of visual space. Merging occurs in stages at different points within the visual pathway. As a result, a unilateral lesion at a more distal point within the pathway will produce different visual deficits from one at a more proximal point because the degree of merging will be different. With sufficient knowledge of central visual pathways, the location of a lesion can be deduced from the type of visual deficit present (for details, see below).

Each retina can be divided into quadrants based on superior, inferior, nasal, and temporal coordinates (Fig. 8). The fovea is located at the posterior pole or intersection point of the quadrants. When light rays enter the pupillary opening, the image projected on the retina is reversed and inverted. Thus, the image of an object located within a given region of the peripheral visual field is projected to the opposite position in the peripheral retinal field. Objects located in the center of the visual field can be focused on foveal regions of both eyes simultaneously. Visual fields are usually tested separately for each eye, with the patient looking straight ahead and the eye immobile. Regions of the visual field are designated as right, left, upper, and lower quadrants.

It is important to note that visual field deficits are always defined in terms of visual field loss, not retinal field location. In the 180° visual field viewed with both eyes, a central zone of about 90° (the binocular zone) can be viewed with both eyes (Figs. 9 and 10). The more peripheral regions of the visual field are visible only by the ipsilateral eye (because the nose gets in the way), creating two temporal, crescent, monocular zones. A blind spot in each retina is created by the optic nerve head or optic disk; here, the exiting fibers of ganglion cells coalesce and become invested with myelin. The optic disc is located along the horizontal meridian and displaced in the nasal direction from the fovea. Even with one eye closed, there is no conscious perception of a blind spot, just as there is no perception of the shadows that are cast by retinal blood vessels. This occurs because the visual cortex is able to compensate for these normal interruptions within the visual field; however, both can be demonstrated during visual exams.

macula lutea blood vescles optic disk temporal -<— — t inferior

FIGURE 8 Fundoscopic view of the human retina. The inner (vitreal) surface of the retina of the right eye is illustrated. Axons of ganglion cells from the entire retina converge and exit the eye at the optic disk, where they penetrate the retina and produce an area with no neuronal elements other than the optic nerve, thus creating a blind spot. Blood vessels enter the eye at the optic disk and spread over the surface of the retina except in the region of the fovea located in the macula lutea. Two hypothetical lines crossing at right angles through the fovea delineate the four quadrants of the retina. The optic disk is located in the inferior nasal quadrant.

Left and Right Visual Hemifields

Optic nerve fibers exit the eye and project to the optic chiasm, where partial decussation occurs (see Fig. 10). Fibers originating from the nasal portions of both retinas cross the midline and merge with uncrossed fibers from the temporal retina of the opposite eye. Fibers proximal superior superior

macula lutea blood vescles optic disk

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