The occipital lobes are the termini of the geniculocalcarine pathways and are essential for visual perception and recognition. This part of the brain has a large medial surface and smaller lateral and inferior surfaces (Fig. 21-1). The parietooccipital fissure creates a noticeable medial boundary with the parietal lobe, but laterally the occipital lobe merges with the parietal and temporal lobes. The large calcarine fissure courses in an anteroposterior direction from the pole of the occipital lobe to the splenium of the corpus callosum; area 17, the primary visual receptive cortex, lies on its banks (see Figs. 21-1 and 21-2). Area 17 is a typical homotypical cortex but is unique in that its fourth receptive layer is divided into two granular cell laminae by a greatly thickened band of myelinated fibers, the external band of Baillarger. This stripe, also called the line or band of Gennari, is grossly visible and has given this area its name, striate cortex. The largest part of area 17 is the terminus of the retinal macular fibers that arrive via the lateral geniculate (see Fig. 12-2). The parastriate cortex (areas 18 and 19) lacks the line of Gennari and resembles the granular unimodal association cortex of the rest of similar areas in the cerebrum. Area 17 contains cells that are activated by the homolateral geniculocalcarine pathway (corresponding, of course, exclusively to the contralateral visual field); these cells are interconnected and project also to cells in areas 18 and 19. The latter are connected with one another and with the angular gyri, lateral and medial temporal gyri, frontal motor areas, limbic and paralimbic areas, and corresponding areas of the opposite hemisphere through the posterior third (splenium) of the corpus callosum.
The occipital lobes are supplied almost exclusively by the posterior cerebral arteries and their branches, either directly in most individuals or through an embryologically persistent branch of the internal carotid arteries (“fetal” posterior cerebral artery). A small area of the occipital pole receives blood supply from the inferior division of the middle cerebral artery. This assumes importance in the clinical finding of “macular sparing,” discussed in
Chap. 12.
The connections among these several areas in the occipital lobe are complex, and the notion that area 17 is activated by the lateral geniculate neurons and that this activity is then transferred and elaborated in areas 18 and 19 is surely not complete. Actually, 4 or 5 occipital receptive fields are activated by lateral geniculate neurons, and fibers from area 17 project to approximately 20 other visual areas, of which only 5 are well identified. These extrastriate visual areas lie in the lingula and posterior regions of the occipital lobes. As Hubel and Wiesel have shown, the response patterns of neurons in both occipital lobes to edges and moving visual stimuli, to on-and-off effects of light, and to colors reflects this complexity. Hence form, location, color, and movement each have separate localizable hierarchical arrangements of neurons in series. The monographs of Polyak and of Miller contain detailed information about the anatomy and physiology of this part of the brain.
Beyond the effects on vision of lesions in the occipital lobes, monkeys with bilateral lesions in the temporal visual zones lose the ability to identify objects; with posterior parietal lesions, there is loss of ability to locate objects.
Clinical Effects of Occipital Lobe Lesions