Cognitive neuroanatomy

The current research focus on the correlation between structure and function in the human brain was stimulated largely by a report by Geschwind and Levitsky' in 1968, based on a sample of 100 autopsy brain specimens. Following leads from early century reports of right-left asymmetry in the lateral (sylvian) fissure and surrounding regions, they documented marked asymmetry, in prevalence and degree, in the superior plane of the posterior temporal lobe (the planum temporale [PTI) and explicitly suggested that the larger PT on the left side may be related to its predominance in the representation of language skills. One of the first quantitative imaging studies2 of the living human brain using pneumoencephalograms also appeared in 1968 in Neurology and reported an association between ventricular asymmetry and hand preference, used as a behavioral index of the pattern of functional asymmetry between the two hemispheres. Soon after, LeMay and Culebras3 reported what is likely the first in vivo imaging study of asymmetry in perisylvian cortical regions in relation to a behavioral index of functional asymmetry. Using ar ter iographs, LeMay and Culebras described a typical pattern of asymmetry consisting of smaller arterial arches in the posterior part of the middle cerebral artery on the left side and, by inference, greater left parietal opercularization. Additionally, they reported that this asymmetry was attenuated in left-handers compared with right-handers, thus providing some support for the hypothesis that asymmetry in the parietotemporal region is a substrate of functional asymmetry. In an accompanying editorial4 to this angiographic report, Geschwind envisioned the importance of imaging studies for future research relating human brain structure and behavior. The 1970s produced numerous studies documenting the reliability of PT asymmetry in the adult, infant, and fetal brain and demonstrating other neuroanatomic asymmetries, based mainly on direct postmortem (PM) examination and a few arteriograph and CT imaging s t u d i e ~ . ~ ~ This body of evidence of anatomic asymmetries led to an interest in the morphology of other brain structures related to interhemispheric connection, such as the cerebral commissures. In the 1980s, there was a major focus on individual differences in the corpus callosum (CC), studied with PM examination and MRI, which had the advantage of extensive background information from ~l in ica l ,~ neuropsychologica1,'O and experimental" research on the function, physiology, and anatomic development of the CC. The human CC varies greatly in shape and size in the adult brain (eg, midmax midsagittal area = 450/906 mm2 [reference 121; 439/905 mm2 [reference 131). Its variation in size is not completely accounted for by variation in brain size. The highest correlation with brain size reported to date is r = 0.48 (n = 50, p < 0.001), i n th i s case with cerebrum weight.12 Whether the variation in CC anatomy is related to variation in subject or behavioral characteristics remains t o be determined. Sex and functional asymmetries are two factors that have received considerable attention. A report by Steinmetz et a1 in this issue of Neurology (1992;42:749-752) considers both sex and hand preference in relation to CC anatomy in a quantitative MRI study that is one of the most technologically advanced to date. Ten years ago, de Lacoste-Utamsing and Holloway reported14 a sex difference in the size and shape of the splenium of the CC (defined as the posterior one-fifth region) in a small sample (n = 14) of PM brains. Women tended to have a greater midsagittal splenial area than men ( p = 0.081, but did show a significantly greater maximal splenial width, indicating a possibly different shape of the posterior bulbous region. What was intriguing about this observation was that a structure related to the cerebral cortex, and thus to behavior other than reproductive sexual activity, showed a sex difference in i t s gross anatomy t h a t was not attributable to the difference in overall brain size between the sexes. The authors suggested14 that the difference might be related to the sex difference