Comparison of brain structure volumes in insectivora and primates. V. Area striata (AS).

Volumes of the area striata (area 17, Brodmann) were measured in 44 species of Primates and Scandentia and some of its laminar components in 25 species. The relative size (expressed by size indices) is on the average distinctly larger in simians than in isoponderous prosimians. The average dimensions of increase are 2.5 times for total area striata (ASV), 3.4 times for white matter (ASW), 2.35 times for grey matter (ASG), 1.9 times for lamina 1 (ASG 1) and 2.4 times for laminae 2-6 (ASG 2-6). The high increase of the white matter and the low increase of lamina 1 both correspond to similar results on the total neocortex (Frahm et al., 1982). The molecular layer (ASG 1) is on the average about 12% of the total area striata grey (ASG). The visual cortex thus has a narrower molecular layer than the neocortex as a whole, in which an average of 14% was found. The size indices of man are within the simian range and generally slightly above the simian average. Tree shrews have relatively small visual cortices, but reach into the lower part of the prosimian range. For Insectivora, ASG volumes were estimated from CGL volumes. According to these estimates, the relative ASG size is about 1/90 in shrews, about 1/40 in tenrecs and about 1/10 in hedgehogs that of isoponderous prosimians. In the European hedgehog, the area striata is about 12% of the total neocortex, a value similarly to that found in area measurements by Brodmann (1913). The relative size of the area striata is discussed with regard to functional, ecoethological and phylogenetic considerations. With increasing body weight, the increase in the visual structures, both cortical and non-cortical, is weak when compared with other brain parts and body organs (strongly negative allometry).