Abstract: As to the problem of stuttering, most researchers were interested in the processes of speech movement and speech planning. However, an important process, which is closely related with speech motor initiating time, is ignored. That is speech motor preparation. Researchers have found that PWS initiated motor cortex early before Broca area; while immidieatly before articulation, the degrees of speech motor preparation were lower for PWS than that for people who do not stutter (PWNS). However, one problem of these studies is they used only one preparatory interval (PI). If optimal PI of PWS is different from that of PWNS, the results above would be challenged. In the present research, the effect of PI on verbal response of PWS was investigated in three experiments.
In experiment 1, six PIs (50, 100, 200, 400, 800 and 1600ms) were applied respectively in naming task. Warning signal was a “+”, imperative signal was a colored square, which need to be named with one word as soon as possible. Average inter-trial interval (ITI) was 7s. Besides, a simple naming task without warning signal was used as baseline. Ten PWS and Ten PWNS participated in this experiment. Results showed that optimal naming PI of PWS was around 400ms, which lasted shorter than that of PWNS (400~800ms); when PI increased from 400ms to 800ms, verbal response time of PWS was significantly prolonged.
Procedure and design of experiment 2 was similar to experiment 1, except that a botton pressing task was applied instead of naming task. Ten PWS and Ten PWNS participated in this experiment, 6 of them had also paticipanted in experiment 1. As a result, no significant difference was observed between PWS and PWNS.
In experiment 3, average ITIs were shortened to 1s to investigate whether results of experiment 1 could be influenced by time pressure. Other procedures were same as experiment1. Fifteen PWS and fifteen PWNS participated in this experiment; none of them had paticipanted in experiment 1 and 2. Interestingly, prominent difference of naming response time between PWS and PWNS were obtained again when PI was increased to 800ms.
In conculsion, no matter how time pressure (ITI) changed, the problem for PWS was that, they were difficult in maintaining or improving reaction speed when PI was increased from 400ms to 800ms. And this phenomenen was only significant in naming task rather than hand moving task. The authors suggested 800ms after “+”, the attentional system of prefrontal cortex is still working to maintain a high degree of attention for PWNS; while for PWS, this system is weak. Then, motor preparation system may be advanced, but other signals (such as phonogical plan) were no arrived. Thus, PWS would have to wait and pronounce some stuttering utterances.

Key words: stuttering, speech motor preparation, preparatory interval