Characteristics of the pupillary light reflex in the alert rhesus monkey

This study investigated the static and dynamic characteristics of the pupillary light reflex (PLR) in the alert rhesus monkey. Temporal characteristics of the PLR were investigated with Maxwellian viewing during sinusoidal changes in illumination of a 36 degrees stimulus in both monkeys and humans. Bode plots of the PLR response were fitted by a linear model composed of a delay combined with a cascaded first- and second-order filter. The Bode magnitude plots conformed to this model with a sharp roll-off above 1.3 Hz for the human PLR and 1.9 Hz for the monkey PLR. Bode phase angle plots were fitted by this model with a delay of 280 ms for humans and 160 ms for monkeys. To investigate the influence of the sympathetic innervation of the iris on steady-state pupil diameter, dynamics of pupillary responses, and the latency of the PLR, we blocked this innervation pharmacologically with a selective alpha-1 adrenoreceptor antagonist. Although there was a resultant miosis (decrease in pupil diameter) from the relaxation of the pupil dilator muscle, no other measures of the PLR, including the dynamics and latency, were significantly affected by this treatment. We examined the pupillary responses evoked by visual stimuli presented either binocularly or monocularly at various locations on a 80 x 60 degrees tangent screen. These pupillomotor fields revealed that, as has been reported for humans, stimuli at the fovea and surrounding macular region of monkeys produce substantially larger pupillary responses than more peripheral stimuli and that binocular responses are substantially greater than can be accounted for by the linear summation of binocular retinal illuminance. In conclusion, we found that the spatial characteristics of the PLR of the rhesus monkey are very similar, in all important aspects, to those reported for humans and that the temporal responses of the PLR are comparable between the two species. The rhesus monkey thus provides an excellent model for experimental studies of the neural control of the pupil.