This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Even when we are silent, we often talk to ourselves in our heads, using words to guide our thoughts and actions. This "inner speech" is thought to be involved in thinking and self-control.
Brain imaging studies have shown that inner speech activates some of the same language networks involved in speaking aloud ("overt speech"). However, it has remained unclear whether inner and overt speech are associated with similar levels of brain arousal. To investigate this question, researchers focused on pupil size as a noninvasive measure of moment-to-moment changes in arousal.
Healthy young adults performed a counting task under three conditions: rest (not counting or thinking about anything in particular), overt speech (counting aloud) and inner speech (counting silently in their heads). Pupil diameter was recorded during three experiments that used different counting methods: simple counting (1, 2, 3 ...), counting with number-noun phrases (one cat, two cats ...) and step counting (e.g., 1, 4, 7 ...), which requires greater cognitive effort. The results showed that the pupils dilated during overt speech across all three experiments.
In contrast, pupil responses during inner speech depended on the task: pupils constricted during simple counting, showed no change during number-noun counting and dilated during step counting. In all cases, the changes in pupil size were smaller than those observed during overt speech. These findings suggest that inner speech is not merely a silent version of overt speech; rather, it is a form of language use associated with lower arousal.
By examining pupil responses as an indicator of brain arousal during inner speech, this study offers a new perspective for understanding how language functions in the mind. The research is published in the journal iScience. Because inner speech cannot be directly observed, prior studies have largely relied on participants' descriptions of their own thoughts, which can be difficult to obtain from young children.
As pupil measurement is noninvasive and does not require verbal responses, it could be used with infants and young children. This approach could therefore help researchers understand how inner speech develops in early childhood and how people use inner speech to calm or motivate themselves, such as when participating in sports. Ryuta Kuwamizu et al., Dissociable pupil-linked arousal during overt and inner speech, iScience (2026).
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