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Temporal and spatial variability in speakers with Parkinson's disease and Friedreich's ataxia.

Article Excerpt
Speech variability in groups of speakers with Parkinson's disease (PD) and with Friedreich's (FA) ataxia was compared with healthy controls. Speakers repeated the same phrase 20 times at one of two rates (fast or habitual). A nonlinear analysis of variability was performed that used some of the principles behind the spatio-temporal index (STI). The STI usually employs variation in lip displacement over repetitions of the same utterance, and a linear analysis of such signals is conducted to represent the combined variation in spatial and temporal control. When working with patients, audio measures (here we used speech energy) are preferred over kinematic ones as they are minimally disruptive to speech. Nonlinear methods allow spatial variability to be estimated separately from temporal variability. The results are tentatively interpreted as showing that PD speakers were distinguished from healthy control speakers in spatial variability, and ataxic speakers were distinguished from controls in temporal variability. These findings are consistent with the speech symptoms reported for these disorders. We conclude that the nonlinear analysis using the speech energy measure is worth investigating further as it is potentially revealing of the differences underlying these two pathologies.

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Parkinson's disease (PD) and ataxia may lead to more variable speech performance in such patients compared to healthy controls. Previous work using variability measures has employed the spatio-temporal index (STI) introduced by Anne Smith (Smith, Goffman, Zelaznik, Ying, & McGillem, 1995). In her original work, Smith applied the STI to a lower lip kinematic signal obtained on a phrase rich in bilabials ("Buy Bobby a puppy"). Participants repeated the phrase as precisely as they could 10-20 times, and lower lip movement was measured. During analysis, the individual lip-movement records were normalized in amplitude by transforming each record to z scores and normalized in time by stretching or compressing them to a common length using a linear scale factor. The standard deviation (SD) was then obtained at 2% intervals on the normalized time axis, and the computed quantities were summed to give the STI score.

A small number of studies have used STI in speakers with dysarthria. Kleinow, Smith, and Ramig (2001) investigated the motor performance of speakers with PD in different rate (habitual, slow, and fast) and loudness conditions (loud and soft voice). All participant groups showed elevated STI levels in the slow rate condition. The PD speakers did not differ significantly from the age-matched healthy control group, but the PD participants were of very mild severity. The STI also showed that the loud voice condition resulted in more consistent articulation in all groups, lending some support to the benefits of LSVT [R] (Ramig, Pawlas, & Countryman, 1995) treatment. In another study McHenry (2003) compared participants with different dysarthria severities across rate conditions. STI values were related to the observed differences in severity. Despite encouraging results for the use of the STI in clinical populations, insufficient data are currently available to make predictions on how STI measures are affected in speech disorders, mainly because the current methodology is too invasive to gain data from a large number of speakers. There is also the question of whether the current STI measure is sufficiently discriminative to reflect differences in the underlying pathology. This exploratory study therefore employed a different methodology that is more suited to...

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