Walsh & Hoyt: Inhibitory Influences on the Pupil

When a light-adapted eye is suddenly exposed to a short period of darkness, the pupils dilate after a short latency period of about 300400 milliseconds. This dilation still occurs, although it is slightly diminished, after the sympathetic innervation to the dilator muscle is interrupted. Thus, it must be partly caused by inhibitory impulses that affect the Edinger-Westphal nucleus. Because darkness is a positive stimulus for the photopic retina and evokes an ""off-response"" in the electroretinogram, it is reasonable to assume that this ""off-stimulus"" produces inhibitory impulses that are relayed to the mesencephalon, where they act on neurons concerned with pupillary constriction. Role of the dark reaction in causing pupil dilation. Cortical and hypothalamic inhibition of pupillary constriction. Brain stem and spinal cord inhibition of pupillary constriction.