Examining the effects of multiple sclerosis on speech production: does phonetic structure matter?

The second formant (F2) is well-known to be important to intelligibility (e.g. [Delattre, P., Liberman, A., & Cooper, F. (1955). Acoustic loci and transitional cues for consonants. Journal of the Acoustical Society of America, 27, 769-774]) and is affected by a variety of dysarthrias [Weismer, G., & Martin, R. (1992). Acoustic and perceptual approaches to the study of intelligibility. In R. Kent (Ed.), Intelligibility in speech disorders (pp. 67-118). Amsterdam: John Benjamins Publishing Company]. This study tests two related hypotheses: (1) dysarthria associated with Multiple Sclerosis (MS) has a greater effect on the ability to produce extreme F2 movement than on typical F2 movement and (2) phonetic stimuli associated with large and/or rapid F2 movement in healthy speakers precipitate larger differences between healthy and dysarthric speech than do stimuli associated with small and/or slow F2 movement. Twelve participants with MS and 16 healthy controls read aloud the Grandfather Passage. F2 slopes were calculated from the F2 tracings (i.e. change in Hz over a 20ms lag). For each sentence, the following measures of F2 movement were calculated: F2 range (maximum F2-minimum F2), median slope, and 95%ile slope. The mean and the maximum observation for each participant were respectively used as measures of typical and extreme productions. A repeated-measures MANOVA detected significantly larger group differences in the slopes of maximum productions than for mean production. Also, group differences in F2 slope were greater in phonetic stimuli associated with the largest F2 slopes in healthy speech than in phonetic stimuli associated with more shallow slopes. No group differences in F2 range were detected. Results indicate dysarthria affects the production of extremely rapid changes in F2 and that some phonetic structures are more useful than others for detecting these impairments. It is concluded that comparison of speakers' best-productions (e.g. maximum F2 slope) yield more useful estimates of the effects of mild to moderate dysarthria on F2 slopes than do typical productions.

The reader will be able to: (1) describe two approaches to measuring typical and extreme acoustic variability that can be applied to connected speech, (2) discuss evidence that MS affects extremely rapid changes in F2.