More Recent Views Of The Organization Of Visual Cortex In Nonhuman Primates

The distinction of three subdivisions of visual cortex was a widely held concept until the 1970s and 1980s, when seminal anatomical and electrophysiological mapping studies demonstrated that primate visual cortex consists of a large number of areas that are not simply belts of cortex surrounding striate cortex. They recognized that area V2 is a constant feature of organization of primate (and mammalian) visual cortex. V2 is seen as a belt of cortex that surrounds striate cortex, V1, and appears to correspond largely to area 18 of Brodmann or area OB of von Bonin. However, area 18 is somewhat larger in width than area V2; therefore, it might contain portions of areas such as V3, VP, and perhaps V3A. When electrophy-siological mapping is compared with studies of inter-hemispheric connections, it is revealed that the border between V1 and V2 is coincident with a dense band of callosal connections that demarcates the superior and inferior vertical meridians. Anterior to this dense callosal band is a large callosal-free region that contains area V2, V3 in dorsal cortex, and VP in ventral cortex. A second dense band of callosal connections is observed along the anterior borders of areas V3 and VP, again coincident with the representation of the inferior and superior vertical meridians, respectively. Therefore, the representation of the horizontal meridian is located within this callosal-free zone because areas V2, V3, and VP contain split representations of the horizontal meridian, which form their common border. Similarly, the mapping studies indicated that the cortex that makes up Brodmann's area 19 contains a number of distinct visual areas rather than a single third-tier area. Specifically, area MT is separated from area V2 by at least one other cortical area, V4 (or DL, the dorsolateral visual area).

Several convergent criteria are generally used to identify cortical subdivisions. Traditionally, a distinctive cyto- or myeloarchitecture has been used to identify specific cortical subdivisions. Second, a representation of visual space, a cortical map, is usually identified with a visual area. Third, areas are defined by a unique set of receptive field properties within their population. Fourth, areas are defined by a specific behavioral loss following their lesion or temporary inactivation. Finally, areas can be defined by a unique pattern of corticocortical and/or cortical-fugal connections.

V2 in nonhuman primates has been characterized by most, if not all, of these criteria. For example, V2 in macaque monkeys has a distinct appearance, most notably its cytochrome oxidase dense stripes. These appear as repeating thin and thick stripes that are separated by cytochrome oxidase pale zones. The distinction of thick and thin stripes is more difficult in macaques than in squirrel monkeys, but the thick stripes can be distinguished by their high immunor-eactivity to the CAT-301 antibody. V2 contains an overall representation of the contralateral visual hemifield with the representation of the vertical meridian forming the posterior border with V1 and the representation of the horizontal meridian forming the anterior border with area V3 in dorsal cortex and VP (V3v) in ventral cortex. Furthermore, each of the cytochrome oxidase stripe compartments (thin, pale, and thick) contains a separate representation of the visual hemifield. Macaque V2 contains neurons with a wide range of stimulus specificities. A high proportion of cells located within the cytochrome oxidase thin stripes demonstrate selectivity for stimulus color and lack orientation selectivity, whereas a high proportion of cells located in the pale and thick stripes demonstrate orientation selectivity. Some cells located within the pale and thick stripes are also selective for illusory contour stimuli. In addition, a substantial proportion of V2 cells in awake fixating macaque monkeys exhibit selectivity for complex visual features such as non-cartesian gratings.

Macaque V2 makes connections with a variety of cortical areas or modules within areas. The cyto-chrome oxidase thin stripes of V2 receive a preferential input arising largely from the cytochrome oxidase blobs of V1. Similarly, the cytochrome oxidase pale stripes receive feedforward input from the interblobs of V1. Finally, the V2 thick stripes receive feedforward input almost exclusively from layer IVB of V1. Thick stripes make feedforward connections primarily with areas V3 and MT. V2 makes feedback connections to each of these V1 compartments. Thin stripes and interstripes make dense projections that remain largely segregated into discreet zones of area V4. Areas V3A and VP receive input from V2 compartments that have not been identified. Finally, V2 makes weak projections to area TEO in posterior inferotemporal cortex. Each of these higher areas makes feedback projections to V2, demonstrating that V2 is located at the second level within a complex cortical hierarchy that contains reciprocal pathways.

The behavioral and perceptual contributions of macaque area V2 have been difficult to assess primarily due to the difficulty in limiting lesions to V2 and the need for appropriate controls. These limitations were overcome in one study that made ibotenic acid lesions of parafoveal lower field V2 and compared them to similar lesions in area V1 of fixating, behaving monkeys. In contrast to area V1, lesions of V2 led to no appreciable deficits in acuity or contrast sensitivity. V2 lesions did lead to significant deficits in orientation discrimination of lines made of collinear dots embedded in noise or of texture elements in an array. These results indicate that V2 is not needed for some low-level discriminations but may be essential for tasks involving more complex spatial discriminations, especially those that involve segregating features from a noisy background.

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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