Interhemispheric anatomical differences in human primary auditory cortex: probabilistic mapping and volume measurement from magnetic resonance scans.

The gyral morphology of the region of the primary auditory cortex (PAC) in the human brain is highly variable, and possible asymmetries between the hemispheres have been noted since the beginning of the century. We mapped the location and extent of PAC as identified from gross anatomical landmarks in magnetic resonance scans that had been transformed into Talairach-Tournoux stereotaxic space. Individual maps were averaged to produce a probabilistic map of the region which can be co-registered with any image of brain structure or function that has been similarly transformed. The map can be used to localize a region of interest, such as a lesion, or an activation focus from position omission tomography or functional magnetic resonance imaging, within a specified range of probability. We also measured the total volume of the region and found a significant L > R asymmetry both on average and in the majority of subjects. Automatic segmentation of the volumes into grey and white matter revealed larger volumes of white, but not grey matter on the left. This larger volume of cortical connecting fibres may be related to the known left-hemisphere dominance for speech, and a preferential role for left PAC in processing temporal aspects of auditory stimuli is suggested.