Fig. 6.

Schematic diagram of the cortical preparation for human vocalization. A diagram illustrating the time course of the cortical preparation for vocalization based on the results of the present and preceding studies. The entire duration of the prevocalization period corresponds to the reaction time of voice measured from the time of visual cue to the onset of voice, which is ∼350 msec in Cracco's study and 250–300 msec in ours. To accommodate all the results from preceding and present studies into a single diagram, the time course was scaled such that the total duration was 300 msec. Visual input to the retina is transferred to the visual cortex in 40–60 msec, which is processed in and travels out of the striate and extrastriate visual cortices by 120 msec (Amassian et al., 1989). Activation of the frontal cortex is apparent at ∼120 msec (Cracco et al., 1996). The early phase of cortical preparation starts as early as 150–200 msec before the expected onset of voice (i.e., 100–150 msec after visual cue presentation), which is followed by a distinct later phase 0–50 msec before EOVt. During the early phase (from 50 to 100 msec before the onset of voice), a mild left hemispheric predominance for inducing the delay is noted. During the later phase, this switches over to robust right hemispheric predominance. During the same period, the volume of voice increases by TMS of high intensity or by bilateral TMS. These almost coincide in time with the muscle contraction of the lateral cricoarythenoid muscle, beginning 80–100 msec before the onset of voice (Hirose and Gay, 1972) and the activation of laryngeal EMG, preceding the onset of voice by 80 msec (Cracco et al., 1996). Activation of the SMA proper region was also apparent during this late phase. The effect of bilateral TMS is greater than that of unilateral TMS during this late period. These results indicate that multiple cortical areas are active during the late phase.