Binocular alternating pulse stimuli: Experimental and modeling studies of the pupil reflex to light

In our experiment, alternating pulse stimuli of both low and high intensity are used to study the pupil reflex to light. When applied monocularly, high intensity stimulation normally results in a sustained contraction; when alternated between the two eyes, it is found to produce small transient responses similar to those obtained with low intensity monocular stimulation. In order to study the mechanisms regulating these binocular responses, a model of the pupillary light reflex is constructed. It includes parallel AC and DC pathways for processing the light stimulus to produce motor signals to the iris muscles, nonlinear parameter control of pathway gains dependent upon internal operating level, binocular summation of DC pathway signals to produce that operating level, equal motor responses of both pupils, and iris neuromuscular delays and lags. The model is found to simulate the experimental data. It shows the binocular transient responses to be due to the canceling by summation of the symmetric DC pathway responses to alternating stimuli, thus allowing the AC pathway signals to become manifest. Therefore the dilatory portion of the transient responses is shown to be due to the lead-lag operator in the AC pathway and not to the off-dilatation elicited by removal of the light stimulus from the eye. Finally the results of our study are used to discuss the Marcus Gunn pupillary sign, a clinical test utilizing this binocular alternating pulse stimulation for detecting unilateral afferent defects.