Neuroanatomical Aspects of Hemisphere Specialization in Humans

Anatomical asymmetries between the right and left hemispheres have been documented in the human brain since the last century. Much of the early work focussed on the Sylvian (lateral) fissure and the observation that this fissure was longer on the left more often than on the right side (e.g., Cunningham, 1892; Eberstaller, 1890). More recent anatomists pursued this morphological difference and looked within the Sylvian fossa in order to measure the horizontal surface of the temporal lobe which lies posterior to the first transverse gyrus (Heschl’s gyrus, the primary auditory receiving area) (e.g., Pfeiffer, 1936; von Economo & Horn, 1930). Early on, the area of this region, called the planum temporale, was found to be larger on the left side (see Figure 1). Much of the current interest in neuroanatomical asymmetry was rekindled by the report of Geschwind and Levitsky (1968) who observed that the linear extent of the lateral edge of the planum temporale was greater on the left side in 65 percent of 100 brain specimens. Other more recent studies (e.g., Wada, Clarke & Hamm, 1975; Witelson & Pallie, 1973) have confirmed this finding. Several reviews of asymmetry in the planum temporale in adults as well as in children are available (see e.g., Witelson, 1977; 1983).