In vivo studies relating anatomical asymmetry and functional laterality

In vivo imaging techniques such as carotid angiography and computerized tomography were initially critical for establishing a relationship between behavioral later-alization and anatomical asymmetry. More recently, magnetic resonance imaging (MRI) studies of cerebral asymmetries in the language areas of living subjects have been performed, confirming the concept of a relationship between anatomical asymmetry and functional laterality in Wernicke's area (Aboitiz and Ide, 1998). In general, the strongest behavioral indicator of anatomical asymmetry in language regions is linguistic laterali-zation to the left and its reduction in left-handers (as determined, for example, by dichotic listening, hemifield tachistoscopy or the sodium amytal test). Indeed, there is a good correlation between right-handedness and left-language dominance (Witelson, 1995). Perhaps language lateralization evolved from a brain already lateralized for manual functions in the left hemisphere and for certain visuospatial functions in the right (Bradshaw and Rogers, 1993). An interesting finding is that, among lefthanders, those who have an inverted writing posture tend to show leftward asymmetry of the planum temporale, while those with noninverted writing have a right-ward asymmetry (Foundas, Leonard, and Heilman, 1995). Using MRI, Habib and colleagues (1995) found two leftward asymmetric parameters: the size of the planum temporale and the distance between the central sulcus and the end of the Sylvian fissure. These two asymmetries were not correlated; that is, in a given individual, presence of asymmetry in one region does not predict that the other region will be asymmetric. Although each of these asymmetry measures correlates with handedness, the two combined showed a much stronger correlation with manual preference. This is consistent with a recent report (Ide et al., in press) showing several anatomical asymmetries in different surface regions of the human brain even though there is no correlation between them. This indicates that the overall morphological pattern rather than that of a specific measure may yield better estimates of lateralization of function.

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