Your eyes dilate depending on your thoughts

Your eyes really can betray your thoughts, but don’t get too self-conscious about it. Researchers in Japan believe that the subtle signs our eyeballs make are more useful to neuroscientists than the interrogation room. Based on their findings recently published in the journal iScience, examining the difference physiological responses from “inner” versus “overt” speech may help us better understand everything from linguistic development in children to the efficacy of silent pep talks.

To learn more, a team at the University of Tsukuba in Honshu enlisted dozens of volunteers for a series of simple counting exercises. After observing the participants at rest while thinking about nothing in particular for a control, researchers then asked them to count sequentially (1, 2, 3…), in number-subject combinations (one dog, two dogs…), and in specific intervals (1, 3, 5…). They also completed each task both silently and aloud. In each phase, the team carefully monitored their pupil dilation.

The results were subtle, but striking. Participants’ pupils dilated during each overt speech task, but this same muscle movement depended across each inner speech directive. Pupils constricted while volunteers silently counted sequentially, dilated for interval counting, and didn’t change at all for number-subject counting. But regardless of the inner speech task, pupil size changes were always smaller than they were during overt speech.

“The results of the present study do not support the initial assumption that inner and overt speech produce equivalent pupillary responses,” the team summarized. “Across the three experiments, overt speech consistently elicited pupil dilation, whereas inner speech did not.”

This collective data indicates that our inner speech isn’t simply the nonverbal counterpart to talking aloud. Instead, it’s a form of communication that appears to correspond to lower levels of brain arousal. If true, measuring pupil dilation may become a valuable tool when attempting to assess internal language and thinking. This could be particularly useful for studying language development in young children and infants who can’t properly describe their experiences to adults yet.