Early Views Of The Organization Of Human Visual Cortex

Early views of the functional organization of visual cortex were based largely on two different types of criteria. First were behavioral criteria evaluated following brain injury to large or more restricted portions

Encyclopedia of the Human Brain Volume 1

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of the occipital lobe. Complete lesions of the occipital cortex in monkeys and dogs led to "Rindeblindheit" or complete cortical blindness. However, small lesions of occipital cortex led to "Seelenblindheit" or psychic blindness, in which dogs were capable of seeing and avoiding objects but no longer recognized what they saw. By the end of the nineteenth century, similar observations were made following brain injury in humans.

Second were the anatomical criteria that included the analyses of the cytoarchitecture, myeloarchitec-ture, pigmentoarchitecture, and, finally, the myelogen-esis of the optic radiations. The most widely recognized study of the cytoarchitecture of the occipital lobe was performed by Brodmann, who described three subdivisions of occipital cortex. He identified an area 17, largely contained within the calcarine sulcus, that was identified by a highly differentiated laminar structure in which layer 4 was subdivided into three sublaminae, IVA, IVB, and IVC. Area 17 is also known as striate cortex primarily because of the dense band of myelin, called the stria of Gennari, which is located in layer 4B. Brodmann also described two belts of cortex that surrounded area 17 that he named area 18, the occipital area, and area 19, the praeoccipital area. Unfortunately, the cytoarchitectural characteristics of areas 18 and 19 in humans were not well-specified in this study.

Von Economo and Koskinas described a tripartite organization of occipital cortex that they based on quantitative measurements of laminar widths and cell-packing density. Their areas OC (area striata), OB (area parastriate), and OA (area peristriata) correspond largely with Brodmann's areas 17, 18, and 19. Area 18 is easily recognized at the border with striate cortex (OC) and is characterized by a high cell density in layers II and IIIa, low density in layer V, and large cells in layer 6. A distinct population of very large pyramidal cells in layer IIIc of area OB characterizes the border of area OB with OC. This region, OBg or limes parastriatus gigantocellularis, is located in the region of OB cortex that contains the representation of the vertical meridian. In macaque monkeys, this region has been shown to contain a population of pyramidal cells that are immunoreactive to SMI-32, many of which were shown to project across the corpus callosum. Furthermore, in humans lacking a corpus callosum, this population of large pyramidal cells in layer IIIc is greatly reduced. The cytoarchitectonic characteristics of area 18 (or OB) at the border with area 17 (OC) are illustrated in Fig. 1A. This border is distinct in many respects, including the changes in

Border Area

Figure 1 Architecture of the 17 (OC)-18 (OB) border. (A) Cytoarchitectonic characteristics of the 17-18 border region. Black arrow-head indicates the border. Open arrowhead in area 17 indicates the limit of the fringe area, Randsaum. Open arrowhead in area 18 indicates the limit of the border tuft region, Grenzbuschel. Layers indicated according to Brodmann. (B) Myeloarchitectonic characteristics of the 17-18 border. Area 17 is characterized by a dense band of fibers in layer IVB, the stria of Gennari. The inner band of Baillarger begins to emerge within 17 and becomes dense within area 18. A dense border tuft of radially oriented fibers is located in area 18 just across the 17-18 border. From Amunts et al. (2000).

Figure 1 Architecture of the 17 (OC)-18 (OB) border. (A) Cytoarchitectonic characteristics of the 17-18 border region. Black arrow-head indicates the border. Open arrowhead in area 17 indicates the limit of the fringe area, Randsaum. Open arrowhead in area 18 indicates the limit of the border tuft region, Grenzbuschel. Layers indicated according to Brodmann. (B) Myeloarchitectonic characteristics of the 17-18 border. Area 17 is characterized by a dense band of fibers in layer IVB, the stria of Gennari. The inner band of Baillarger begins to emerge within 17 and becomes dense within area 18. A dense border tuft of radially oriented fibers is located in area 18 just across the 17-18 border. From Amunts et al. (2000).

width, density, and sublamination of layer IV, the density of layer VI, and increased in density in layer II. This border region is quite complex when viewed in sections stained for myelin (see Fig. 1B). Although the 17-18 border can be recognized by the change in myelin density in the infragranular layers with the emergence of the inner band of Baillarger, the myeloarchitecture of the more superficial layers is more complex. The region of area 17 just within the 17-18 border is known as the fringe area Randsaum (Rs) of Sanides and Vitzthum. Within the fringe area, the inner band of Baillarger, which is not prominent within area 17 proper, begins to emerge. On the other side of the 17-18 border, a narrow band of radially oriented fibers becomes prominent. This region is known as the border tuft, or Grenzbuschel (Gb), of Sanides and Vitzthum. This border tuft is located within von Bonin and Koskinas' area OBg, which corresponds to the callosal recipient-origin region of OB.

The anterior border of area OB is more difficult to describe, although von Economo and Koskinas were able to use quantitative criteria to describe the overall organization and possible subdivisions within area OA (OAi, OA2, and OAm). More recently, a clear separation of layers II and IIIA in 19 and a decrease in the cell density of layer IIIB in 19 have been used to distinguish area 18 from area 19.

The location and extent of Brodmann's areas 17,18, and 19 and von Economo and Koskinas' areas OC, OB, and OA are illustrated in lateral and medial views of the human brain in Fig. 2. Similarly, the striate, parastriate, and peristriate subdivisions of occipital cortex, based on the pigmentoarchitectonic of Braak (see later discussion), are illustrated. In the medial view of the hemisphere, Brodmann's area 18 is wider than von Bonin and Koskinas' area OB. Similarly, on the lateral occipital surface, area 18 is described to occupy a b 1 cm wide swath, whereas area OB barely extends onto this surface.

Although the border of area 18 with striate cortex is easily distinguished in fresh brain tissue due to the presence of the densely myelinated stria of Gennari in layer IVB and in Nissl-strained sections due to the characteristic sublamination of layer IV, the anterior border of area 18 has proven difficult to discern using traditional cytoarchitectonic criteria. However, the pigmentoarchitectonic method was able to characterize the lipofuscin distribution in striate cortex and distinguish several subdivisions of extrastriate cortex. This method identified parastriate cortex that forms a belt or horseshoe that surrounds V1 except at its most

What Striate Peristriate
Figure 2 Cytoarchitectonic maps of the human visual cortex. (A, B) Brodmann, 1909; (C, D) von Economo and Koskinas 1925, and (E, F) myeloarchitectonic map by Eliot Smith, 1907. From Zilles and Clarke (1997).

anterior extent. The most striking characteristic of parastriate cortex was the presence of a tripartite external teania (pigmentoarchitectonic layer IV). The upper portion (pigmentoarchitectonic IVA or pIVA) is lightly stained, the middle portion (pIVB) is densely stained, and the lower portion (pIVC) is lightly stained. This pattern is distinctive, forms a precise border with striate cortex, and extends for approximately 10 mm from the V1 border. The pigmentoarch-itectonic characteristics of the border between striate and parastriate regions are illustrated in Fig. 3. The parastriate region corresponds largely to area 18 of Brodmann, so it is unclear whether it only contains area V2 or whether it contains additional visual areas

Limes

Area Parastriata Parastriata Area Striata

Area Striata

Figure 3 Pigmentoarchitectonics of the border region between area striata and area parastriata. Area striata (right) is characterized by a densely pigmented layer 4Cb that ends abruptly at the border with area parastriata (left). Area parastriata is characterized by a tripartite layer 4 divided into pIVa, pIVb, and pIVc. The border region (limes parastriatus) contains a collection of densely pigmented cells in layers IVc and Va. From Braak (1980).

Figure 3 Pigmentoarchitectonics of the border region between area striata and area parastriata. Area striata (right) is characterized by a densely pigmented layer 4Cb that ends abruptly at the border with area parastriata (left). Area parastriata is characterized by a tripartite layer 4 divided into pIVa, pIVb, and pIVc. The border region (limes parastriatus) contains a collection of densely pigmented cells in layers IVc and Va. From Braak (1980).

such as V3 and VP (see later discussion). In order to address this question, it will be necessary to combine this pigmentoarchitectonic approach with studies of interhemispheric connections in post mortem brain tissue or to combine in vivo topographic mapping with post mortem pigmentoarchitectonic analyses.

The third anatomical criterion that distinguished a tripartite subdivision of visual cortex was based on the developmental pattern of myelination of the optic radiation. Three zones were described: a projection zone that was myelinated at birth, an intermediate zone that myelinated 1 month later, and finally a terminal zone that myelinated later yet. The projection zone was shown to correspond to the area containing the stripe of Gennari, the striate cortex. The surrounding two zones together were considered to contain visual association cortex.

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  • regina labingi
    What is striate vs peristriate?
    8 years ago

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