Pupil size reveals differences between speaking aloud and inner speech

Pupil size can reveal differences between speaking aloud and inner speech, a study published in iScience has found. Even when people are not speaking, they often engage in inner speech, using words to guide their thoughts and actions. This mental conversation is thought to aid in thinking and self-control. Brain scans have shown that inner speech and speaking aloud both activate language networks in the brain, yet it was unclear if they involve similar levels of brain activity.

To explore this, researchers measured pupil size as a noninvasive indicator of arousal. Healthy young adults were asked to perform a counting task in three conditions: rest, speaking aloud, and silent counting. Pupil diameter was recorded during these tasks. The results showed that pupils dilated during speaking aloud across all three experiments.

However, pupil responses during silent counting varied depending on the task: they constricted during simple counting, showed no change during counting with number-noun phrases, and dilated during step counting, which required more cognitive effort. Importantly, these pupil changes were consistently smaller than those observed during speaking aloud, indicating lower arousal during silent counting.

These findings suggest that inner speech is not simply a silent version of speaking aloud but is a form of language use associated with lower brain arousal. By using pupil responses as an indicator of brain activity during inner speech, this study provides a new method for understanding how language functions in the mind. The research could be particularly useful for studying inner speech in infants and young children, who may have difficulty describing their thoughts verbally.

Pupil measurement, being noninvasive and not requiring verbal responses, could help researchers gain insights into how inner speech develops in early childhood and how people use it to calm or motivate themselves in various situations, such as sports.