Evidence for a Modular Organization of Human Area V2

The distinction of area V2 from V1 and the identification of functional subunits in both of these areas have been facilitated greatly through the examination of the differential distribution of cytochrome oxidase histo-chemistry in nonhuman primates. Cytochrome oxidase blobs or puffs have been identified in supra-granular striate cortex that represents regions of higher degree metabolic activity than the surrounding interblob zones. Subsequent investigations using mi-croelectrode, 2-deoxyglucose autoradiography and optical recording techniques have linked these different anatomical modules within V1 with distinct physiological properties. Cells in V1 blobs tend to be monocular, express a lack of orientation selectivity, and possess a higher degree of color selectivity. In addition, V1 blobs tend to be located at the center of ocular dominance stripes.

V2 is also characterized by a unique pattern of cytochrome oxidase histochemistry in a wide range of nonhuman primate species. In V2, increased densities of CO are observed to form elongated stripes that are oriented perpendicular to the V1-V2 border. In squirrel monkeys, these stripes can readily be observed to alternate in a regular thick-thin pattern, separated by intervening interstripe zones of low CO activity. The alternation in stripe width as well as the overall regularity of stripe alternation is less obvious in macaque monkeys as well as in humans (see later discussion).

A periodic pattern of cytochrome oxidase activity in human visual area V2 was first described in 1984. This pattern was similar to that observed in macaque monkeys but differed in size, periodicity, and overall homogeneity. Like nonhuman primates, area V2 can be identified by the presence of CO-dense stripes in layers III-V (see Fig. 8). These stripes form a regular pattern of dense stripes that are oriented perpendicular to the V1 border. The dense stripes are 1.0-2.75 mm in width and are separated by CO-pale zones that are 1.52.75 mm in width.

Unlike nonhuman primates, human V2 CO stripes are more globular than homogeneous in structure. These globular zones are 580-1400 mm in width and form stripes that have a spacing of 1500-2300 mm. These CO density dimensions were reported to be rather variable across individuals. Interestingly, CO stripes in V2 can be observed in neonatal tissue at a time when CO blobs are not present in area V1. Furthermore, although the CO densities formed rough stripes oriented perpendicular to the V1 border, the demarcation of thick and thin stripes, which is readily observed in squirrel monkeys and less so in macaque monkeys, generally is not possible in humans. However, it is possible to distinguish these stripes from each other in humans through the use of CAT-301 immu-noreactivity.

CAT-301 is a cell-surface proteoglycan that is observed at high density in several levels of the macaque visual system that are related to the magno-cellular thalamocortical pathway. The distribution of

Figure 8 Cytochrome oxidase stripes in human V2. Three prominent stripes are seen in this section through layer 4. Individual stripes consist of dense patches separated by pale zones that are aligned in rows. Bar=2 mm. From Tootell et al. (1993).

CAT-301 immunoreactivity has been observed in cross-sectional and tangential tissue blocks of human visual cortex. Labeled cells in V2 are distributed in two distinct bands: one located in layer 3 and a second in layer 5 (see Fig. 9A). The periodicity of CAT-301 is best revealed in tangentially sectioned tissue. CAT-301 stained dense clusters of neurons in V2, which formed stripes that were 1-3 mm wide and 1.5-3 cm long and oriented perpendicular to the V1-V2 border (see Fig. 9B). These dense stripes were separated by approximately 4-8 mm. Because CAT-301 immunoreactivity identifies the CO-dense thick stripes of macaque area V2, it seems likely that this CAT-301-ir in human V2 also recognizes thick stripes. It is important to point out that these CAT-301-dense regions appear distinctly stripelike, whereas studies of V2 using CO tend to identify CO-dense clusters rather than stripes.

In addition to cytochrome oxidase and CAT-301 staining, myelin staining of unfolded, tangentially sectioned portions of V2 with the Gallyas stain reveals stripes that are 6-8 mm apart. This spacing is consistent with the labeling of one set of stripes, either

Transition Striate

Figure 9 CAT-301 immunoreactivity in human area V2. (A) Laminar distribution of CAT-301-ir neurons in V1 (left) and V2 (right). V1 contains two bands of labeled cells corresponding to layers IV and VI. The laminar pattern changes at the V2 border, where CAT-301-ir cells are located in a dense band in layer III and a lighter band in layer V. Arrows indicate the V1-V2 border. Bar= 1 mm. (B) Tangential distribution of CAT-301-ir cells in area V2. Thick bands of CAT-301-ir cells run orthogonal to the V1-V2 border. Bar=1 mm. From Hockfield et al. (1990).

Figure 9 CAT-301 immunoreactivity in human area V2. (A) Laminar distribution of CAT-301-ir neurons in V1 (left) and V2 (right). V1 contains two bands of labeled cells corresponding to layers IV and VI. The laminar pattern changes at the V2 border, where CAT-301-ir cells are located in a dense band in layer III and a lighter band in layer V. Arrows indicate the V1-V2 border. Bar= 1 mm. (B) Tangential distribution of CAT-301-ir cells in area V2. Thick bands of CAT-301-ir cells run orthogonal to the V1-V2 border. Bar=1 mm. From Hockfield et al. (1990).

thin or thick stripes. Alternatively, the myelin staining may reflect the pale CO compartments that have been reported to contain higher myelin densities in squirrel monkey V2. The pattern of myelin staining in unfolded V2 is illustrated in Fig. 10.

The distinctiveness and regularity of CO densities end at the anterior border of V2 and allow for an assessment of the overall size of V2 in humans. At the foveal representation, V2 is relatively narrow at 1-1.5 cm wide. In both dorsal and ventral cortex, the width

Figure 10 Myelin staining in human area V2. Composite image of four aligned sections spanning layers 3-5. Three myelin dense stripes are observed running orthogonal to the V1-V2 border (dashed line). Bar=2 mm. From Tootell and Taylor (1995).

of V2 increased to 1.8-3.4 cm at parafoveal eccentricities. These features of the modular organization of area V2 are summarized in Table I.

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