Asymmetry of the Diaphragmatic Motion during Vocalization may Cause the Laterality of the Speech Center

It is unknown why the speech center is located in the left cerebrum. There are no apparent structural asymmetries in organs related to vocalization including the respiratory system. However, beneath the diaphragm, the solid liver is located right, and the deformable air-containing stomach is located left. During vocalization, abdominal muscles contract, elevate the diaphragm through the displacement of those abdominal organs, and control expiratory airflow. We hypothesized that this mechanical asymmetry beneath the diaphragm could be related to the control of vocalization. We investigated diaphragmatic motion with dynamic MR (Magnetic Resonance) images, and found apparent difference between right and left diaphragmatic motions only at the beginning of vocalization, at which left diaphragmatic motions were often paradoxical. Intentional vocal segmentation is thought to be the origin of speech. The speech center in the left cerebrum may be the result that mechanical information of the right diaphragm is preferentially included in the speech circuit.