A paper by a research group including Ryuta Kuwamizu (A01), Ryoichi Watanabe and Yusuke Moriguchi (A03) has been published in the international journal iScience.

 In this study, healthy young adults counted numbers either aloud (“overt speech”) or silently in their heads (“inner speech”), and changes in pupil size were compared across three experiments using different counting methods. The pupils dilated during overt speech in all three experiments. In contrast, pupil responses during inner speech varied with the task—constricting, showing no change, or dilating—and in every case remained smaller than those during overt speech.

 Inner speech is thought to support thinking and self-control, but it had been unclear whether the brain operates at the same level of arousal during inner and overt speech. This study shows that inner speech is not merely a silent version of overt speech, but a form of language use associated with lower arousal. The findings add a new perspective—brain arousal—to research on inner speech, and the approach may also help clarify how inner speech develops in early childhood.

 The paper is open access. We encourage you to read it. Press releases on this study have been issued by the University of Tsukuba and Ritsumeikan University.

Paper Information

Authors: Ryuta Kuwamizu, Ryoichi Watanabe, Nozomi Yamamoto, Kota Otani, Yusuke Moriguchi

Title: Dissociable pupil-linked arousal during overt and inner speech

Journal: iScience, Volume 29, Issue 11, Article 117825 (2026)

DOI: https://doi.org/10.1016/j.isci.2026.117825

Summary

 Inner (covert) speech, the silent production of language in one’s mind, plays a central role in human cognition and engages speech-related cortical networks partly overlapping with those for overt speech. Because cortical language processing unfolds within arousal states set by the ascending systems, we asked whether inner and overt speech are accompanied by similar pupil-linked arousal patterns. Using pupillometry across three counting-task experiments, we found a dissociation between inner and overt speech. While overt speech consistently elicited pupil dilation across conditions, inner speech produced a variable dilation profile relative to baseline: pupil constriction (simple counting), no deviation (noun-phrase counting), and clear dilation (step counting; high cognitive load). Importantly, pupil size during inner speech remained smaller than that during overt speech despite task-dependent dynamics. These findings suggest that task-elicited inner speech is accompanied by lower pupil-linked arousal than overt speech, adding a state-level physiological dimension to cortical accounts of verbal processing.

Press Releases

University of Tsukuba: Pupil Size Reveals Differences Between Speaking Aloud and Inner Speech 

Ritsumeikan University (in Japanese): Pupil Size Reveals Differences Between Speaking Aloud and Inner Speech