Humans speak and produce symbolic gestures. Do these two forms of communication interact, and how? First, we tested whether the two communication signals influenced each other when emitted simultaneously. Participants either pronounced words, or executed symbolic gestures, or emitted the two communication signals simultaneously. Relative to the unimodal conditions, multimodal voice spectra were enhanced by gestures, whereas multimodal gesture parameters were reduced by words. In other words, gesture reinforced word, whereas word inhibited gesture. In contrast, aimless arm movements and pseudo-words had no comparable effects. Next, we tested whether observing word pronunciation during gesture execution affected verbal responses in the same way as emitting the two signals. Participants responded verbally to either spoken words, or to gestures, or to the simultaneous presentation of the two signals. We observed the same reinforcement in the voice spectra as during simultaneous emission. These results suggest that spoken word and symbolic gesture are coded as single signal by a unique communication system. This signal represents the intention to engage a closer interaction with a hypothetical interlocutor and it may have a meaning different from when word and gesture are encoded singly.
