Appendixes Showing the Logistic Psychometric Function, Detailing the Pilot Study, and Providing Additional Analyses Showing the Influence of a Motor Speech-Based Therapy (Neef et al., 2012)

Purpose In individuals who stutter (IWS), speech fluency can be enhanced by altered auditory feedback, although it has adverse effects in control speakers. This indicates abnormalities in the auditory feedback loop in stuttering. Current motor control theories on stuttering propose an impaired processing of internal forward models that might be related to a blurred auditory-to-motor translation. Although speech sound perception is an essential skill to form internal models, perceptual acuity has not been studied in IWS so far. The authors tested the stability of phoneme percepts by analyzing participants' ability to identify voiced and voiceless stop consonants.
Method Two syllable continua were generated by systematic modification of the voice onset time. The authors determined speech perceptual acuity by means of discriminatory power in 25 IWS and 24 matched control participants by determining the phoneme boundaries and by quantifying the interval of voice onset times for which phonemes were perceived ambiguously.
Results In IWS, discriminatory performance was weaker and less stable over time when compared with control participants. In addition, phoneme boundaries were located at longer voice onset times in IWS.
Conclusion Persistent developmental stuttering is associated with less reliable phonological percepts, supporting current theories regarding the sensory–motor interaction in human speech.