Susceptibility to delayed auditory feedback and dependence on auditory or oral sensory feedback.

Studies of the delayed auditory feedback (DAF) effect have consistently reported marked individual differences in susceptibility to DAF among normal speakers. It has been suggested that speakers showing extreme susceptibility to DAF may be differentially dependent on auditory feedback in regulating their speech production. Assuming this is the case, then a reduction in sensory feedback would also be expected to produce differential effects on these speakers. To test this hypothesis, normative data on the actual range of susceptibility to 180 msec DAF were first obtained from a group of 400 normal male speakers. Subjects from the extremes of this distribution were then tested under conditions that selectively reduces sensory feedback. Auditory masking, whispering and local anaesthesia were used separately and in combination to achieve a reduction in one or more feedback channels (air-conducted, bone-conducted and oral sensory feedback). The results obtained did not support the feedback dependence hypothesis. Measures of reading duration, disfluency and correct syllabel rate revealed similar changes in the speech of high and low susceptible speakers when auditory and/or oral feedback was reduced. Alternative explanations of individual differences in DAF susceptibility are considered.