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Humans rarely speak without producing co-speech gestures of the hands, head, and other parts of the body. Co-speech gestures are also highly restricted in how they are timed with speech, typically synchronizing with prosodically-prominent syllables. What functional principles underlie this relationship? Here, we examine how the production of co-speech manual gestures influences spatiotemporal patterns of the oral articulators during speech production. We provide novel evidence that words uttered with accompanying co-speech gestures are produced with more extreme tongue and jaw displacement, and that presence of a co-speech gesture contributes to greater temporal stability of oral articulatory movements. This effect–which we term coupling enhancement–differs from stress-based hyperarticulation in that differences in articulatory magnitude are not vowel-specific in their patterning. Speech and gesture synergies therefore constitute an independent variable to consider when modeling the effects of prosodic prominence on articulatory patterns. Our results are consistent with work in language acquisition and speech-motor control suggesting that synchronizing speech to gesture can entrain acoustic prominence. 

Co-speech gestures are timed to occur with prosodically prominent syllables in several languages. In prior work in Indo-European languages, gestures are found to be attracted to stressed syllables, with gesture apexes preferentially aligning with syllables bearing higher and more dynamic pitch accents. Little research has examined the temporal alignment of co-speech gestures in African tonal languages, where metrical prominence is often hard to identify due to a lack of canonical stress correlates, and where a key function of pitch is in distinguishing between words, rather than marking intonational prominence. Here, we examine the alignment of co-speech gestures in two different Niger-Congo languages with very different word structures, Medʉmba (Grassfields Bantu, Cameroon) and Igbo (Igboid, Nigeria). Our findings suggest that the initial position in the stem tends to attract gestures in Medʉmba, while the final syllable in the word is the default position for gesture alignment in Igbo; phrase position also influences gesture alignment, but in language-specific ways. Though neither language showed strong evidence of elevated prominence of any individual tone value, gesture patterning in Igbo suggests that metrical structure at the level of the tonal foot is relevant to the speech-gesture relationship. Our results demonstrate how the speech-gesture relationship can be a window into patterns of word- and phrase-level prosody cross-linguistically. They also show that the relationship between gesture and tone (and the related notion of ‘tonal prominence’) is mediated by tone’s function in a language.  

Fluid conversation depends on conversation partners’ ability to make predictions about one another’s speech in order to forecast turn ends and prepare upcoming turns. One model used to explain this process of temporal prediction is the coupled oscillator model of turn-taking. A generalization that the model captures is the relative scarcity of interruption in turn-taking, as it predicts partners’ turns should be counter-phased to one another, with minimal pause time between turns. However, in naturalistic conversation, turns are often delayed, rather than occurring in perfect succession. We hypothesize that these delays are not of arbitrary duration, but are structured in their timing, just as between turns with immediate transitions. We demonstrate that relative timing of prosodic events occurring at turn ends is key to modelling pause duration between turns, providing evidence that interturn pauses exist in a temporal trading relation with the final syllable and prosodic word of immediately preceding turn. 

Text-setting patterns in music have served as a key data source in the development of theories of prosody and rhythm in stress-based languages, but have been explored less from a rhythmic perspective in the realm of tone languages. African tone languages have been especially under-studied in terms of rhythmic patterns in text-setting, likely in large part due to the ill-understood status of metrical structure and prosodic prominence asymmetries in many of these languages. Here, we explore how language is mapped to rhythmic structure in traditional folksongs sung in Medʉmba, a Grassfields Bantu language spoken in Cameroon. We show that, despite complex and varying rhythmic structures within and across songs, correspondences emerge between musical rhythm and linguistic structure at the level of stem position, tone, and prosodic structure. Our results reinforce the notion that metrical prominence asymmetries are present in African tone languages, and that they play an important coordinative role in music and movement.