A model of the phonatory response time of stutterers and fluent speakers to frequency-modulated tones

Stutterers and fluent speakers tracked frequency-modulated tones by humming. The response time (RT) to the first corrective change in fundamental frequency in response to linear ramps of increasing and decreasing frequency was measured. The results demonstrate that RT is a function of the stimulus ramp velocity. A model of this dependency is provided which consists of parameters of threshold frequency and a fixed time delay. The estimated threshold frequency for the fluent speakers is 2.029 Hz with 95% confidence interval: (1.70 Hz, 2.35 Hz) whereas that of the stutterers is 3.937 Hz (3.28 Hz, 4.60 Hz). These threshold frequencies are significantly different (p < 0.0001). This implies that stutterers are slower to respond to changes in frequency than are fluent speakers. The fixed time delays for the two groups are not significantly different. This means that it is possible for the stutterers to respond as fast as the fluent speakers (i.e., their basic "reflexes" are the same); however, they spend more time in the detection of the change in a tracking signal. This supports the model of the stuttered event as being triggered by an instability in a multiloop speech motor control system.