Pupillometric Evaluation of the Dynamics of the Pupillary Response to a Brief Light Stimulus in Healthy Subjects

PURPOSE: To investigate the correlation between measurements of amplitude (A) and peak velocity (V) of constriction in the pupil light reflex of normal subjects, and to determine the effects of stimulus intensity, pupil size, and age on this relationship. METHODS: The pupil response to a variable intensity 1.0 second light stimulus presented under open-loop conditions (Maxwellian optics) was measured using infrared video techniques in 43 healthy subjects aged 20 to 75 years old. RESULTS: In response to the "standard-intensity" light stimulus, mean measurements of A and V were 1.92 mm (SD 0.39) and 5.65 mm/s (SD 1.17), respectively. Over a 4.0 log unit range of stimulus intensities measurements of A and V were seen to co-vary with the data being best fit by the equation V=0.86+2.65A (linear regression coefficient, R = 0.919, P < 0.001). In each subject the regression plot was used to normalize the velocity estimates for A = 1.0 mm; these normalized velocity estimates showed no significant correlation with the starting size of the pupil or the age of the subject. CONCLUSIONS: There is a strong linear correlation between amplitude and peak velocity of constriction for the pupil light reflex in normal subjects. This relationship is unaffected by the stimulus intensity, size of the pupil, or age of the subject. Clinicians and researchers must keep this interdependence in mind when drawing inferences from the observed (or measured) speed of the pupillary constriction to light.