Natke, U., & Kalveram, K.Th. (2001) Effects of frequency shifted auditory feedback on fundamental frequency in long stressed and unstressed syllables. Journal of Speech, Language, and Hearing Research, 44, 577-584.

Abstract
Twenty-four normally speaking subjects had to utter the test word /tatatas/ with different stress patterns repeatedly. Auditory feedback was provided by headphones and was shifted downwards in frequency during randomly selected trials while speaking the complete test word. If the first syllable was long stressed, fundamental frequency of the vowel significantly increased by 2 Hz (corresponding to 25.5 cents) under frequency shifted auditory feedback of -1/2 octave, while under a shift of -1/10 octave a trend of an increase could be observed. If the first syllable was unstressed, fundamental frequency remained unaffected. Regarding the second syllable, significant increases or a trend for an increase of fundamental frequency was found in both shifting conditions. Results indicate a negative feedback mechanism which controls the fundamental frequency via auditory feedback in speech production. However, within a syllable a response could only be found if syllable duration was long enough. Compensation for frequency shifted auditory feedback still is quite imperfect. It is concluded, that control of fundamental frequency is rather important on a suprasegmental level.