Modulating the assessment of semantic speech–gesture relatedness via transcranial direct current stimulation of the left frontal cortex

BACKGROUND Co-verbal gestures are crucial for communication. Neuroimaging studies suggest that the left frontal lobe may be especially important for processing metaphoric co-verbal gestures. However, so far, the specific functional relevance of the left frontal lobe in metaphoric (abstract sentence content) co-verbal gesture processing compared to iconic (concrete sentence content) co-verbal gesture processing has not been demonstrated. OBJECTIVE We investigated the functional relevance of the left frontal lobe for processing metaphoric co-verbal gestures using transcranial direct current stimulation (tDCS). We hypothesised a polarisation dependent effect of left frontal tDCS on reaction times and ratings in a speech-gesture semantic relatedness assessment task. METHODS We applied anodal, cathodal and sham stimulation to the frontal (F3/F4), parietal (CP3/CP4) and frontoparietal (F3/CP4) areas. During stimulation, seventeen subjects were presented with videos of an actor saying concrete or abstract sentences accompanied by related or unrelated iconic or metaphoric gestures and rated to what extent gestures were related to the sentence content. RESULTS We found electrode localisation- and polarisation-dependent changes in reaction times and ratings for metaphoric co-verbal gestures compared to iconic gestures. Post-hoc tests revealed a specific polarisation effect for frontoparietal stimulation sites: compared to cathodal stimulation, anodal stimulation of the left frontal lobe decreased reaction times and relatedness assessments for metaphoric conditions only. CONCLUSION Using tDCS, we demonstrated the functional relevance of the left frontal lobe for processing metaphoric co-verbal gestures. Thus, tDCS may possibly constitute an approach to facilitate metaphoric co-verbal gesture-processing in patients with specific deficits.