High verbal working memory load impairs gesture-speech integration

While previous studies have shown the importance of visuo-spatial working memory in the processing of co-speech iconic gestures, clear evidence for a potential involvement of the verbal working memory (vWM) is currently lacking. To address this issue, participants in the present study were presented with a dual task paradigm. The main outcome variable was the performance on a Stroop-like gesture task which provides a behavioural index of gesture-speech integration. Participants performed this task under conditions of either high or low concurrent vWM load. Unlike in previous studies, the number of words to remember in the high load condition was determined by their individual verbal span rather than being fixed. Results showed reaction time costs in the form of longer reaction times for semantically incongruent gesture-speech combinations as compared to congruent combinations. However, this semantic congruency effect disappeared when the vWM load increased. This result suggests a causal involvement of verbal working memory capacity in gesture-speech integration.