This study examined progressive speech intensity decay using two speech tasks: sustained vowel phonation (Experiment 1) and sentence reading (Experiment 2).
Parkinsonian speech intensity has often been clinically observed to fade out or trail off. This gradual diminution of intensity is not unlike the well-documented progressive reduction of force underlying (upper limb) micrographic parkinsonian handwriting and (lower limb) marche à petit pas. Motor instability in speech intensity has yet to be investigated in a controlled experimental setting, however.
Thirteen Parkinson disease (PD) patients and their matched controls participated in Experiment 1; data from 6 PD patients and controls who naturally (i.e., without prior instruction) read the target sentence within a breath span were included in the analysis for Experiment 2. Participants were instructed to inhale maximally before vocalizing, and the extent of intensity declination over the breath span was measured.
Parkinson disease patients demonstrated a consistently greater level of progressive intensity decay compared with matched controls for both speech tasks. This successful documentation and analysis of fading speech was interpreted as evidence for motor instability within the speech motor system in PD.
It was concluded that the control of force in complex motor sequences involving speech and limb movement is affected by a common deficit in the frontostriatal circuit.