We measured the surEdce area of the cerebral cortex and its gross morphological subdivisions in 10 pairs of monozygotic twins. Cortical surface area was estimated in vivo using magnetic resonance imaging and threedimensional computer models of the intra- and extrasulcal pial surface. The means and standard deviations of regional (e.g., gyral), lobar, hemisphere, and total cortical surface area were tabulated for the entire population of 20 young, right-handed adults (10 females, 10 males). To determine whether genotypic differences were associated with morphometric differences, analyses of variance were carried out on each measure across unrelated twin pairs (genotype factor) and within co-twins (birth order factor). Across unrelated pairs, there was wide variation in regional cortical surface area for the left hemisphere (normalized by total cortical surface area, p ≤ 0.0001) but not for the right hemisphere (normalized, p = 0.12). More variation in lobar surface area was also observed for the left hemisphere (normalized, p = 0.05) than for the right (normalized, p = 0.48). Within co-twins, no signifcant variation in regional surface area or lobar surface area was found for the left or right hemisphere. Although normalized regional and lobar surface area in the left hemisphere differed across unrelated pairs, overall left hemisphere surface area normalized by total cortical surface area did not (p = 0.73). Total cortical surface area normallzed by body weight varied across unrelated pairs (p = 0.001) but not within co-twins (p = 0.39). The effects observed across unrelated pairs were not attributable to sex differences. These results suggest: 1) both the total area and folding of the cortical surface are heavily influenced by genetic factors in humans; and 2) the cerebral hemispheres may be differentially affected by genetic influences on cortical morphogenesis, with the languagedominant left cerebral cortex under stronger genetic control than the right.

We describe an in vivo method for the quantitative analysis of human necrotical anatomy. The technique allows unfolded regions of functional and morphological interest to be measured planimetrically. Two-dimensional cortical maps and surface area determinations derived from magnetic resonance images of monozygotic twins are presented. In addition, reconstructions and measurements of published post-mortem human and rhesus monkey hemispheres are reported. Potential applications for the study of brain organization in relation to cognitive, motor, and perceptual performance in normal and neurological populations are considered.