Natke, U., & Kalveram, K.Th. (2001) Fundamental frequency and vowel duration under frequency shifted auditory feedback in stuttering and nonstuttering adults. In H.-G. Bosshardt, J. S. Yaruss & H. F. M. Peters (Eds.), Fluency Disorders: Theory, Research, Treatment and Self-help. Proceedings of the Third World Congress of Fluency Disorders in Nyborg, Denmark. Nijmegen: Nijmegen University Press, 66-71.

Abstract
Twelve stuttering and twelve nonstuttering subjects had to utter a test word repeatedly with different stress patterns. In randomly selected trials auditory feedback provided by headphones was shifted in frequency by -1/2 octave. Nonstuttering subjects responded with an increase by 2 Hz of fundamental frequency of the first syllable vowel, if it was long stressed. Stuttering subjects did not show this effect. In both groups unstressed syllables remained unaffected. Vowel duration of syllables did not change substantially in any group. Results indicate a compensatory mechanism that controls fundamental frequency via auditory feedback in speech like productions, but even in long stressed syllables it is unable to compensate for interferences completely. In stuttering people, this control mechanism might be not as effective as in nonstuttering people. Results indicate, that the fluency enhancing effect of continuous frequency shifting is not based on modified vocalization.