Approximately 80 percent of the fibers of the optic tract terminate in the lateral geniculate nucleus of the thalamus and synapse with the six laminae of its neurons. Three of these laminae (1, 4, 6) receive crossed (nasal) fibers from the contralateral eye, and three (2, 3, 5) receive uncrossed (temporal) fibers from the ipsilateral eye. Selective occlusion of either component of the dual blood supply to the lateral geniculate, consisting of the anterior and posterior choroidal arteries, is infrequent but when it does occur, produces a characteristic “multiple sectoral field defect.” Occlusion of the anterior choroidal artery produces a contralateral quadruple sectoranopia, meaning homonymous sectoral defects in the upper and lower quadrants of both eyes. Occlusion of the posterior (lateral) choroidal artery produces a contralateral homonymous horizontal sectoranopias.
The geniculate cells project via the optic radiations to the visual (striate) cortex of the occipital lobe, also called area 17 (Brodmann classification) or V1 (Figs. 12-4 and 12-5).
In their course through the temporal lobes, the fibers from the lower and upper quadrants of each retina diverge. The lower ones arch around the anterior pole of the temporal horn of the lateral ventricle before turning posteriorly (forming Meyer’s loop), and their disruption causes a contralateral superior quadrant field deficit. The upper ones follow a more direct path through the white matter of the uppermost part of the temporal lobe (Fig. 12-4), and probably of the adjacent interior parietal lobe; their disruption causes a contralateral inferior field deficit. Both groups of fibers merge posteriorly at the internal sagittal stratum (see Fig. 12-2E and F).
It is in Brodmann area 17, embedded in the medial lip of the occipital pole, that cortical processing of the retinogeniculate projections occurs. The receptive neurons are arranged in columns, some of which are activated by edges and forms and others by moving stimuli or by color. The neurons for each eye are grouped together and have concentric, center-surround receptive fields. The seminal studies of Hubel and Wiesel have elucidated much of this visual cortical anatomy and physiology and their papers, for which they were awarded the Nobel Prize, should be consulted for a fuller appreciation of the organization of the visual cortex. The deep neurons of area 17 project to the secondary and tertiary visual areas of the occipitotemporal cortex of the same and opposite cerebral hemispheres and also to other multisensory parietal and temporal cortices. These complex extrastriate connections are still being elucidated.
Portions of the visual systems are specialized for the perception of motion, color, stereopsis, contour, and depth perception. As Ungerleider and Mishkin proposed, the flow of secondary visual processing can be conceptually divided into a dorsal stream, which carries predominantly spatial information to the parietal lobe (“the where”) and a ventral stream, which carries shape and color information to the temporal lobe (“the what”) as proposed by Goodale and Milner in 1992 and further articulated by Levine and colleagues.
The normal development of the connections described previously requires that the visual system be activated through several critical periods of development. The early deprivation of vision in one eye causes a failure of development of the geniculate and cortical receptive fields of that eye. Moreover, in this circumstance the cortical receptive fields of the seeing eye become abnormally large and usurps the monocular dominance columns of the blind eye (Hubel and Wiesel). In children with a congenital cataract, if the opacity is removed after a critical period of development, the eye will remain amblyopic. A severe strabismus in early life, especially an esotropia, will have the same effect (amblyopia ex anopsia).