Semantics is crucial for the right-hemisphere involvement in metaphor processing: evidence from mouth asymmetry during speaking.

Research on the neural basis of metaphor provides contradicting evidence about the role of right and left hemispheres. We used the mouth-opening asymmetry technique to investigate the relative involvement of the two hemispheres whilst right-handed healthy male participants explained the meaning of English phrases. This technique is based on the contralateral cortical control of the facial musculature and reflects the relative hemispheric involvement during different cognitive tasks. In particular, right-handers show a right-sided mouth asymmetry (right side of the mouth opens wider than the left) during linguistic tasks, thus reflecting the left-hemisphere specialization for language. In the current study, we compared the right-sided mouth asymmetry during metaphor explanation (e.g., explain the meaning of the phrase "to spin a yarn") and concrete explanation (e.g., explain the meaning of the phrase "to spin a golf ball") and during the production of content and function words. The expected right-sided mouth asymmetry reduced during metaphorical compared to concrete explanations suggesting the relative right-hemispheric involvement for metaphor processing. Crucially, this right-sided mouth asymmetry reduction was particularly pronounced for the production of content words. Thus, we concluded that semantics is crucial to the right-hemispheric involvement for metaphorical speech production.