Figure 6.

Gray matter (GM) thickness is a function of brain weight and neuron density. (A) Variation in GM thickness can be significantly explained by brain weight [F_{(2, 37)} = 22.58, P = 3.9 × 10⁻⁷] and neuron density [F_{(2, 20)} = 7.96, P = 0.003], but not by either GI [F_{(2, 38)} = 0.066, P = 0.936] or astrocyte density [F_{(2, 20)} = 2.37, P = 0.119]. The insets suggest a strong phylogenetic signal (Pagel, 1999), tantamount to a random walk, in the scaling of GM thickness as a function of brain weight (lambda = 0.89^(+0.07)_(−0.09)) and neuron density (lambda = 0.88^(+0.12)_(−0.17)). (B,C) Ln-transformed phylogenetically independent contrasts with regression through the origin for GM thickness as a function of (B) brain weight and (C) neuron density. GM thickness scales positively as a function of brain weight (e^{0.136 ± 0.027}) and negatively as a function of neuron density (e^{−0.276 ± 0.098}). Cell densities pertain to gray matter counts in the visual cortex from Lewitus et al. (2012). See (Lewitus et al., 2013) and Table A1 for neuroanatomical data.