In a surprising discovery, a new study has overturned a long-held belief about how our pupils respond to breathing.

For years scientists have believed that pupils — those little black circles in the middle of our eyes — dilate when we inhale and contract to their smallest size when we exhale.

However, this new research presents evidence that refutes this notion.

A team of researchers from the Department of Clinical Neuroscience at Karolinska Institutet conducted three separate experiments to get to the bottom of this mystery.

They wanted to see how pupil size changed during different types of breathing and activities. Two of these experiments were “pre-registered,” meaning the researchers planned and recorded their methods in advance to ensure their findings were as reliable as possible.

The experiments included both resting tasks and visual perception tasks, and the participants breathed through their nose and mouth under various conditions. The results were clear and consistent: the pupils were indeed smallest around the beginning of inhalation and largest around the peak of exhalation.

This pattern was observed across all conditions, whether participants were resting or engaged in some task.

This phenomenon has been named the inspiratory-pupil phase effect (RPPE). It directly contradicts the old belief that pupils are largest during inhalation and smallest during exhalation. According to the researchers, the processes of dilation and contraction overlap with both phases of breathing, which adds an extra layer of complexity to understanding how our bodies work.

The consistency and significance of these findings are prompting calls for more research. Scientists want to understand the underlying mechanisms of RPPE and its potential impact on human behavior. For example, how might this discovery impact our understanding of attention, stress, or even mental health?

As researchers dig deeper into rPPE, we may soon learn even more about the complex connections between our physical responses and our daily lives.

This research has been uploaded to the preprint server bioRxiv and is awaiting review.