Hemispheric lateralization in the cortical motor preparation for human vocalization

To investigate the cortical information processing during the preparation o f vocalization, we performed transcranial magnetic stimulation (TMS) over t he cortex while the subjects prepared to produce voice in response to a vis ual cue. The control reaction time (RT) of vocalization without TMS was 250 -350 msec. TMS prolonged RT when it was delivered up to 150-200 msec before the expected onset of voice (EOV). The largest delay of RT was induced bil aterally over points 6 cm to the left and right of the vertex (the left and right motor areas), resulting in 10-20% prolongation of RT. During the ear ly phase of prevocalization period (50-100 msec before EOV), the delay indu ced over the left motor area was slightly larger than that induced over the right motor area, whereas, during the late phase (0-50 msec before EOV), i t was significantly larger over the right motor area. Bilateral and simulta neous TMS of the left and right motor areas induced delays not significantl y different from that induced by unilateral TMS during the early phase, but induced a large delay well in excess of the latter during the late phase. Thus, during the cortical preparation for human vocalization, alternation o f hemispheric lateralization takes place between the bilateral motor cortic es near the facial motor representations, with mild left hemispheric predom inance at the early phase switching over to robust right hemispheric predom inance during the late phase. Our results also suggested involvement of the motor representation of respiratory muscles and also of supplementary moto r cortex.