The differentiation of iconic and metaphoric gestures: common and unique integration processes.

Recent research on the neural integration of speech and gesture has examined either gesture in the context of concrete [iconic (IC) gestures] or abstract sentence content [metaphoric (MP) gestures]. However, there has not yet been a direct comparison of the processing of both gesture types. This study tested the theory that left posterior temporal and inferior frontal brain regions are each uniquely involved in the integration of IC and MP gestures. During fMRI-data acquisition, participants were shown videos of an actor performing IC and MP gestures and associated sentences. An isolated gesture (G) and isolated sentence condition (S) were included to separate unimodal from bimodal effects at the neural level. During IC conditions, we found increased activity in the left posterior middle temporal gyrus and its right hemispheric homologue. The same regions in addition to the left inferior frontal gyrus (IFG) were activated during MP conditions in contrast to the isolated conditions (G&S). These findings support the hypothesis that there are distinct integration processes for IC and MP gestures. In line with recent claims of the semantic unification theory, there seems to be a division between perceptual-matching processes within the posterior temporal lobe and higher-order relational processes within the IFG.