Descriptive Anatomy

1. Gross Anatomy of the Occipital Lobe

Figure 1 illustrates the location and extent of the human occipital lobe. Viewed from the medial surface of the cerebral hemisphere, it is bounded anteriorly by the parieto-occipital sulcus (POS). Ventrally and laterally there are no major anatomical features that clearly and consistently demarcate the anterior extent of the occipital lobe. For practical purposes, an imaginary line running laterally from the dorsal tip of the POS to the preoccipital notch is considered the effective boundary separating the occipital lobe from the parietal and temporal lobes.

Medially, the most prominent feature of the occipital lobe is the calcarine sulcus, which is flanked above by the cuneus and below by the lingual gyrus (cf. Fig. 1, top). The lingual gyrus is separated from the more laterally placed fusiform gyrus by the collateral sulcus. The fusiform gyrus is then bounded laterally by the occipitotemporal sulcus (OTS), although this sulcus tends to be variable and interrupted as it extends posteriorly toward the occipital pole. The lateral surface of the occipital lobe is especially variable from individual to individual. Consequently, the lateral occipital gyrus can be irregular and can be split by the lateral occipital sulcus. This latter fissure has several important variants, sometimes appearing as one, two, or even three small sulci running approximately in the anterior-posterior direction. Most dorsally on the lateral surface is the transverse occipital sulcus, which often forms the most posterior end of the intraparietal sulcus. A thorough treatment of individual variants of the occipital anatomy can be found in the atlas of Ono, Kubik, and Abernathy.

2. Cytoarchitecture, Myelination, and Histochemistry

Traditionally, occipital cortex has been subdivided into anatomically distinct regions based on cytoarch-itecture. As for all neocortex, the occipital gray matter is clearly laminated, though the criteria for identifying and naming different layers have varied considerably. Figure 2A illustrates one of the more popular laminar numbering schemes. Primary visual cortex is the most cytoarchitecturally distinct region of the occipital lobe. It is also known as striate cortex due to the stria of Gennari, which can be identified even in a cross section of freshly cut tissue. This band of myelinated axons running horizontally in layer 4B demarcates the extent

Figure 1 Gross anatomy of the occipital lobe in humans and macaque monkeys. Top: Medial, ventral, and lateral views of the occipital lobe with medial and lateral views of the whole cerebral cortex (below) from the Visible Man database. Middle: Identification of cortical lobes in the macaque monkey. Color code same as for top. Bottom: Computer brain model reconstructed from the Talairach and Tournoux Co-Planar Stereotaxic Atlas of the Human Brain showing the standard Talairach coordinate system, with the origin located at the anterior commissure (not shown) and the y axis passing through both the anterior and posterior commissures (not shown). Polarity of axes indicates conventions used in this article. Abbreviations: AC-PC, anterior commissure-posterior commissure reference line; Cal S, calcarine sulcus; POS, parieto-occipital sulcus; F, frontal lobe; P, parietal lobe; T, temporal lobe. (Figures of Visible Man and Macaque monkey brains courtesy of David Van Essen.)

Figure 1 Gross anatomy of the occipital lobe in humans and macaque monkeys. Top: Medial, ventral, and lateral views of the occipital lobe with medial and lateral views of the whole cerebral cortex (below) from the Visible Man database. Middle: Identification of cortical lobes in the macaque monkey. Color code same as for top. Bottom: Computer brain model reconstructed from the Talairach and Tournoux Co-Planar Stereotaxic Atlas of the Human Brain showing the standard Talairach coordinate system, with the origin located at the anterior commissure (not shown) and the y axis passing through both the anterior and posterior commissures (not shown). Polarity of axes indicates conventions used in this article. Abbreviations: AC-PC, anterior commissure-posterior commissure reference line; Cal S, calcarine sulcus; POS, parieto-occipital sulcus; F, frontal lobe; P, parietal lobe; T, temporal lobe. (Figures of Visible Man and Macaque monkey brains courtesy of David Van Essen.)

of striate cortex within, and adjacent to, the calcarine sulcus (Fig. 2B). Cytoarchitectural differences among visual areas outside the striate cortex (known as extrastriate cortex) tend to be less obvious and more inconsistent, perhaps accounting for significant differ ences in the accounts of cytoarchitectonic parcellation of occipital cortex by early investigators.

One of the most widely used cytoarchitectonic schemes for subdividing cerebral cortex has been that of Brodmann (Fig. 2D). His scheme was primarily

Primary Visual Cortex Cytoarchitectonic

Figure 2 (A) Example of the laminar architecture of primary visual cortex visible with common Nissl stain (left) and with histochemical stain for cytochrome oxidase activity (right). Arrows indicate cytochrome oxidase dense puffs in layers 2-3. Note that different stains reveal different features. (Adapted from Horton, 1992.) (B) Example of myeloarchitecture near the border of areas V1 and V2 marked by a large vertical arrow. Abbreviations: bt, border tuft; fa, fringe area. A star marks the stria of Gennari. Small arrowhead on the left marks the inner band of Baillarger. (Adapted from Zilles and Clarke, 1997.) (C) Illustration of dense cytochrome oxidase histochemistry in human MT visual area. (From Tootell and Taylor, 1995.) (D) Brodmann's parcellation (numbers) of the human occipital lobe and adjacent cortex based on cytoarchitectonic features. (From Zilles and Clarke, 1997.)

Figure 2 (A) Example of the laminar architecture of primary visual cortex visible with common Nissl stain (left) and with histochemical stain for cytochrome oxidase activity (right). Arrows indicate cytochrome oxidase dense puffs in layers 2-3. Note that different stains reveal different features. (Adapted from Horton, 1992.) (B) Example of myeloarchitecture near the border of areas V1 and V2 marked by a large vertical arrow. Abbreviations: bt, border tuft; fa, fringe area. A star marks the stria of Gennari. Small arrowhead on the left marks the inner band of Baillarger. (Adapted from Zilles and Clarke, 1997.) (C) Illustration of dense cytochrome oxidase histochemistry in human MT visual area. (From Tootell and Taylor, 1995.) (D) Brodmann's parcellation (numbers) of the human occipital lobe and adjacent cortex based on cytoarchitectonic features. (From Zilles and Clarke, 1997.)

based on differences in cell morphology (e.g., stellate-pyramidal), density, and distribution, as well as the overall thickness, proportion, and development of different layers within the cortical mantle. According to this plan, striate cortex is designated area 17 and is immediately surrounded by area 18, which in turn is bounded by area 19. Portions of Brodmann's area 37 may also be included in what we have defined here as occipital cortex (though Brodmann considered it part of the temporal cortex). Except for visual areas 17 and 18, Brodmann's areas often correspond poorly with functional distinctions identified by more modern techniques such as neuroimaging. This has resulted in diminished use of Brodmann's nomenclature as a basis for describing the functional organization of visual cortex.

Although some of Brodmann's original criteria for differentiating occipital cortex have not been functionally diagnostic, other cytoarchitectural features have been found to fare better in some cases. Indeed, researchers have argued in favor of a renewed interest in architectonic data as a useful correlate of function. Histological stains that reveal the distribution of myelinated fibers within the cortical gray matter have been successful in delineating some functionally defined visual areas. This is certainly the case for the stria of Gennari, which distinguishes the primary visual cortex. Dense myelin staining also characterizes the middle temporal visual area, hMT + , located in lateral occipital cortex near the confluence of the occipital, temporal, and parietal lobes (Fig. 2C). In macaque monkeys, the third visual area, V3, can also be identified by heavy myelin staining, though it is not clear whether this is also true in humans.

One alternative to the study of cytoarchitecture that can provide functionally relevant information is to examine the distribution of chemically distinct molecules within the cortex. This has been accomplished through a wide variety of histochemical and immuno-logical techniques, though, historically, enzyme visualization has been especially useful in this respect. The discovery of circumscribed zones of high cytochrome oxidase activity in striate cortex (puffs, blobs) and extrastriate cortex (V2 "stripes", V4 "patches") triggered the identification and characterization of functionally distinct "modules" within occipital visual areas that had previously been thought to be functionally homogeneous. These discoveries led to the concept that individual cortical visual areas can contain multiple, distinct processing pathways or streams, thereby extending (though not necessarily reiterating) the organizational principle of multiple processing pathways found in the retina and LGN (outlined later). To a large extent, the features of cortical organization revealed by these studies are on a spatial scale that, until very recently, had been beyond the resolution of human neuroimaging and electro-physiological techniques, and so are outside the scope of this article.

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