Pupillary light reflex in humans: Evidence for an unbalanced pathway from nasal retina, and for signal cancellation in brainstem

Abstract

The pupillary light reflex pathway is a simple binocular system that shows both convergence and divergence of information. We studied pathway organization with stimuli delivered to restricted portions of the retina of either eye. When a stimulus turned off at one location and on at another, small bilateral pupil contractions occurred. Response size was minimum for simultaneous onset and offset, suggesting cancellation of signals in the brainstem. When a stimulus fell on nasal retina, the response was greater in the stimulated eye. The difference between responses was much less sensitive to stimulus timing than was the average response size. All of our results can be economically explained by a simple proposed scheme for pathway organization.