FIG 2. Heterotypical sensory isocortex.

A–C, MRM image (A) of the calcarine cortex with corresponding Nissl (B) and Luxol Fast Blue (C) stains. The thin, prominent, sharply defined intracortical band of low signal intensity corresponds to the highly myelinated plexus designated the external band of Baillarger (line of Gennari) (Layer IVB) (curved arrow in A and C). The prominent granule cells (long arrow in B) seen in layer II on the Nissl stain appear as a gray band (long arrow in A) on the MRM image. Note that the cortex is thicker at the crowns of the gyri and thinnest at the depths of the sulci. The changing relationship of layer IVB to the underlying white matter shows that the variation in cortical thickness results from thinning of the deep layers V and VI (magnification x5).

D–G, Signal intensity profiles oriented perpendicular to the cortical surface, color coded by location on the cortex. Focal "dips" in signal intensity at the line of Gennari (1) and within the subcortical white matter (2) correspond to the presence of heavily myelinated fibers, confirmed by histologic staining (B and C). The lower signal intensity in the subcortical white matter compared with that in the line of Gennari may reflect not only the greater concentration of myelin in the U fibers but also the increased concentration of iron within them. The dip in the middle of the blue graph (E) identifies the sulcus (S) between two adjacent gyri.