Pupillary pathways, cross-section view. The dotted line represents the efferent pathway; the solid line represents the afferent pathway. A light stimulating the left retina generates impulses that travel up the left optic nerve and divide at the chiasm. Some impulses continue up the left tract; some cross and continue up the right tract. The impulses arrive at each pretectal nucleus and stimulate cells, which in turn send impulses down the third cranial nerve to each iris sphincter, causing each pupil to contract. It is because of the double decussation, the first in the chiasm and the second between the pretectal nuclei and the Edinger-Westphal nuclei, that the direct pupil response in the left eye equals the consensual response in the right eye.