In the Pupillary Light Reflex
, the fibres of the optic tract terminate at the pretectal nucleus in the midbrain and not at the LGN of the thalamus. The nasal fibres cross at the optic chiasm and transfer the signal to the contralateral pretectal nucleus, whereas the temporal fibres relay the information to the ipsilateral pretectal nucleus. Each pretectal nucleus projects bilaterally to the Edinger-Westphal nuclei (of cranial nerve III). This begins the efferent limb of the reflex by sending preganglionic parasympathetic axons within the oculomotor nerve to the ciliary ganglion. Subsequently, the short ciliary nerves arising from the ciliary ganglion stimulate the pupillary sphincter muscle and cause pupillary constriction. Even when the light is shined in only one eye, a consensual response occurs in the other eye too because of 
crossing fibres and 
bilateral fibres leaving the pretectal nucleus. This test is useful in examining unconscious patients.