Distribution and Timing of Verbal Backchannels in Conversational Speech: A Quantitative Study

This paper explores backchannels, short listener responses such as “mhm”, which play an important role in managing turn-taking and grounding in spontaneous conversation. While previous work has largely focused on their acoustic cues or listener’s behavior in isolation, this study investigates if and when backchannels occur by taking into account the prosodic characteristics together with the communicative functions of the interlocutor’s speech preceding backchannels. Using a corpus of spontaneous dyadic conversations in Austrian German annotated with continuous turn-taking labels, we analyze the distribution of backchannels across different turn-taking contexts and examine which acoustic features affect their occurrence and timing by means of Conditional Inference Trees and linear mixed-effects regression models. Our findings show that the turn-taking function of the interlocutor’s utterance is a significant predictor of whether a backchannel occurs or not: Backchannels tend to occur most frequently after longer and syntactically complete utterances by the interlocutor. Moreover, prosodic features such as utterance duration, articulation rate variability and rising or falling intensity affect the timing of listener responses, with significant differences across different turn-taking functions. These results highlight the value of using continuous turn-taking annotations to investigate conversational dynamics and demonstrate how turn-taking function and prosody jointly shape backchannel behavior in spontaneous conversation.