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) and a carefully designed paradigm modified from Cardillo et al. (2012) to investigate how neural activation varies by sentence type (metaphorical versus literal sentences) and novelty (completely novel versus familiarized phrases). Activity was significantly greater for metaphorical over literal sentences in the left inferior frontal gyrus, pars triangularis (LIFGtri), 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, RAG, and right middle temporal gyrus (RMTG) compared to novel 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 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 higher LMTG activity for novel over more familiar metaphors. Taken together, these findings suggest that metaphors--particularly novel metaphors--do engage right hemisphere cortex more than other phrase types (literal sentences, more familiar metaphors) but that the effects are weaker than condition differences within canonical left language network and domain-general multiple demand network regions.