Abstract
Controlled semantic retrieval to words elicits co-activation of inferior frontal (IFG) and left posterior temporal cortex (pMTG), but research has not yet established (i) the distinct contributions of these regions or (ii) whether the same processes are recruited for non-verbal stimuli. Words have relatively flexible meanings – as a consequence, identifying the context that links two specific words is relatively demanding. In contrast, pictures are richer stimuli and their precise meaning is better specified by their visible features – however, not all of these features will be relevant to uncovering a given association, tapping selection/inhibition processes. To explore potential differences across modalities, we took a commonly-used manipulation of controlled retrieval demands, namely the identification of weak vs. strong associations, and compared word and picture versions. There were 4 key findings: (1) Regions of interest (ROIs) in posterior IFG (BA44) showed graded effects of modality (e.g., words>pictures in left BA44; pictures>words in right BA44). (2) An equivalent response was observed in left mid-IFG (BA45) across modalities, consistent with the multimodal semantic control deficits that typically follow LIFG lesions. (3) The anterior IFG (BA47) ROI showed a stronger response to verbal than pictorial associations, potentially reflecting a role for this region in establishing a meaningful context that can be used to direct semantic retrieval. (4) The left pMTG ROI also responded to difficulty across modalities yet showed a stronger response overall to verbal stimuli, helping to reconcile two distinct literatures that have implicated this site in semantic control and lexical-semantic access respectively. We propose that left anterior IFG and pMTG work together to maintain a meaningful context that shapes ongoing semantic processing, and that this process is more strongly taxed by word than picture associations.
Highlights
Semantic cognition is inherently multimodal, allowing us to assign meaning to things we encounter in different modalities and to map between these modalities (Jefferies, 2013; Lambon Ralph et al, 2008; Patterson et al, 2007)
Since our aim was to establish how the semantic control network previously established for verbal tasks might be modulated by modality, we examined possible interactions between difficulty and modality or task in the following Regions of interest (ROIs) that were highlighted by that review of the literature: posterior, mid and anterior inferior frontal gyrus (IFG) in the left and right hemispheres and left posterior middle temporal gyrus (pMTG), dorsal angular gyrus (dAG), and ventral Angular Gyrus (AG) (vAG)
The results showed (1) graded modality effects across the hemispheres in posterior IFG: left pIFG made a greater contribution to challenging verbal semantic decisions, while right pIFG responded more to picture-based decisions
Summary
Semantic cognition is inherently multimodal, allowing us to assign meaning to things we encounter in different modalities (words, pictures, actions, smells, etc.) and to map between these modalities (Jefferies, 2013; Lambon Ralph et al, 2008; Patterson et al, 2007). On encountering the word “piano”, we can retrieve the full range of multimodal features for this concept, including visual properties (large size, black and white keys), actions (finger presses) and sounds (musical notes). Semantic cognition involves the interaction of (i) a store of multimodal semantic information, accessed by inputs in different modalities and (ii) control processes that shape semantic retrieval according to high-level goals established by the context or task (Jefferies and Lambon Ralph, 2006).
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