Hierarchy within human SI: supporting data from cytochrome oxidase, acetylcholinesterase and NADPH-diaphorase staining patterns

The human primary somatosensory cortex consists of four cytoarchitectonic subdivisions (3a, 3b, 1 and 2) that are likely to contain distinct somatosensory representations. The intraareal organization of these areas as well as that of the primary motor cortex (area 4) has been analyzed using histochemical stains of cytochrome oxidase, acetylcholinesterase and NADPH-diaphorase activity in normal human brains. Cytochrome oxidase activity was revealed in individual cortical neurons and neuropil. Areas 4, 3a and 3b were on average darker than areas 1 and 2. The laminar distribution of cytochrome oxidase activity varied in different areas. A prominent dark band was present in layers IV and lower III in areas 3a and 3b and in layer III in areas 1, 2 and 4. Acetylcholinesterase staining revealed fibers and pyramidal cells in layers III and V; stained layer III pyramids were rare in areas 3a and 3b and numerous in areas 1, 2 and 4. NADPH-diaphorase positive elements included Golgi-like stained non-pyramidal neurons and Nissl-like stained pyramidal neurons; the former were found, in small numbers, in layer II of areas 4, 3a, 3b and 1, and the latter in layers III and V of areas 4 and 3a and in layer V of areas 1 and 2. The dark cytochrome oxidase staining of layer IV and the paucity of acetylcholinesterase positive pyramids in areas 3a and 3b resemble the pattern found in primary visual and auditory areas, whereas the dark cytochrome oxidase staining in layer III and abundance of acetylcholinesterase positive pyramids in areas 1 and 2 that of association areas. These results suggest that the four areas included in human SI constitute hierarchical stages of cortical processing, with 3a and 3b corresponding to primary and 1 and 2 to secondary areas.