Acoustic analyses of voice and speech in individuals with PD have confirmed the perceptual descriptions of hypokinetic dysarthria. Fox and Ramig (33) documented reduced vocal sound pressure level (vocSPL) by 2 to 4 dB (at 30 cm) on a number of speech tasks in 29 individuals with PD, compared with age- and gender-matched controls, which is equal to a 40% change in vocal loudness. Ho et al. (34) found vocSPL in PD to decay much faster than in neurologically normal speakers. They interpreted this fading as symptomatic of frontostriatal dysfunction. Rosen et al. (35) examined intensity decay in the phonation of persons with and without PD on various speech tasks. They found that vocSPL declined more rapidly in PD than in normal, age-matched speakers during syllable repetition [speech diadochokinesis (DDK)]. They also found that in some of the individuals with PD, there were abnormally abrupt changes in vocSPL during conversation. However, during sustained vowel phonation, vocSPL did not show decay more than that of normal controls. Some early studies (36,37) did not confirm a reduction in vocSPL even though the speech of individuals with PD was perceptually characterized by reduced loudness. The reasons for these discrepant findings are not clear. The presence or absence of vocal decay in parkinsonian speech is related, at least partially, to the specific speech task being performed (35), as well as to the severity of hypokinetic dysarthria (34).
Prosodic pitch inflection in speech, measured acoustically as fundamental frequency (F0) variability, or standard deviation from the mean F0 (SDF0) has been reported to be consistently lower in individuals with PD when compared with controls. These findings are consistent with the perceptual characterization of parkinsonian speech as monotone or monopitch (31,32). A reduction in maximum fundamental frequency range has also been observed in the dysarthric speech of individuals with PD, when compared with the normal speech of healthy speakers (38).
Voice quality is measured in terms of jitter (random cycle-to-cycle variation in the periodicity of the voice waveform), shimmer (random cycle-to-cycle variation in the amplitude of the voice waveform), and harmonics-to-noise ratio. These are acoustic indices of short-term phonatory stability. Such instability has been documented in the speech of individuals with PD, consistent with various perceptual characteristics of disordered voice quality (e.g., hoarse, breathy, harsh) (39,40). Long-term phonatory instability, reflected mainly in rhythmic changes in F0, has also been documented in individuals with PD (40).
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