The key facts
- In three experiments involving 80 students, pupils dilated steadily during counting aloud, but during silent counting the response depended on the difficulty of the task.
- When numbers were spoken aloud in the difficult task, dilation reached about 0.4 mm and lasted more than ten seconds.
- The authors emphasize: one cannot read other people's thoughts from the eyes; only differences in responses to specific tasks were identified.
Pupils react differently to speech aloud and inner dialogue
Human pupils dilate when words are spoken aloud, but during inner dialogue their response depends on the difficulty of the mental task. This is the conclusion reached by scientists from the University of Tsukuba and Ritsumeikan University; their study was published in the journal iScience, ScienceAlert reports.
How the experiments were conducted
The researchers carried out three experiments involving 80 healthy students who performed counting exercises. Volunteers either said numbers aloud or counted them silently in sequence while instruments recorded changes in pupil size; the order of the aloud and silent tasks was randomized each time. First, participants simply counted in order from one, then object names — cats, balls, or books — were added to the numbers, and in the final part the sequence was increased in steps of two, three, or four: for example, adding three produced the series 1, 4, 7, 10, and so on. The non-invasive method made it possible to study physiological responses associated with mental activity.
Key findings
The difference between overt and inner speech proved noticeable. When answers were spoken aloud, the pupils dilated steadily regardless of the nature of the exercise. During silent counting the picture was different: in the simplest task there was even constriction, in the second no significant changes were recorded, and only the most complex calculations caused an increase in diameter — less pronounced than when speaking aloud. The response was especially strong in the third experiment: in volunteers who said the numbers aloud, dilation reached about 0.4 millimeters and lasted more than ten seconds. According to the authors, this figure is comparable to physiological changes during intense exertion, including cycling at about 80% of maximum oxygen consumption.
Explanation and limitations
Neuroscientist Ryuta Kuwamizu of the University of Tsukuba and developmental psychologist Ryoichi Watanabe of Ritsumeikan University explained that inner monologue engages the brain's frontotemporal network responsible for speech production: it partially overlaps with the areas active during ordinary conversation but does not fully coincide with them. The scientists suggest that speaking words requires additional resources — the brain has to simultaneously coordinate movements of the speech apparatus, breathing, auditory control, and attention, which may activate the arousal systems of the brainstem. The authors also recalled a previous study that found a correlation between pupil dilation and how well people later remembered the words they had spoken. At the same time, the researchers do not claim that one can read other people's thoughts from their eyes: the experiment revealed only differences in responses to specific tasks.
Why it matters
The work clarifies how the inner voice is connected to the nervous arousal systems — mechanisms that, as the authors admit, have been studied less than the frontotemporal network itself, which has been researched for several decades. In practical terms, this is a non-invasive way to assess mental workload from pupil responses, although transferring the results beyond counting tasks has not yet been demonstrated: only 80 healthy students took part in the experiments, and there are no data on applicability to other groups.
