Losing Semantic Control: Anodal Stimulation of the Left-Inferior Frontal Gyrus Reduces Metaphor Comprehensibility

Understanding metaphors (e.g., Passion is a storm) requires semantic control processes to select relevant semantic properties (i.e., intense) while inhibiting irrelevant ones (i.e., weather). Here, we targeted the left-inferior frontal gyrus (LIFG) with high-definition transcranial direct-current stimulation (HD-tDCS) to study its functional role in metaphor comprehension. Participants received either anodal stimulation, which increases the excitability of neurons, or sham stimulation, and subsequently rated metaphorical (e.g., Passion is a storm), literal (e.g., A gorilla is an ape), or nonsensical (e.g., Depression is a party) sentences on comprehensibility. Participants who received anodal stimulation rated metaphors to be less comprehensible than participants who received sham stimulation. No effects were observed for literal or nonsensical sentences. In addition, word-level semantic characteristics affected metaphor comprehensibility ratings but not literal or nonsensical sentences. We interpret our findings to demonstrate that the LIFG plays a causal role in metaphor comprehension, and that excitatory stimulation overworks semantic control processes, leading to selection and inhibition impairments during comprehension.