Anodal stimulation of the left inferior frontal gyrus reduces metaphor comprehensibility

Understanding novel metaphors (e.g., Passion is a storm) requires semantic control processes to activate relevant semantic properties (i.e., intense) rather than irrelevant properties (i.e., weather). The degree to which the left inferior frontal gyrus (LIFG), which is traditionally considered a literal language semantic control region, is functionally involved in metaphor processing remains unresolved in neurocognitive theories of semantic processing. Here, high-definition transcranial direct current stimulation (HD-tDCS) was applied to the LIFG to examine its functional role in metaphor comprehension. Participants received either anodal stimulation (n = 20), which increases the excitability of neurons, or sham stimulation (n = 20), and subsequently rated metaphorical (e.g., Passion is a storm), literal (e.g., A gorilla is an ape), and 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. These findings demonstrate that the LIFG plays a causal role in the controlled retrieval of semantic properties that is necessary for metaphor comprehension.