Optic Tract

The fibers exiting from the chiasm proceed circumferentially around the diencephalon lateral to the hypothalamus and in contact with the ambient cistern (see Fig 1-11). Just prior to the LGN, the fibers involved in pupillary pathways exit to the pretectal nuclei; other fibers exit to the superficial layers of the superior colliculus via the brachium of the superior colliculus. These fibers originate from ipRGCs and are likely the sole source of pupillomotor input from the retina to the midbrain. These ipRGCs also project to the suprachiasmatic nucleus of the hypothalamus, which is probably responsible for light-induced diurnal rhythms.

The incongruous nature of optic tract visual field defects is explained by the lack of close proximity between corresponding fibers from the right and left eyes. Most of the axons that originate in the retinal ganglion cells terminate within the LGN.

The LGN is located in the posterior thalamus below and lateral to the pulvinar and above the lateral recess of the ambient cistern. The LGN is a peaked, mushroom-shaped structure that is divided into 6 levels. The 4 superior levels are the termini of P-cell axons, which are the ganglion cells with smaller receptive fields and are responsible for mediating maximal spatial resolution and color perception. The 2 inferior layers receive input from the M-cell fibers, which are the ganglion cells with larger receptive fields and are more sensitive to detecting motion. Axons originating in the contralateral eye terminate in layers 1, 4, and 6; the ipsilateral fibers innervate 2, 3, and 5. As the fibers approach the LGN, the superior fibers move superomedially and the inferior fibers swing inferolaterally. Overall, the retinal representation rotates almost 90°, with the superior fibers moving medially and the inferior fibers laterally. The macular fibers tend to move superolaterally. Cortical and subcortical pathways may modulate activity in the LGN. In addition, the cortex, superior colliculus, and pretectal nuclei project back to the LGN.

Cortex

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 (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 the radiations produces a homonymous hemianopia, a contralateral visual field defect that respects the vertical midline. If the corresponding fibers from the 2 eyes are in close proximity, the field defect is identical in each eye (congruous). Congruous field defects occur with 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.

Figure 1-21 Primary visual cortex and corresponding visual field representation. A, Left occipital cortex showing the location of striate cortex within the calcarine fissure. The blue color 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 a Goldmann perimeter, corresponds to the regions of the striate cortex in A. The black circle marks the region of striate cortex corresponding to the contralateral eye’s blind spot. The stippled area corresponds to the monocular temporal crescent, which is mapped in the most anterior ~8% of

striate cortex. (Illustrations b y Christine Gralapp.)

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 of the 6 layers in 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.

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 on the basis of visual field defects. In addition, the anteriormedial 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 receives direct input from V1. 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 capable of responding to more than one stimulus dimension, suggesting that visual integration occurs in this region. V4, 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. The V5 area, 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 here encode the speed and direction of moving stimuli. This sensory area is likely the origin of pursuit movements and thus links the afferent and efferent pathways. Compared with those of V1, the receptive fields are larger. The superior colliculus receives afferent input both directly from the anterior visual pathways and secondarily 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. The occipitotemporal, 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, or “where,” pathway begins in the striate cortex and projects to the posterior parietal and superior temporal cortex, concerned with visuospatial analysis.

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