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Where is the semantic network?
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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)
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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)
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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
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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)
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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)
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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)
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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)
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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
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Semantic representation versus control Whitney, Jefferies, & Kircher (2011) Semantic relatedness judgement task
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Semantic representation versus control Whitney, Jefferies, & Kircher (2011) Semantic relatedness judgement task LION
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Semantic representation versus control Whitney, Jefferies, & Kircher (2011) Semantic relatedness judgement task LIONSTRIPE
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Semantic representation versus control Whitney, Jefferies, & Kircher (2011) Semantic relatedness judgement task LIONSTRIPETIGER
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Semantic representation versus control Whitney, Jefferies, & Kircher (2011) Semantic relatedness judgement task LIONSTRIPETIGER Is this word related to any of the last two words? YESNO
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Semantic representation versus control Prime-target relationships: 1.Ambiguous double-related (game-dance-ball) 2.Unambiguous double-related (lion-stripe-tiger) 3.Single-related superordinate (game-pillow-ball) 4.Single-related subordinate (dance-clock-ball)
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Ambiguous > unambiguous trials
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High > low semantic control demands
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Proposed research To identify the brain region (within the temporal lobe) that stores semantic representations Three-phase research project: 1.Acquisition of novel word meanings 2.Mapping semantic representations (fMRI) 3.Validation (TMS)
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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
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Acquisition of novel words Masked semantic priming task: Have the new meanings been fully consolidated? Presence of the priming effect #######
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Acquisition of novel words Masked semantic priming task: Have the new meanings been fully consolidated? Presence of the priming effect FRECKTON#######
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Acquisition of novel words Masked semantic priming task: Have the new meanings been fully consolidated? Presence of the priming effect FRECKTON#######MOUSE
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Acquisition of novel words Masked semantic priming task: Have the new meanings been fully consolidated? Presence of the priming effect FRECKTON#######MOUSE DOG WHISKERS
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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
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Mapping semantic representations (fMRI) Mixed/blocked design Comparison of the activation loci pre and post the acquisition of novel words ROI analysis (temporal lobe)
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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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