Two Interacting Neural Processes Support Speech Planning during Naturalistic Conversation

Conversation requires speakers to plan their responses while still listening to their partner, yet the neural dynamics supporting this overlap remain unclear. In particular, competing accounts differ on whether behaviourally relevant preparation emerges early during comprehension or only close to speech onset. Here, we recorded EEG from pairs of participants engaged in natural, face-to-face conversation and tested whether neural activity during listening predicts both when speakers begin their response (latency) and how long they speak (duration). Using event-related potentials, oscillatory analyses, and multivariate decoding, we show that pre-speech neural activity carries robust information about upcoming behaviour more than one second before articulation. The strongest and earliest effects tracked response duration, with sustained ERP components and alpha/beta power modulations predicting how long participants would speak, even after controlling for behavioural variables. In contrast, neural predictors of response latency were more temporally restricted and partly aligned with partner turn boundaries. Temporal generalisation analyses further revealed stable neural patterns linking early and late stages of preparation. Together, these findings indicate that conversational planning unfolds during listening and that early neural activity reflects the maintained specification of response extent, followed by later processes related to commitment and initiation. This supports a temporally structured account of turn taking in which preparation involves both early maintenance and late motor engagement.