Whether there are gender differences in the size or the shape of the CC is a long-standing question. The first author who evaluated gender differences in callosal area size was Bean (1906), who found a larger genu in males. Shortly thereafter, Mall (1909), using a modern approach including a blind procedure (not used by Bean), failed to find differences in the CC that could be attributed to gender. This debate was reestablished by a postmortem study by De Lacoste-Utamsing and Holloway (1982), who reported that women might have a wider and more bulbous splenium than men and that even the overall size of the CC might be absolutely larger in women (Holloway and De Lacoste, 1986). The majority of follow-up studies failed to replicate these results (Weber and Weis, 1986; Kertesz et al., 1987; Oppenheim et al., 1987; Byne, Bleier, and Huston, 1988; Demeter et al., 1988; Clarke et al., 1989, 1993b; Hayakawa et al., 1989; Weis et al., 1989; Elster, Di Persio, and Moody, 1990; Going and Dixson, 1990; Allen et al., 1991; Emory et al., 1991; Habib et al., 1991; Aboitiz et al., 1992c; Steinmetz et al., 1992, 1995; Holloway et al., 1993; Pujol et al., 1993; Johnson et al., 1994; Pozzilli et al., 1994; Rauch and Jinkins, 1994). For a thorough discussion of this issue, see also Bishop and Wahlsten (1997). Nevertheless, most authors found a larger relative CC in women (i.e., CC relative to brain or skull size) or larger relative posterior portions of the CC in women (i.e., splenium or isthmus relative to total CC) (Kertesz et al., 1987; Reinarz et al., 1988; Clarke et al., 1989; Witelson, 1989; De Lacoste et al., 1990; Elster et al., 1990; Allen et al., 1991; Habib et al., 1991; Steinmetz et al., 1992, 1995; Holloway et al., 1993; Johnson et al., 1994; Clarke and Zaidel, 1994). A typical result based on our previously described sample of 120 young and healthy subjects is shown on Figure 3.3a. This figure illustrates that there is no gender difference in callosal size, either for the total CC or for the CC subareas (which are defined according to criteria suggested by Witelson (1989) and Jancke and coworkers (1997). However, callosal size related to FBV revealed a stronger gender difference, with women showing the largest CC ratios (Figure 3.3b). The common interpretation of this sexual dimorphism would be that it reflects increased interhemispheric connectivity due to increased female ambilaterality especially for tem-poroparietal cognitive functions (McGlone, 1980).
In light of the data presented in the preceding paragraph, one might ask whether there is a true gender effect on relative CC area size or whether the relative CC size might be affected by a more general brain volume effect. To examine whether there was a true gender difference in the CC ratios (CC/FBV), we divided our sample into FBV quintiles, with 24 brains per quintile (Figure 3.4). For each FBV quintile, t-tests were calculated to compare FBV and CC measurements between the genders. They revealed no significant gender differences, except for the third quintile, made up of brains with FBV values of approximately 1.0 liter. For this group we found a larger total CC and CC ratio in women. Subsequent analyses revealed that this gender
ant. middle isthmus splenium a third third
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