Language production in Parkinson's disease: acoustic and linguistic considerations

The hypokinetic dysarthria of Parkinson's disease (PD) has been described extensively. In contrast, patterns of hesitation and the language structure in spontaneous speech of the PD patient have not been investigated, although several studies have shown language-related abnormalities in word naming, word generation, and verbal recall. In the present study, 10 male Parkinson's patients and 10 normal male speakers were compared in a reading and spontaneous speaking paradigm for acoustic and linguistic features. Among acoustic measures, fundamental frequency and relative intensity differentiated PD from control subjects, consistent with reported features of hypokinetic dysarthria. The striking observations among linguistic measures differentiating PD from control subjects were an increase in the number of (a) silent hesitations per minute, (b) abnormally long silent hesitations, (c) words per silent hesitation, (d) open class phrases, and (e) optional open phrases per speech sample, and a decrease in the number of modalizations and interjections. An increase in the number of filled hesitations occurring per minute, as well as a decrease in syntactic complexity separated moderate from mild Parkinson's patients. Our interpretation of the data favors the hypothesis that changes in the structure of spontaneous language production with increasing severity of dysarthria reflect PD patients' adaptation to their disease.