Number of neurons in individual laminae of areas 3B, 4 gamma, and 6a alpha of the cat cerebral cortex: a comparison with major visual areas.

The number of neurons per mm3 of tissue (number per volume) and the number under 1 mm2 of cortical surface (number per column) have been estimated for each lamina of seven cytoarchitectural areas of the cat cortex by using a method of size frequency distribution. The areas studied consisted of four visual areas (the binocular and monocular portions of area 17: 17B and 17M; area 18; and the posteromedial lateral suprasylvian area: PMLS), a somatosensory area (3B), and two motor areas (4 gamma and 6a alpha). For both series of measurements, significant differences could be demonstrated among the seven areas studied (one-way ANOVA; P less than .001). The number of neurons per volume in the binocular and monocular regions of area 17 (approximately 49,000/mm3) is 85% greater than that of each of the other regions (approximately 27,000) with a P less than .01 on an a posteriori Tukey test, but there are no significant differences between the latter areas. The number of neurons per column is greater in the binocular portion of area 17 (78,000 under 1 mm2 of cortical surface) than in any other area (P less than .01). Other sensory areas (17M, 18, PMLS, and 3B) have fewer neurons per column (P less than .01) and the numbers do not vary significantly between these regions (range from 56,100 to 61,900). Areas 4 gamma and 6a alpha have still fewer neurons (approximately 44,000; P less than .01, except P less than .05 when compared to PMLS). Thus, the seven areas studied fall under three different categories. Motor areas have the smallest number of neurons per column, sensory areas have more, and the greatest number is found in the binocular region of area 17. It appears that these differences are principally (but not exclusively) due to variations in the number of neurons in layer IV: These variations are largely responsible for the differences that we have found between the binocular portion of area 17 and other sensory areas as well as between the latter and motor areas. We thus cannot confirm the view of Rockel et al. (Brain 103:221-244, '80) that there is a basic uniformity of the number of neurons per unit of cortical surface in different cortical areas of the cat.