Change in the diameter of the pupil of the eye in response to stimuli. Pupil size is regulated by the sphincter pupillae muscles in the iris, which respond to parasympathetic stimulation, and the dilator pupillae muscles, innervated by the sympathetic nervous system. Dilation can result from sympathetic nervous system stimulation or the suppression of the parasympathetic nervous system. Pupil dilation has been investigated by several researchers as an index of stress and continues to be a phenomenon of interest in PDD. See: Bradley & Janisse (1981); Webb, Honts, Kircher, Bernhardt & Cook (2009).