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132 JCPSLP Volume 14, Number 3 2012 Journal of Clinical Practice in Speech-Language Pathology
dysarthria (P2), and flaccid dysarthria (P3) (Duffy, 2005).
Acoustic measures are presented in Table 3 along with
comparative data from healthy age- and sex-matched
adults. The nonparametric Wilcoxin Matched Pairs Signed
Ranks test was used to compare the PVI for each syllable
pair in a patient’s sample with those for the matched
control. The relationships between perceptual and acoustic
measures are reported below.
Participant 1
Vocal quality
Duration of sustained ah was reduced, consistent with the
reduced respiratory-phonatory control and short phrase
length. P1’s average f0 was higher than normal, although
low pitch has been more often associated with the
increased laryngeal tone of spasticity (Duffy, 2005). While
variability of f0, jitter, and shimmer for the sustained ah
production were within normal limits, the harmonics-to-
noise ratio (HNR) was slightly below the recommended
threshold, indicative of mild vocal hoarseness. This likely
relates to the perception of P1 having a strained-strangled
voice quality.
Speech rate and prosody
P1’s speech rate was perceived as mildly slow, consistent
with AMR and SMR rates being about 1 syllable/sec below
the normal range and 1.7 syllables/sec slower than normal
for reading. Prosodic variation in the reading task was
measured with the Pairwise Variability Index. PVI_Dur was
significantly reduced compared to the control sample,
consistent with the perception of mild equalisation of stress.
PVI_f0 and PVI_dB were not significantly different to the
control, despite the perception of reduced pitch and
loudness variation in the reading sample.
Participant 2
Vocal quality
Duration of sustained ah was well below the average
expected for healthy speakers, suggestive of poor
respiratory-phonatory control. P2 displayed irregular pitch
breaks and vocal tremor. Average f0 was within the normal
range but standard deviation of f0 was very high, possibly
influenced by brief pitch breaks. Jitter was below the
Acoustic measurements
Vocal quality. Vocal quality was assessed during sustained
phonation, which represents stable vocal performance with
minimal demands for vocal tract adjustments. First, the
average duration (msec) was measured over three
successive attempts at sustained phonation. Second, a
3-second stretch of the sustained vowel was selected for
measurement from the middle of the sustained phonation,
not including the first 25 msec or the terminal part of the
phonation (Kent et al., 2000). The PRAAT Voice Report
function was used to calculate average f0, standard
deviation of f0, jitter (local), shimmer (local), and HNR
e.g.,
Kent et al., 2000).
Speech rate and prosody. Speech rate was measured as
syllables spoken per second for AMR, SMR, and reading. In
connected speech, stress variability was measured with the
Pairwise Variability Index (PVI). Instructions for calculating
PVI_Dur, PVI_f0 and PVI_dB for the first 20 syllables/vowels
in the Grandfather reading are given in Table 2 and Figure 1.
Higher PVI values represent greater variation; PVI values
close to zero indicate equal stress, monopitch, or
monoloudness (i.e., dysprosody).
Reliability of measurement
Inter-rater reliability was calculated on all manual
measurements using intra-class correlation coefficients
(inter-rater reliability: ICC 2, 1, absolute agreement, single
measures). Inter-rater reliability was excellent (≥0.75;
Cicchetti, 1994) for vowel duration (ICC: 0.78, 95% CI 0.51
–0.89), vowel peak f0 (ICC: 0.78, 95% CI 0.62 –0.86) and
vowel peak dB (ICC: 0.75, 95% CI 0.02 –0.92). Absolute
agreement resulted in the wide CI band for vowel peak dB
however, the average difference in dB measures was not
clinically significant at 2.07 dB (SD = 1.12). Intra-rater
reliability was also high for the three measures (ICC: 0.85,
95% CI 0.72 –0.93; ICC: 0.92, 95% CI 0.84 –0.96; and
ICC: 1.0, 95% CI 0.99 –0.1, respectively).
Results
Each participant presented with prototypical perceptual
features consistent with spastic dysarthria (P1), ataxic
Figure 1. Waveform and spectrogram for the word “vegetables”, as displayed in PRAAT, with the first vowel highlighted in the
waveform (upper panel). In the spectrogram (lower panel), the top overlaid dotted line represents the fundamental frequency as it
changes over the word (displayed in blue within PRAAT), the bottom overlaid line represents the vocal intensity (displayed in yellow
within PRAAT). Note the erroneous pitch values just prior to the ‘b’ and at the onset of the final schwa.