Metaphor comprehension in neurotypical adults: A scoping review of event-related potential studies

In this scoping review, we examined event-related potential (ERP) studies on verbal metaphor processing in neurotypical adults. While it is commonly found that metaphors elicit a more negative N400 (a negativity in the 200-600 ms time window post word onset) than literal counterparts, results vary greatly in the 500-1,000 ms time window. Some studies report a late positivity (LP) and others late negativity (LN). Our main objectives were to explain the metaphor-related N400 and to investigate methodological choices that could account for the varying findings at the later time window. For this, we focused on stimulus properties (novelty, concreteness, emotionality, and context) and task demands. We reviewed 62 different experiments published in 52 research papers that have 1) investigated verbal metaphor comprehension in a healthy, neurotypical adult populations, 2) used ERPs, 3) included a literal condition for comparison, 4) presented original data, and 5) were written in English. Our review indicates that the N400 is more commonly found for novel metaphors compared to literal counterparts, especially when presented within limited context, reflecting meaning retrieval effort. For the later time window, while the exact function of the LP is still unclear, based on the available evidence, we associate it with contextual richness and stimulus emotionality. Finally, we suggest that the LN is associated with re-analysis or evaluation of stimuli prompted by metaphor novelty and task complexity.