Young adults' pupils dilated every time they counted aloud and only once when they counted silently
Ryuta Kuwamizu's team reports in iScience that young adults' pupils dilated while counting aloud in all three experiments. Silent counting always produced smaller pupil changes, and their direction depended on the task.
The Scientist · Science desk
What happened
- Earlier brain imaging showed that inner speech activates some of the same language networks as speaking aloud, but whether the two raise arousal alike was unclear.
- Healthy young adults completed a counting task in three conditions: resting, counting aloud and counting silently in their heads.
- Each of three experiments used a different counting method: plain counting, number-noun phrases such as one cat, two cats, and step counting such as 1, 4, 7.
- During silent counting, pupils constricted in the simple task, did not change in the number-noun task and dilated in step counting.
Why it matters
- capability If pupils can stand in for self-report, researchers could study inner speech in infants and young children, according to the summary, and look at self-calming or motivating talk such as in sports.
- constraint Any pupil-based claim about inner speech now has to name the counting task it was measured in, because the same silent activity gave three different readings.
- contradiction The authors' summary line, lower arousal, is a comparison with overt speech. It does not place inner speech below rest, and in step counting the pupil data put it above.
The comparison is built on shared content. The aloud and silent conditions use the same counting tasks, and a rest condition sits beside them as a baseline [3]. That matters because inner speech cannot be observed directly, and earlier studies leaned largely on what participants said about their own thoughts [13]. A pupil measure sidesteps the report, and the authors treat it as a moment-to-moment index of arousal [2].
Across the three experiments the pattern is sharper for speaking aloud than for silence. Overt counting dilated pupils in three of three experiments, and silent counting did so in one of three [11]. That one was step counting, the task the source describes as requiring greater cognitive effort [12].
In our view, effort is the likeliest explanation for that dilation. But the experiments changed the counting method, and effort moved with it [4]. From the summary alone, the pupil could be following either.
The phys.org write-up does not report how many people took part or how large the pupil changes were, and it does not say how the authors separated arousal from the act of articulating. Speaking aloud adds more to a task than arousal, and the gap between overt and inner speech may include some of that.
The thing this doesn't tell you is how silent self-talk behaves outside counting. The summary raises self-calming and motivating talk, as in sports, as something the method could help study [15]. Participants in these experiments rested or counted [3].
What to watch
- The full iScience paper's participant count and effect sizes, which the phys.org summary leaves out.
- Whether the authors or others test a design that varies effort within a single counting rule, to see if silent dilation tracks effort.
- Any pupil study of inner speech in infants or young children, the use the summary proposes but the experiments did not test.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence45
- Adoption
- Insufficient
- Hype gap+15
- Incentives
- Insufficient
- Confidence40
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
Brain imaging studies have shown that inner speech activates some of the same language networks involved in speaking aloud, but it remained unclear whether inner and overt speech are associated with similar levels of brain arousal.
- [2]
Researchers used pupil size as a noninvasive measure of moment-to-moment changes in arousal.
- [3]
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).
- [4]
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 ...).
- [5]
Step counting requires greater cognitive effort than the other counting methods.
- [6]
Pupils dilated during overt speech across all three experiments.
- [7]
During inner speech, pupils constricted during simple counting, showed no change during number-noun counting and dilated during step counting.
- [8]
In all cases, the changes in pupil size during inner speech were smaller than those observed during overt speech.
- [9]
The findings suggest that inner speech is not merely a silent version of overt speech but a form of language use associated with lower arousal.
- [10]
The research by Ryuta Kuwamizu et al., titled 'Dissociable pupil-linked arousal during overt and inner speech', is published in iScience (2026).
- [11]
Overt speech dilated pupils in 3 of 3 experiments, while inner speech dilated pupils in 1 of 3 (step counting).
- [12]
The only experiment in which silent counting dilated pupils, step counting, is the one the source describes as requiring greater cognitive effort.
- [13]
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.
- [14]
Pupil measurement is noninvasive and does not require verbal responses, so it could be used with infants and young children.
- [15]
The approach could 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.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgPupil size reveals differences between speaking aloud and inner speech
1 article · October 15, 2026
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