Speech planning depends on next-speaker selection: evidence from pupillometry in question–answer sequences in naturalistic triadic conversation

Next-speaker selection, which controls who should speak next, is fundamental to turn taking. While it is central in Conversation Analysis, little is known about its cognitive repercussions. We draw on turn-taking and pupillometric data in triadic interaction, investigating open-floor questions, which license more than one participant to respond, and closed-floor questions, which license only a single participant to answer. Comparing pupil size changes in answerers versus not-answerers at turn transitions, we find that answerers’ pupils dilate irrespective of question type, while not-answerers’ pupils dilate only in open-floor questions, indicating that in closed-floor questions not-selected participants do not prepare a response, whereas in open-floor questions both recipients engage in response planning, even if only one responds. Mutual gaze, by contrast, is not found to have an effect on pupil size at turn transitions. We propose an extension to the current model of language processing in turn taking, including next-speaker selection as a relevant variable impacting interlocutors’ behavior and their mental processes.