The pigment architecture of the human occipital lobe

With the aid of steromicroscopical examination of pigment preparations uo to 1,000 micronm thick the various areas of the occipital lobe of the human brain are described. In the occipital lobe there are three main cortical types, forming the striate, the parastriate area, and the peristriate zone. The parastriate area is a horseshoe-shaped fringe area adjacent to the primary visual field. It is conspicuously marked by a relatively dark stripe of pigmented granule cells dividing the fourth layer into a light upper, a pigmented middle, and a light lower zone, a unique feature of the parastriate area, which is not to be found in any other isocortical area. This three-fold layer disappears abruptly as one passes from the parastriate to a peristriate area, permitting the definitive determination of this boundary. The peristriate zone is quite large in the inferior and the lateral parts of the occipital lobe and comparatively small in the superior parts. Its peripheral boundaries do not coincide with the limits of the occipital lobe. Parts of the cuneus on the one side are covered by parietal and temporal areas, whereas peristriate areas on the other side penetrate widely into the temporal lobe along the medial occipitotemporal and to a lesser extent the parahippocampal gyrus. As regards pigment architecture, the peristriate zone is divisble into ten areas. A relatively simply organized area accompaines the parastriate area. The structure of the cortex changes gradually until one reaches the various fringe areas adjacent to the parietal and temporal areas. The limit adjoining the temporal areas is sharply traceable, since the lower of two light cortical stripes vanishes abruptly when passing from a peristriate to a temporal area. The most highly developed field, the area peristriata magnopyramidalis, occupies part of the inferolateral margin of the occipital lobe. It displays a wealth of large pyramids in the lower reaches of the third layer, which contain tightly packed and densely stained pigment granules forming large rounded aggregates in the basal cytoplasm. Pyramids of this type give the field a close resemblance to association fields such as the posterior speech area, where they are commonly encountered as the predominant neurons of the third layer.