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Understanding the Semantic Network: Structure, Function, and Cognitive Representation

The semantic network is a mental framework that represents knowledge of objects, people, concepts, and word meanings. The spreading-activation theory, developed by Quillian and furthered by Collins & Loftus, explains how semantic processing occurs. Various models explore how knowledge is organized in the mental lexicon, incorporating phonological and syntactic information. Neuroimaging studies reveal a broad neural representation related to semantic tasks and highlight distinct processes in semantic cognition. Understanding these relationships has profound implications for treating semantic processing impairments in conditions like dementia and aphasia.

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Understanding the Semantic Network: Structure, Function, and Cognitive Representation

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  1. Where is the semantic network?

  2. 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)

  3. 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)

  4. 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

  5. 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)

  6. 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)

  7. 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)

  8. 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)

  9. 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

  10. Semantic representation versus control • Whitney, Jefferies, & Kircher (2011) • Semantic relatedness judgement task

  11. Semantic representation versus control • Whitney, Jefferies, & Kircher (2011) • Semantic relatedness judgement task LION

  12. Semantic representation versus control • Whitney, Jefferies, & Kircher (2011) • Semantic relatedness judgement task LION STRIPE

  13. Semantic representation versus control • Whitney, Jefferies, & Kircher (2011) • Semantic relatedness judgement task LION STRIPE TIGER

  14. 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

  15. 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)

  16. Ambiguous > unambiguous trials

  17. Ambiguous > unambiguous trials High > low semantic control demands

  18. 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)

  19. 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

  20. Acquisition of novel words • Masked semantic priming task: • Have the new meanings been fully consolidated? • Presence of the priming effect #######

  21. Acquisition of novel words • Masked semantic priming task: • Have the new meanings been fully consolidated? • Presence of the priming effect ####### FRECKTON

  22. Acquisition of novel words • Masked semantic priming task: • Have the new meanings been fully consolidated? • Presence of the priming effect ####### FRECKTON MOUSE

  23. Acquisition of novel words • Masked semantic priming task: • Have the new meanings been fully consolidated? • Presence of the priming effect ####### FRECKTON MOUSE DOG WHISKERS

  24. 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

  25. Mapping semantic representations (fMRI) • Mixed/blocked design • Comparison of the activation loci pre and post the acquisition of novel words • ROI analysis (temporal lobe)

  26. 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

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