New Study Reveals Distinct Pupil Size Differences Between Speaking Aloud And Inner Speech

Phys.org Science•Published: Oct 15, 2026 07:30 PM IST

Researchers led by Ryuta Kuwamizu have discovered that pupil dilation serves as a reliable, noninvasive indicator of brain arousal levels when comparing overt speech to inner speech. By monitoring healthy young adults performing counting tasks, the study published in iScience on October 15, 2026, found that inner speech consistently triggers lower arousal levels than speaking aloud.

Experimental Methodology: Participants performed three counting tasks—simple, number-noun phrases, and step counting—while researchers measured pupil diameter to track moment-to-moment brain arousal.
Key Physiological Findings: While overt speech caused consistent pupil dilation, inner speech responses varied by task, ranging from constriction during simple counting to dilation during complex step counting.
Research Implications: The study suggests inner speech is a distinct cognitive process rather than a silent version of overt speech, offering a new tool to study language development in children.

The Context & Background

Prior to this study, understanding inner speech relied heavily on subjective self-reporting, which is often unreliable or impossible to obtain from infants. This new noninvasive pupil-tracking method provides a breakthrough for developmental psychology and cognitive science research.

Key takeaway:This research provides a new, objective window into the human mind, allowing scientists to track cognitive development in children who cannot yet verbalize their thoughts. By understanding how inner speech functions, researchers may eventually unlock new ways to help individuals use self-talk to improve focus, motivation, and emotional regulation in high-pressure environments like sports.

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