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A 2026 study found that speaking aloud reliably dilates the pupils, while silent inner speech produces smaller, task-dependent changes. Simple silent counting caused pupil constriction, counting phrases produced no clear change, and more demanding step counting caused dilation. The result suggests that inner speech is not simply overt speech with the sound removed: it is linked to lower, more variable arousal.
What the study tested
Researchers from the University of Tsukuba and Ritsumeikan University compared pupil responses during rest, overt speech and inner speech in healthy young adults. Their report, “Dissociable pupil-linked arousal during overt and inner speech,” appeared in iScience in 2026 (volume 29, issue 11, article 117825; DOI 10.1016/j.isci.2026.117825).
Participants completed three counting formats. In the overt condition they counted aloud. In the inner-speech condition they performed the same kinds of counting silently, without moving their lips or producing a voice. The researchers used pupil diameter as a non-invasive, moment-to-moment indicator of arousal rather than as a direct readout of thoughts.
What happened to pupil size
| Condition | Task | Pupil response | What it indicates |
|---|---|---|---|
| Overt speech | Counting aloud | Dilation in all three experiments | Speech production was consistently associated with increased pupil-linked arousal. |
| Inner speech | Simple numerical counting, such as 1, 2, 3 | Constriction | Low-demand silent counting did not reproduce the overt-speech response. |
| Inner speech | Number-noun phrases, such as “one cat, two cats” | No clear change | The pupil effect depended on the form of the task. |
| Inner speech | Step counting, such as 1, 4, 7 | Dilation | Greater cognitive demand was associated with increased arousal. |
Across every experiment, the inner-speech changes were smaller than the changes during overt speech. The direction of the silent response therefore depended on what participants had to calculate or maintain, whereas speaking aloud produced a more consistent increase.
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Why inner speech is not just silent talking
Inner speech is language generated without vocal output. It can involve rehearsing words, planning an action, counting, or narrating an experience. Overt speech adds several demands that silent speech does not: coordinating breathing and vocal muscles, monitoring sound, and producing an externally observable signal. Those added demands can help explain why aloud counting produced a stronger and more reliable pupil response in this experiment.
The findings also show why “inner speech” should not be treated as one uniform mental state. Straightforward counting required little manipulation, while step counting required participants to apply a rule and keep track of a changing sequence. The dilation in that condition is consistent with pupil-linked arousal increasing as cognitive effort rose. The number-noun phrase task fell between those cases, with no reported pupil change.
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What pupil size can—and cannot—tell you
An indirect arousal signal
Pupils respond to arousal and cognitive demand, but also to lighting and other physiological factors. In this study, pupil diameter was used to compare moment-to-moment responses under controlled task conditions. A larger pupil does not identify a particular thought, sentence, memory or emotion.
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Eye tracking cannot decode the literal content of a person’s private sentences from this result. At most, a measured response can contribute to group-level inferences about arousal during a defined task. The study did not establish that researchers can determine what an individual is saying internally, nor did it directly test whether exactly the same neural networks are active during overt and inner speech.
Why the method matters for developmental research
Researchers have traditionally asked people to describe their own thoughts. That is difficult with infants and very young children, who may not be able to give reliable verbal reports. Because pupil measurement does not require participants to speak, it could provide a way to study language-related mental activity in populations that cannot complete a self-report task.
This is a research opportunity, not a validated clinical diagnostic tool. The reported work involved healthy young adults, and the available official summaries do not state the participant count or exact effect sizes. Further studies would be needed before applying the pattern to children, older adults, different languages, clinical groups or ordinary conversation.
How to interpret the result in everyday life
- If your pupils change while you silently rehearse words or count, that does not mean the eyes are revealing the words themselves.
- A demanding internal task may produce more arousal than an easy one, even when both are silent.
- Speaking aloud can create a larger pupil response because vocal production adds physical and monitoring demands.
- Lighting, fatigue, medication, emotion and other factors can also influence pupil diameter, so a single observation has no reliable standalone interpretation.
Key takeaway
The strongest conclusion is a distinction, not a decoder: overt speech consistently increased pupil size in these experiments, while inner speech generated smaller responses whose direction changed with task difficulty. Pupil measurements may help researchers study otherwise private language processes, but they remain an indirect measure of arousal—not a window that displays the contents of a person’s thoughts.
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