Quantifying variability in the planum temporale: A probability map

The acquisition of definitive evidence for systematic hemispheric asymmetri es in the size of the planum temporale (PT) has been restricted by difficul ties in identifying, standardizing and measuring the region of interest. In this paper an operational definition for identifying the problematic poste rior border of the PT on magnetic resonance imaging (MRI) scans is proposed . An interactive voxel-painting program was used to identify and label the PT simultaneously in horizontal, sagittal and coronal planes in MRI scans, transformed into the standardized Talairach-Tournoux stereotaxic space, fro m 50 normal right-handed volunteers. Both grey matter volume and cortical s urface area of the PT were measured, while controlling for individual varia tion in overall brain shape and volume. The labeled tissue was averaged tog ether to produce a probability map in standardized space of the region of i nterest The PT region is highly variable, with no single voxel being labele d with a probability of >65%. In this study there were no significant hemis pheric differences in volume or area of the PT. An asymmetry in area and vo lume was introduced by using an alternative method the 'knife-cut' method - for identifying the posterior border. Implications for functional neuroima ging of the PT are discussed.