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)

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 LIONSTRIPE

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

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

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)

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: 1.Acquisition of novel word meanings 2.Mapping semantic representations (fMRI) 3.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