Morphologic cerebral asymmetries and handedness. The pars triangularis and planum temporale

A. L. Foundas, C. M. Leonard and K. M. Heilman
Department of Psychiatry and Neurology, Tulane University School of Medicine, New Orleans, La, USA.

OBJECTIVE: To explore the relationship between morphologic cerebral asymmetries of the pars triangularis (PTr) and the planum temporale (PT) measured on three-dimensional, gradient-echo, magnetic resonance imaging scans of healthy right- and left-handed subjects. DESIGN: (Blinded) comparison of healthy right- and left-handed subjects who underwent magnetic resonance imaging. SETTING: The Seimens 1-T Magnetom (Seimens, Iselin, NH) at the University of Florida Health Science Center, Gainesville. SUBJECTS: Healthy right-handed (n = 8) and left-handed (n = 8) subjects matched for age, sex, and educational level. MAIN OUTCOME MEASURES: (1) Average length of the PTr (anterior ascending and anterior descending rami), (2) average length of the PT, (3) asymmetry quotients ([left-right]/[(left+right) (0.5)]) of the PTr and PT, and (4) combined asymmetry quotients of the PTr and PT. RESULTS: There was a significant leftward asymmetry of the PTr and PT in the right-handed subjects, but there was no significant asymmetry in the left-handed subjects. When the combined asymmetry quotient of the PTr and PT was calculated, the right-handed subjects had a leftward asymmetry (87.5% [n = 7]) or left was equal to right (12.5% [n = 1]), and the left-handed subjects had a leftward asymmetry (62.5% [n = 5]), left was equal to right (12.5% [n = 1]), or a rightward asymmetry (25% [n = 2]). In the left-handed subjects, writing posture seemed to predict these combined asymmetry quotients, ie, left-handed subjects using an inverted writing posture had a leftward asymmetry and left-handed subjects using a non-inverted writing posture had a rightward asymmetry of the perisylvian speech-language regions. CONCLUSIONS: On three-dimensional, gradient-echo, magnetic resonance imaging scans, we found a significant leftward asymmetry of the PTr and PT. When the groups were divided into right- and left-handed subjects, the former had a significant leftward asymmetry of the PTr and PT, while the latter did not. Measurements of the PTr and PT appear to be important indexes of the known functional asymmetries of the perisylvian speech-language regions.