Effects of delayed auditory feedback and frequency-shifted feedback on speech control and some potentials for future development of prosthetic aids for stammering.

It has been known for at least a hundred years that the speech of a person who stammers becomes more fluent when alterations are made to the speaking environment. Alterations that lead to an improvement in fluency include a) noises that prevent a speaker hearing his or her own voice, and b) manipulations to the sound of a speaker's voice before it is heard. Examples of manipulations that have been made are introducing a delay, and shifting the voice up or down in frequency. The influences all these alterations have on fluent speakers and speakers who stammer, that have been established over the last century, are reviewed. In addition, the ways in which these phenomena have been explained for both fluent speaker and speakers who stammer are outlined. Several previous findings have potential significance for ways in which the fluency-enhancing effects of these alterations in speakers who stammer could be employed in clinical settings. These are highlighted and discussed, mainly in connection with the SpeechEasy prosthetic device for treating stammering.