Natke, U., Grosser, J., & Kalveram, K.Th. (2001) Fluency, fundamental frequency, and speech rate under frequency shifted auditory feedback in stuttering and nonstuttering persons. Journal of Fluency Disorders, 26, 227-241.
Ten stuttering and ten nonstuttering persons produced spontaneous speech, while their auditory feedback provided by headphones was shifted in frequency by +1/2 octave and -1/2 octave. In persons who stutter the downward shift condition led to a significant fluency enhancement of 25 % measured by the percentage of discontinuous speech time (PDST, corresponds to loss of time due to disfluencies). Although in the upward shift condition fluency measured by PDST was enhanced by 21 %, the significance level was not reached. In nonstuttering persons a difference in global fundamental frequency between the non altered auditory feedback condition and the upward shift condition in the opposite direction of the frequency shift indicates a slight compensating response. This change in global fundamental frequency corresponds to 36 cents (100 cents equals one semi tone). Persons who stutter did not show such a change as a group. However, some stuttering individuals showed a change in global fundamental frequency up to 2 semi tones, but change in global fundamental frequency does not correlate with fluency enhancement measured by PDST. The downward and the upward frequency shift do not affect global fluent speech rate, neither in stuttering nor in nonstuttering persons. It is concluded, that fluency enhancement by frequency shifted auditory is not based on global changes in speech production.