A new study by researchers at Utrecht University and the University of Amsterdam has found that people with synesthesia, a neurological condition in which the senses blend together, react differently to different colors. The study found that synesthesia does not just experience these colors in their minds but also produces measurable differences in pupil reactions. The findings suggest that the brain processes internally generated colors and real visual input in a similar way. The researchers tested whether pupil size could provide a physical sign of colors perceived internally. They found that the colors reported seeing when they look at gray letters or numbers are linked to real physiological changes in their eyes. These findings could lead to better ways to identify synesthesia using physiological measures.