The neural correlates of novel versus familiar metaphors in healthy young adults: A functional near-infrared spectroscopy study.

Despite extensive investigation, the neural correlates of metaphor processing remain debated. Poor theoretical and experimental control of variables that drive metaphor activation-particularly the constructs of novelty and familiarity-may be the reason for past discrepancies between studies. To address this issue, we used functional near-infrared spectroscopy (fNIRS) to investigate how neural activation varies by sentence type (metaphorical versus literal sentences) and novelty (completely novel versus familiarized phrases) in 28 young adults without known neurological or psychiatric diagnoses. Activity was significantly greater for metaphorical over literal sentences in the bilateral inferior frontal gyrus, pars triangularis (IFGtri), left inferior parietal cortex, right IFG, pars opercularis (RIFGop), and right angular gyrus (RAG). Novel metaphors to which participants had no prior exposure had significantly higher (albeit weak) effects within RIFGop and right middle temporal gyrus (RMTG) compared to metaphors to which participants were exposed just prior to the fNIRS experiment. Pre-exposed, more familiar metaphors significantly activated a wider network of regions compared to completely novel metaphors, including bilateral middle frontal gyrus (MFG), bilateral IFGtri, and LMTG. A greater response time difference between conditions was associated with less LMFG activity for metaphors over literal sentences, but this effect had weak explanatory power. Taken together, these findings suggest that metaphors-particularly novel metaphors-somewhat engage right hemisphere cortex more than other phrase types (literal sentences, more familiar metaphors) but depend primarily on a distributed network of canonical language and domain-general multiple demand network regions, consistent with current neuropragmatic frameworks.