"Pupil response" refers to the way the pupils of the eyes react to changes in light. When exposed to bright light, the pupils constrict, or become smaller, to limit the amount of light entering the eye. In dim light, the pupils dilate, or enlarge, to allow more light in, helping to improve vision in low-light conditions. This response is controlled by the iris, the colored part of the eye. The pupil response is an important indicator of neurological function and can be assessed during medical examinations. Abnormal pupil responses may signal issues such as brain injury, drug effects, or other health conditions. Healthcare professionals often use a flashlight to test pupil reaction as part of a routine check-up.