Temporal speech characteristics of individuals with multiple sclerosis and ataxic dysarthria: 'scanning speech' revisited.

'Scanning speech' has been used as a description of a prominent characteristic of the dysarthria of multiple sclerosis (MS) as well as of ataxic dysarthria in general. It is thought to be measurable as equalized syllable durations. There are seemingly contradictory prosodic-temporal characteristics of ataxic dysarthria: perceptually characterised as prosodic excess as well as phonatory-prosodic insufficiency and acoustic characteristics including signs of isochrony as well as variability. This study investigates the temporal characteristics at two levels, duration and variability of syllable durations and the durations of interstress intervals. A group of 14 individuals with MS and ataxic dysarthria as well as 15 control subjects were studied. It was concluded that individuals with ataxic dysarthria and MS showed (a) for syllables: significantly increased durations and decreased intrautterance variability (more isochrony or syllable equalization) as well as significantly increased interutterance variability; (b) for interstress intervals: significantly increased durations and increased variability (less isochrony). The results point to inflexibility as well as instability of temporal control, which could contribute to the explanation of why the perceptual characteristics are contradictory.