Tongue shape complexity for liquids in Parkinsonian speech

Parkinson’s disease (PD) is a neurological disorder characterized by the degeneration of dopaminergic neurons. Speech impairments in PD are characterized by slowed muscle activation, muscle rigidity, variable rate, and imprecise consonant articulation. Complex muscular synergies are necessary to coordinate tongue motion for linguistic purposes. Our previous work showed that people with PD had an altered rate of change in tongue shape during vowel to consonant transitions, but differences were small, perhaps due to the simplicity of the speech task. In order to test sentences, four PD participants and three older controls were imaged using ultrasound. They repeated sentences from the Rainbow Passage. Tongue shape complexity in liquids and adjacent vowels was assessed by their bending energy [Young et al., Info. Control 25(4), 357–370 (1974)]. Preliminary results show that bending energy was higher in liquids than in vowels, and higher in controls than PD speakers. Production of liquids typical requires a flexible tongue shape; these PD speakers show reduced flexibility that is nonetheless compensated sufficiently for the production of intelligible speech. Implications for speech motor control and for PD evaluation will be discussed.