Prosodic Cues to Backchanneling in Igbo Oral Storytelling

Listener backchannels coordinate spoken interaction, yet most of the acoustic work on the prosodic cues that invite them has examined non-tonal (non-African) languages, in which fundamental frequency (F0) is freely available for pragmatic signalling. This study provides an acoustic investigation of backchannel-inviting prosody in an African tone language, using a corpus of dyadic oral storytelling (akukọ ifo) from eight native speakers of Igbo. We examine which prosodic configurations in the narrator’s speech predict backchannels, how these configurations interact with the lexical tone system of Igbo, and how the prosody of backchannel tokens varies with interactional function. In a logistic mixed-effects model, three configurations each independently predict backchannels: pitch plateaus, final lengthening combined with a low (L%) boundary tone, and an intervening pause; backchannel probability rises from under 4% when none is present to nearly 70% when all three co-occur. Crucially, low boundary tones significantly raise backchannel probability even on lexically high-toned final syllables, and narrators prepare these boundaries through a staged high–mid–low descent on the penultimate syllable (“terraced final descent”) that is far more frequent before backchannels than elsewhere. On the listener side, backchannel tokens are significantly differentiated by function in F0 and duration. We argue that Igbo speakers achieve the cross-linguistically attested low-F0 backchannel-inviting environment through a phonologically more elaborate route that reconciles lexical tonal specification with pragmatic intonational marking.