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.
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.
Figure 3-8 The visual pathways.
(Illustration by Dave Peace.)
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).
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).
Figure 3-9 Lateral geniculate nucleus (LGN). A, The LGN receives the fibers of the corresponding optic tract. Layers 1, 4, and 6 receive input from the crossed fibers of the optic tract; layers 2, 3, and 5 receive input from the uncrossed fibers. Layers 1 and 2 represent the magnocellular pathways, which are concerned with detection of movement. The remaining 4 layers represent the parvocellular pathways, which are responsible for color vision and visual acuity. B, The hilum represents central (macular) vision and is perfused by the posterior choroidal artery, the medial horn represents inferior vision, and the lateral horn represents superior vision. These areas are perfused by the anterior choroidal artery.
(Redrawn with permission from Liu GT, Volpe NJ, Galetta SL. Neuro-Ophthalmology: Diagnosis and Management. 2nd ed. New York: Elsevier; 2010:299–300. Illustration by Mark Miller.)
Figure 3-10 Optic radiations. A, Axial view of the brain demonstrating the optic chiasm, optic tract, and optic radiations, which connect the LGN to the occipital lobe. B, Schematic of the optic radiations, sagittal view. The lower radiations (subserving the superior visual field) course anteriorly before looping posteriorly in the temporal lobe. The upper radiations course dorsally in the parietal lobe to terminate in the occipital lobe above the calcarine fissure.
(Part A reproduced with permission from Sherbondy AJ, Dougherty RF, Napel S, Wandell BA. Identifying the human optic radiation using diffusion imaging and fiber tractography. J Vis. 2008;8(10):12.1–11, Figure 1. Part B redrawn with permission from University of Texas at Dallas. Illustration by Mark Miller.)
Figure 3-11 Primary visual cortex and corresponding visual field representation. A, Left occipital cortex showing the location of the striate cortex within the calcarine fissure. Blue represents the macula (central visual field); green represents the inferior visual field; and orange represents the superior visual field. The most peripheral fibers are represented by the stippled colors. B, Right visual hemifield, plotted with kinetic perimetry, corresponds to the regions of the striate cortex in part A. The stippled area corresponds to the monocular temporal crescent, which is mapped in the most anterior 8%, approximately, of the striate cortex.
(Illustrations by Christine Gralapp.)
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.