Where is the semantic network?

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) • Other models also theorise how the knowledge is represented within the mental lexicon; along with phonological and syntactic information (Roelofs, 1992; Bock & Levelt, 1994)

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

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)

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)

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)

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)

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

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

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

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

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

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

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)

Ambiguous > unambiguous trials

Ambiguous > unambiguous trials High > low semantic control demands

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)

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

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

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

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

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

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

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

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

Where is the semantic network?