Consonant Acoustics in Parkinson's Disease and Multiple Sclerosis: Comparison of Clear and Loud Speaking Conditions.

Purpose: The impact of clear speech or an increased vocal intensity on consonant spectra was investigated for speakers with mild dysarthria secondary to multiple sclerosis or Parkinson's disease and healthy controls.

Method: Sentences were read in habitual, clear, and loud conditions. Spectral moment coefficients were obtained for word-initial and word-medial /s/, /ʃ/, /t/, and /k/. Global production differences among conditions were confirmed with measures of vocal intensity and articulation rate.

Results: Static or slice-in-time first moments (M1) for loud differed most frequently from habitual, but neither loud nor clear enhanced M1 contrast for consonant pairs. In several instances, the clear and loud conditions yielded stable or nonvarying fricative M1 time histories. Spectral contrast was reduced for word-medial versus word-initial consonant pairs.

Conclusion: The finding that the loud and especially clear condition yielded fairly subtle changes in consonant spectra suggests these global techniques may minimally enhance consonant segmental production or contrast in mild dysarthria. The robust effect of word position on consonant spectra indicates that this variable deserves consideration in future studies. Future research also is needed to investigate how or whether consonant production bears on the improved intelligibility previously reported for these global dysarthria treatment techniques.