Topographic Organization of the Visual Pathway
The fibers from individual regions of the retina occupy specific positions in the different parts of the optic system. To illustrate this in a simple way, the retina is subdivided into four quadrants that all share the center, namely, the macula with the central fovea (area of highest visual acuity). The fibers of the fovea show a regular point-to-point connection between fovea, lateral genicu-late body, and striate area.
The halves of the visual fields of each eye (visual hemifields) (A1) are projected onto the respective contralateral halves of the re-spectiveretina (hemiretinas) (A2). Immedi-ately after the exit of the optic nerve from the eyeball (A3), the macular fibers lie on the lateral side of the nerve, with the fibers from the nasal half of the macula lying in the cen-ter, surrounded by fibers of the temporal half of the macula. Further along the optic nerve, the macular bundle comes to occupy the center (A4).
The fibers of the nasal hemiretinas (con-tinuous lines) cross to the opposite side in the optic chiasm (A5). While doing so they take a strange course. The medial fibers cross, then run a short distance into the con-tralateral optic nerve, and finally turn in a right angle into the contralateral optic tract. The lateral fibers run a short distance into the ipsilateral optic tract and then turn abruptly into the contralateral tract. The fibers of the temporal hemiretinas (broken lines) do not cross but continue in the ipsi-lateral tract.
The optic tract (A6) thus contains the fibers of the corresponding halves of both retinas: the left tract contains the fibers of the left hemiretinas, the right tract contains the fibers of the right hemiretinas. The fibers of the two superior retinal quadrants lieven-tromedially, those of the two inferior quad-rants lie dorsolaterally, while the fibers of the macula take a central position. Prior to radiating into the lateral geniculate body (A7), the fibers rearrange so that the macu-lar fibers form a central wedge, the fibersfrom the upper retinal quadrants come to lie medially, and the fibers from the lower reti-nal quadrants lie laterally.
The fibers in the lateral geniculate body (A8) terminate in the same arrangement. The central wedge of the terminal macular fibers makes up almost half of the genicu-late body. The fibers from the periphery of the retina terminate in the most anterior and ventral regions of the lateral geniculate body. The terminals of the ipsilateral and contralateral fibers in the geniculate layers are shown schematically in light gray and dark gray (A9) (see also p. 257, A). The geniculate cells of the central wedge project to the posterior region of the striate area (A10). The area of highest acuity, which in the human retina measures slightly more than 2 mm in diameter, is represented by the largest portion of the visual cortex. Ros-trally to it lie the much smaller areas for the remaining parts of the retina. The upper quadrants of each retina are represented in the upper lip of the calcarine sulcus, and the lower quadrants in the lower lip.
Clinical Note: Corresponding to the arrange-ment of fibers, injury to the visual pathway in specific segments results in various patterns of loss of vision. It should be taken into considera-tion that the lower halves of each retina register the input from the upper halves of the visual fields, while the upper halves of each retina regis-ter the input from the lower halves of the visual fields. The same is true for the left and right halves of each retina. Injury to the optic tract, lateral geniculate body, or visual cortex on the left side affects the left halves of each retina and the right halves of each visual field. The result is ho-monymous hemianopia on the right side. In case of bitemporal heteronymous hemianopia, injury to the crossing nasal fibers of both retinae (e.g., in case of tumors of the hypophysis near the optic chiasm) results in bilateral loss of the temporal halves of the visual fields. Damage of both visual cortices causes visual agnosia.
A11 Blind spot (papilla of the optic nerve).