LUTZ JÄNCKE AND HELMUTH STEINMETZ
abstract Applying in vivo magnetic resonance morphometry in healthy adults and reviewing published work on postmortem brains, we uncovered that the relationship between forebrain volume and the midsagittal size of the corpus callosum (CC) follows a geometrical rule according to which larger brains have a relatively smaller midsagittal CC. This allometric relationship is taken as support for the hypotheses of Ringo and coworkers suggesting that brain size may be an important factor influencing interhemispheric connectivity and lateralization. According to this theory, larger brains would be more lateralized than smaller brains. Because female and male brains on average differ in terms of brain size, this sexual dimorphism might explain the well-known gender differences with respect to functional lateralization.
The corpus callosum (CC) is the main fiber tract connecting the cerebral hemispheres, and it has been estimated that about 200—350 million fibers run through this structure in the human brain (Aboitiz et al., 1992a, 1992b). The CC seems to be important in the transfer and facilitation of sensorimotor and associative information between the hemispheres. It is thought that the cross-sectional size of the CC may indicate the number of small-diameter fibers crossing through (Aboitiz et al., 1992a), implying that a larger callosal area may indicate a higher capacity for interhemispheric transfer of associative information. Because the midsagittal CC size is so easy to measure either in postmortem material or on magnetic resonance images (MRI), it is one of the human brain structures to receive particular attention. There is some evidence suggesting that the morphology of the CC may be related to language dominance (Clarke
Lutz jäncke Department of Psychology, Division of Neuropsychology, University of Zurich, Zurich, Switzerland. helmuth Steinmetz Department of Neurology, Johann Wolfgang Goethe University, Frankfurt, Germany.
et al., 1993a; Clarke and Zaidel, 1994), gender (De Lacoste-Utamsing and Holloway, 1982; Oka et al., 1999), handedness (Witelson, 1985, 1989; Witelson and Goldsmith, 1991; Witelson and Nowakowski, 1991), Down syndrome (Wang et al., 1992), dysphasia (Njio-kiktjien and Sonneville, 1991), schizophrenia (Woodruff, McManus, and David, 1995), depression (Wu et al., 1993), epilepsy (Atkinson et al., 1996), Tourette syndrome (Baumgardner et al., 1996; Moriarty et al., 1997), posttraumatic stress disorder (De Bellis et al., 1999), Alzheimer's disease (Hampel et al., 1998; Lakmache et al., 1998), alcohol abuse (Estruch et al., 1997; Oishi, Mochizuki, and Shikata, 1999), and dyslexia (Hynd et al., 1995). The sometimes conflicting results in CC morphology were interpreted in two ways. The common interpretation has been that a larger CC midsagit-tal area (total CC or CC subarea) reflects increased interhemispheric connectivity resulting in (or due to) increased ambilaterality (Witelson, 1989). This interpretation is at variance with the interpretation by Clarke and coworkers (1993a) that the CC size indicates the number of fibers that subserve inhibiting or interfering processes located in the dominant hemisphere. In the light of these controversies and considering the enormous variability in CC size across the subgroups tested, we employed whole-brain in vivo magnetic resonance morphometry to investigate the anatomical relationship between CC midsagittal size and forebrain volume. This approach may also provide an empirical evaluation of the recently suggested relationship between brain size and lateralization (Ringo, 1991; Ringo et al., 1994), and it might help to explain the large interindividual variability in callosal size. In particular, we were interested in answering the following questions:
1. Is there an allometric relationship between callosal size and brain size?
2. If there is a relationship between callosal size and brain size, does this relationship follow a geometrical rule?
3. Is there a true influence of gender in this relationship?
4. Is handedness or brain lateralization related to callosal size?
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