Following a synapse in the LGN, the axons travel posteriorly as the optic radiations to terminate in the primary visual (calcarine) cortex in the occipital lobe (Fig 1-21). The most inferior of the fibers first travel anteriorly, then laterally and posteriorly to loop around the temporal horn of the lateral ventricles (called the Meyer loop) (see Fig 1-17). More superiorly, the fibers travel posteriorly through the deep white matter of the parietal lobe. The macular (central) fibers course laterally, with the peripheral fibers concentrated more at the superior and inferior aspects of the radiations. Injury to fibers within a radiation produces a homonymous hemianopia, a contralateral visual field defect that respects the vertical midline. If the corresponding fibers from the 2 eyes are close to each other, the field defect is identical in each eye, or congruous. Congruous field defects occur as a result of lesions involving the calcarine cortex. More anterior involvement often produces incongruous field defects, suggesting that the corresponding fibers lie farther apart more anteriorly in the visual pathways.
The primary visual cortex (known variously as V1, striate cortex, or Brodmann area 17) is arrayed along the horizontal calcarine fissure, which divides the medial surface of the occipital lobe. Fibers of the optic radiations terminate in the fourth layer of the 6 layers of the primary visual cortex. The macular fibers terminate more posteriorly. Fibers from the most lateral (temporal crescent) visual field (originating only in the contralateral eye) terminate most anteriorly.
Figure 1-21 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 hemifield, plotted with kinetic perimetry, corresponds to the regions of the striate cortex in A. The stippled area corresponds to the monocular temporal crescent, which is mapped in the most anterior ∼8% of the striate cortex.
(Illustrations by Christine Gralapp.)
The cortex is heavily weighted to central retinal activity, with 50%–60% of the cortex responding to activity within the central 10° and approximately 80% of the cortex devoted to macular activity (within 30°). The superior portion of the cortex continues to receive information from the inferior visual field in a retinotopic distribution. This retinotopic mapping throughout the afferent visual pathways allows lesions to be localized based on visual field defects. In addition, the anteromedial portion of the striate cortex represents the far monocular temporal visual field of the contralateral eye (temporal crescent). Therefore, a far monocular temporal visual field defect localizes the lesion to the contralateral anterior occipital cortex (see Fig 1-21).
The parastriate cortex (also called V2, or Brodmann area 18) is contiguous with the primary visual cortex and receives its input from V1. Area V3 lies primarily in the posterior parietal lobe and also receives direct input from V1. Area V3 has no sharp histologic delineation from V2 and sends efferent information to the basal ganglia (pulvinar) and the midbrain. Cells in this area are thought to be able to respond to more than one stimulus dimension, suggesting that visual integration occurs in this region. Area V4, which is located within the lingual and fusiform gyrus, seems to be particularly sensitive to color. Damage to this area is probably responsible for most cases of cerebral achromatopsia. Anterior and lateral to area V4, V5 (posterior and within the superior temporal sulcus and gyrus subangularis) is very sensitive to movement and direction (Fig 1-22). The underlying white matter is heavily myelinated. Area V5, which corresponds to the medial temporal visual region, receives ipsilateral input from V1 and direct input from the M-cell layers of the LGN. The neurons in this area encode the speed and direction of moving stimuli. This sensory area is likely the origin of pursuit movements and thus connects the afferent and efferent pathways. The receptive fields of V5 are larger than those of V1. The SC receives afferent input both directly from the anterior visual pathways and indirectly from the calcarine cortex. The superficial layers contain a retinotopic map that overlies the deeper layers, which are primarily concerned with saccadic generation.
Figure 1-22 Parallel visual processing pathways in the human brain. The occipitotemporal (ventral, or “what”) pathway begins in the striate cortex (V1) and projects to the angular gyrus for language processing, to the inferior temporal lobe for object identification, and to the limbic structures. The occipitoparietal (dorsal, or “where”) pathway begins in the striate cortex and projects to the posterior parietal cortex and the superior temporal cortex, and is responsible for visual–spatial analysis. MT = medial temporal visual area.
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Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.