cently performed (Ide et al., 1996). Anatomically, the Sylvian fissure divides into an anterior segment that ends posteriorly in the gyrus of Heschl (containing the primary auditory area). Behind the gyrus of Heschl there is a horizontal segment containing the lateral extension of the planum temporale and the Sylvian fossa. More posteriorly, the Sylvian fissure bifurcates into two, usually more superficial ascending and descending rami (see Figure 2.5). There are two main types of Sylvian fissure. In the standard type, the ascending ramus is longer and deeper than the descending one. Two subtypes can be distinguished within the standard type, according to the relative length of the horizontal segment (compare types A1 and A2 in Figure 2.5), type A2 being more common in the right hemisphere (Table 2.1). Ide and colleagues
(1996) have also described a type of fissure (Figure 2.5B) that is characterized by a long horizontal segment and a sometimes small, but at other times relatively long, fissure that is directed upward and forward (instead of upward and backward). This type of fissure is more common in the left hemisphere (Table 2.1), and corresponds in many cases to what other authors have described as the absence of an ascending ramus. The interpretation is that the upward/forward-directed ramus described by Ide and colleagues (1996) is a distorted and sometimes vestigial superior branch.
Distinct fissurization patterns may result from differences in the relative growth of specific cortical regions, which in turn may be directly associated with divergent modes of cortical processing. In particular, the position of the planum temporale and area Tpt may differ in the distinct fissure types reported here, an issue of relevance for studies that incorporate imaging analyses. Witelson and Kigar (1992) and later Ide and colleagues (1996) have suggested that in some cases (particularly in the case of the fissure type A2 in Figure 2.5, which may correspond to Witelson and Kigar's "V-type"), the pla-num temporale perhaps runs into the ascending branch of the Sylvian fissure. In the other cases (especially type B) it appears that the planum temporale is restricted to the horizontal segment of the Sylvian fissure. Since type A2 is more common on the right, this position of the planum may reflect the development of cortical areas related to right hemisphere skills or the reduction in the size of areas related to left hemisphere skills, while the type B fissure, which is more common on the left, may be related to the development of cortical areas involved in linguistic skills. In this context, although many studies highlight the size difference between the left and right plana (Beaton, 1997; Shapleske et al., 1999), the above and other recent findings (e.g., Loftus et al., 1993; Jancke et al., 1994) emphasize a difference in the position of the planum temporale (or the Sylvian fossa) in the two sides rather than an asymmetry in overall planum size. Additional studies are urgently needed to settle the question of a correspondence between variability at the level of gross anatomy and variability at the level of cortical cy-toarchitectonics.
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