Anatomy, physiology and blood supply

Axons of the retinal ganglion cells project through the optic nerves, chiasm and tracts. Each optic tract carries visual information from the contralateral hemifield of both eyes, projecting mainly to the lateral geniculate nucleus. Smaller projections subserving the pupillary light reflex exit just prior to the termination of the tract, heading to the pretectal nuclei, and there are minor projections to hypothalamic circadian nuclei and brainstem ocular motor structures. The tract lies ventral to the brain, coursing just dorsal to the hippocampus. The fibres from the two eyes are not well aligned initially, but the correspondence improves gradually along the course of the tract. The retinotopic map also becomes tilted, so that the macula is represented dorsally, superior quadrant laterally, and inferior quadrant medially (Hoyt & Luis, 1963). The optic tract is supplied by branches of the anterior choroidal artery, which originates from the supraclinoid segment of the internal carotid artery. The anterior choroidal artery courses posterolaterally, at first inferior and lateral to the optic tract, and later on its medial side, giving penetrating branches to the optic tract and to the lateral geniculate nucleus.

The lateral geniculate nucleus contains the terminations of the optic tract fibres and the neurons which give rise to the optic radiations. It is not merely a relay nucleus, but receives inputs from visual cortex (Sillito & Murphy, 1988) and brainstem (Harting et al., 1991) that modulate its information transfer. It is a subnucleus in the ventro-posterolateral thalamus. Neighbouring subnuclei are the medial geniculate nucleus ventromedially, the ventral posterior nucleus dorsomedially, and the pulvinar dorsally. The acoustic radiations pass dorsomedially on their way to auditory cortex.