Neural correlates of speech preparation in interactive turn-taking: An early start?

In psycholinguistic experiments on language processing, researchers have traditionally focused on either comprehension or production. However, real-life, communicative language use happens most often in an interactive setting, involving rapid turn-taking between interlocutors. In such a setting, listening to a turn probably overlaps with preparing an answer to this turn. In the current EEG experiment, participants answered quiz questions, asked by the experimenter. Unknowingly to participants, these questions were pre-recorded, while the experimenter gave live feedback on participants’ answers. Questions appeared in two different conditions. Participants could confidently guess the answer to the question either halfway through the question (e.g., "Which character, also known as James Bond, appears in the famous movies?"), or only when they heard the last word(s) (e.g., "Which character, who appears in the famous movies, is also known as James Bond?"). Participants took longer to respond to the latter than the former question type, indicating that they start response preparation already during the question if they can, but leaving open when exactly production planning starts. ERP results showed a small N400 effect (Kutas & Hillyard, 1984), followed by a large positivity time-locked to the moment within the question that the answer started to become apparent (the critical point), relative to an equivalent position in the other condition. The N400 effect likely reflects the comprehension of the question, caused by a difference in the predictability of the words. In contrast, the positivity is more likely to be triggered by production processes, which was supported by source localisations of this effect in language production areas (e.g., Broca’s area and the temporal lobe, Indefrey & Levelt, 2004). In the frequency domain, less power in the alpha/mu band was found, starting within 500 milliseconds after the critical point. A follow-up control-experiment in which participants only listened to the questions and tried to remember them, was necessary to determine to what extent the positivity in the ERPs and the alpha/mu decrease indeed reflected production processes. Such a control-experiment showed a qualitatively similar pattern in the ERPs. However, the N400 was larger and the positivity was smaller and not localized in production areas, in contrast to the positivity in the main experiment. The effect in the alpha/mu band was absent or at least very much reduced. In combination with the localisations from the main experiment, we tentatively interpret the relative decrease in alpha/mu power as a signal of a shift of attention from comprehension to production-related processing. In all, both this effect and the positivity in the ERPs suggest that response preparation in interactive turn-taking situations starts quickly (within half a second) after an appropriate response can be retrieved. References Indefrey, P., & Levelt, W. J. (2004). The spatial and temporal signatures of word production components. Cognition, 92, 101-144. Kutas, M., & Hillyard, S. A. (1984). Brain potentials during reading reflect word expectancy and semantic association. Nature, 307, 161–163