A 2026 study found that speaking aloud consistently dilates the pupils, while silent inner speech produces smaller responses whose direction depends on the mental task. The result suggests that inner speech is not simply overt speech with the sound removed: it is linked to lower, task-sensitive arousal.
What the study measured
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,” was published in iScience in 2026 (volume 29, issue 11, article 117825; doi:10.1016/j.isci.2026.117825).
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Pupil diameter changes from moment to moment with arousal and other factors such as light. In this experiment, it served as a non-invasive physiological measure while participants either counted aloud or counted silently in their heads. The summaries do not report the participant count or exact effect sizes, so the findings should be read as directional results rather than precise population estimates.
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| Condition | Pupil response | What it indicates in this study |
|---|---|---|
| Overt speech: counting aloud | Dilation in all three experiments | A consistent increase in pupil-linked arousal when speech was produced vocally |
| Inner speech: simple numerical counting (1, 2, 3…) | Constriction | Lower arousal response during the easiest counting format |
| Inner speech: number-noun phrases (“one cat, two cats…”) | No clear change | No measurable pupil shift in this task |
| Inner speech: step counting (for example, 1, 4, 7…) | Dilation | Greater cognitive demand was accompanied by increased arousal |
Across every experiment, the inner-speech response was smaller than the response during overt speech. Silent counting therefore did not reproduce the strength or consistency of the vocal response.
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Why inner speech behaves differently
Overt speech requires coordinated breathing, vocal-cord activity, articulation and auditory monitoring. Those actions add physiological and attentional demands that can enlarge the pupil response. Inner speech uses language without producing an audible voice, so it may engage fewer of those output-related demands.
The task still matters. Simple counting can be relatively automatic, whereas step counting requires holding a rule in mind and repeatedly applying it. The dilation seen during step counting is consistent with increased cognitive effort, but pupil size alone cannot identify the exact thought or prove which neural circuits were active.
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Does this mean pupils can read thoughts?
No. Pupillometry is an indirect arousal measure. It can show that a group of participants had a physiological response under a particular task, but it cannot decode the words in a person’s private monologue, determine whether someone is thinking in sentences, or distinguish a specific belief from another thought with the same effort level.
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Why the finding matters for developmental research
Most studies of inner speech ask participants to describe what they were thinking. That approach is difficult with infants and very young children, who may not be able to provide a reliable verbal report. Because pupil measurement does not require speaking, a carefully controlled version of this method could give developmental researchers an additional window onto language-related mental activity.
That is a research opportunity, not a validated clinical test. The report comes from one newly published research team, and broader samples plus replication are needed before extending the result across ages, languages, cultures or clinical populations.
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How to interpret the result in everyday terms
- Talking aloud: in these experiments, vocal counting reliably increased pupil size.
- Talking silently to yourself: the pupil response was weaker and depended on what the person had to calculate.
- More difficult inner speech: the rule-based step-counting task produced dilation, consistent with greater mental effort.
- Simple inner speech: basic counting produced constriction, while the number-noun task produced no clear change.
- What cannot be inferred: pupil changes do not reveal the literal content of private thoughts.
What remains unknown
The available official summaries do not establish an exact participant number, numerical effect sizes, or how the responses would look during spontaneous conversation rather than laboratory counting. They also do not test whether overt and inner speech rely on exactly the same neural networks. Future work will need independent replications, varied tasks and participants from different age and language groups.
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Frequently Asked Questions
Why might my pupils change when I silently talk to myself?
Inner speech can alter arousal and mental workload, and pupil diameter reflects those changes indirectly. The study found that the direction and size of the response depended on the task, not on a universal signature of silent speech.
Can an eye tracker detect what I am thinking?
It can measure pupil and eye movements, but it cannot read the words or meaning of private thoughts. The reported method distinguishes task-related arousal patterns under controlled conditions.
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