For short wavelength stimuli that have a high melanopsin excitation, the pulsed PIPR amplitude was 14.73% higher in PD participants (median = 80.32%, IQR = 23.16%) compared to controls (median = 65.59%, IQR = 20.52%) (p = 0.018), indicating reduced melanopsin contributions to this process (i.e., closer to baseline diameter in the PD group than controls). Here, OPN4 is linked to Parkinson disease.