In an effort to provide quantitative criteria for the distinction of area V2 from other cortical areas, image-processing techniques have been used to assess the cytoarchitectonic and myeloarchitectonic organizations of area V2 and to compare them with those from areas V1 and V4. According to this approach, Nissl-stained sections are digitized and the profiles of cell bodies are segmented according to gray level. A gray level index is then used to quantify the areal fraction of cortex occupied by stained cell bodies. Figure 5A illustrates the gray level index in areas V1, V2, and V4.
V1 is characterized by a bimodal GLI that peaks in layers II-III, drops in layer IVB, peaks again in layer IVC, and finally falls off in layers V and VI. In area 18 (V2, just across the V1 border), a different pattern is observed with multiple peaks extending across the depth of the cortex and no distinctive cell-sparse zone as observed in layer IVB of V1. Finally, a similar multipeaked pattern was observed in area V4 of the fusiform gyrus.
Similar quantitative techniques have been used to quantify the distribution of myelin in sections of areas V1, V2, and MT stained by the Gallyas method for myelin. All three areas show an increase in the density of myelin from the upper to the lower cortical layers. In V1, a distinct peak is found in the middle of cortex corresponding to the stria of Gennari. V2 can easily be distinguished from V1 by the overall lower density of myelin and the clear lack of the stria of Gennari (see Fig. 5B).
Quantitative cytoarchitectonic and myeloarchitec-tonic methods have been used to determine the variability of areas 17 and 18 when transformed into the Talairach stereotaxic coordinate system. For example, five male and five female brains were obtained at autopsy, fixed in 4% formalin for several months, and imaged with MRI at high resolution. The brains were then cut at 20 mm and sections were analyzed quantitatively every 1200 mm. The borders between areas 17 and 18 and between areas 18 and 19 were determined using the gray level index to quantify neuronal density across cortical layers. These histological borders were then transferred to MRI sections using both linear and nonlinear fluid transformations. Finally, the MRI-reconstructed brains were transformed into a standard format. Figure 6 illustrates the surface rendering of areas 17 and 18 on sagittal views of the MRI reconstructions of the 10 brains used in this study. It is apparent that area 18 is highly variable in location and extent across these 10 brains. This variability can be expressed by the center of gravity and standard deviation of the Talairach coordinates for area 18. Thus, area 18 has a maximum variability of the center of mass of 5 mm in the z plane, and this variability is almost half as large in the x plane.
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