task
      • Meaning retrieval task