Structure, functional and cerebral asymmetry: in vivo morphometry of the planum temporale.

Using high-resolution magnetic resonance (MR) imaging, the normal left-right asymmetry of the planum temporale (PT) can be quantified accurately and reliably in the intact human brain. The following main results have emerged from MR measurements of individual direction and degree of PT asymmetry. (1) Normal left-handers are less left-lateralized than normal right-handers, without significant gender effects. This confirms a structural-functional correlation in cerebral asymmetry. (2) The aforementioned handedness difference in PT asymmetry is also found in pairs of normal monozygotic twins discordant for handedness. Thus, structural brain asymmetry may not be genetically determined. (3) Whereas right-handed patients with developmental deficits of phonological processing have been reported to show decrease PT asymmetry, musicians with perfect pitch display exaggerated leftward asymmetry. Thus, increasing leftward asymmetry of auditory-related cortices covering the PT may be correlated with the processing capacity for certain auditory features. Studies of the PT as a structural marker of cerebral asymmetry will continue to contribute to a better understanding of the phylogeny and ontogeny of laterality.