ABSTRACT
Those new to psychophysiology might be surprised to learn that the small opening at the middle of the eye (the pupil) can offer interesting information about behavioral events. Charles Darwin, in his book on expression of emotions in humans and animals, speculated that the brain is affected by the powerful emotion of fear and causes the pupils to dilate. Much before Darwin, the pupils of the eyes had been referred to as “windows to the soul.” Although this romantic notion is untrue, scientists have discovered some fascinating things about pupillary size changes and behavior. Pupillometry refers to the measurement of variations in the diameter of the pupillary aperture of the eye.
