Left planum temporale: an anatomical marker of left hemispheric specialization for language comprehension

We report on a study aimed at investigating the relationships between handedness, anatomical data and functional data related to speech processing. Twenty subjects with variable handedness (Edinburgh score ranging from -100 to 100) underwent both anatomical magnetic resonance imaging (MRI) and Positron Emission Tomography (PET) during story listening and rest. The surface areas of the left and right planum temporale (PT) were measured on each subject's MRI scan. A multiple regression analysis of PET data was conducted using these PT surface areas as well as handedness scores as predictors. The surface of the left PT explained a significant part of the functional variability. We observed that subjects who had the larger left PT were likely to show a larger leftward functional asymmetry of several perisylvian areas, namely the inferior parietal lobule outside the supra-marginal gyrus (the angular gyrus and the cortex above), Heschl's gyrus, the rolandic operculum, and the temporal pole. The size of the right PT explained only a little part of functional variability and we found no evidence that the anatomical asymmetry of the PT explained functional variability. In addition, we could not evidence any relationship between handedness and functional data. These results, which confirm previous work, argue for a perceptive origin of hemispheric specialization for language comprehension as has been suggested by others.