Visual Pathway
The visual pathway begins in the retina; impulses from the photoreceptors are transmitted to the optic chiasm via the optic nerve of each eye. Within the chiasm, the retinal fibers segregate into the right and left optic tracts. Each optic tract carries information for its respective field of vision. For example, the right optic tract consists of fibers from the ipsilateral temporal retina and the contralateral nasal retina. The corresponding hemifields represent the left half of the visual field for each eye. The optic tracts, whose cell bodies lie in the ganglion cell layer of the retina, go on to synapse at the lateral geniculate nucleus. The subsequent fibers further divide as they travel to the primary visual cortex (known variously as V1, striate cortex, or Brodmann area 17), where they terminate; the most inferior of the fibers (subserving the superior visual field) take one path and the more superior fibers (subserving the inferior visual field) follow a different one (Fig 3-8). Lesions at different locations along the visual pathway produce characteristic visual field defects that help localize the site of damage. Structures of the visual pathway are described further in the following sections and in BCSC Section 5, Neuro-Ophthalmology.
Optic chiasm
The optic chiasm makes up part of the anterior inferior floor of the third ventricle. It is surrounded by pia and arachnoid mater and is richly vascularized. The chiasm is approximately 12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick.
The extramacular fibers from the inferonasal retina cross anteriorly in the chiasm at the “Wilbrand knee” before passing into the optic tract. Extramacular superonasal fibers cross directly to the opposite tract. Extramacular temporal fibers pursue a direct course through the chiasm to the optic tract as a bundle of uncrossed fibers. The macular projections are located centrally in the optic nerve and constitute 80%–90% of the total volume of the optic nerve and the chiasmal fibers. Nasal macular fibers cross in the posterior part of the chiasm. Approximately 53% of the optic nerve fibers are crossed, and 47% are uncrossed.
Optic tract
Each optic tract is made up of fibers from the ipsilateral temporal retina and the contralateral nasal retina. Fibers (both crossed and uncrossed) from the upper retinal projections travel medially in the optic tract; lower projections move laterally. The macular fibers are dorsolateral within the optic tracts.
Lateral geniculate nucleus
The lateral geniculate nucleus (LGN) is the synaptic zone for the higher visual projections. It is a mushroom-shaped structure in the posterior thalamus that receives approximately 70% of the optic tract fibers within its 6 alternating layers of gray and white matter (the other 30% of the fibers go to the pupillary nucleus). Layers 1, 4, and 6 of the LGN contain axons from the contralateral optic nerve. Layers 2, 3, and 5 arise from the ipsilateral optic nerve. The 6 layers, numbered consecutively from inferior to superior, give rise to the optic radiations (Fig 3-9).
Optic radiations
The optic radiations connect the LGN with the visual cortex of the occipital lobe. From the LGN, inferior fibers (which subserve the superior visual field) travel anteriorly, then laterally and posteriorly, looping around the temporal horn of the lateral ventricles in the temporal lobe (Meyer loop). Superior fibers (which subserve the inferior visual field) travel posteriorly through the parietal lobe (Fig 3-10).
Primary visual cortex
The primary visual cortex, the thinnest area of the human cerebral cortex, has 6 cellular layers and occupies the superior and inferior lips of the calcarine fissure (also called calcarine sulcus) on the posterior and medial surfaces of the occipital lobes. Macular function is extremely well represented in the visual cortex and occupies the most posterior position at the tip of the occipital lobe. The most anterior portion of the calcarine fissure is occupied by contralateral nasal retinal fibers only (Fig 3-11).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.