Rate Modulation Abilities in Acquired Motor Speech Disorders

Purpose: The purpose of this study was to describe, compare, and understandspeech modulation capabilities of patients with varying motor speech disorders(MSDs) in a paradigm in which patients made highly cued attempts to speakfaster or slower. Method: Twenty-nine patients, 12 with apraxia of speech (AOS; four phoneticand eight prosodic subtype), eight with dysarthria (six hypokinetic and two spasticsubtype), and nine patients without any neurogenic MSD completed a standard motor speech evaluation where they were asked to repeat words and sentences,which served as their “natural” speaking rate. They were then asked torepeat lower complexity (counting 1–5; repeating “cat” and “catnip” 3 times each)and higher complexity stimuli (repeating “catastrophe” and “stethoscope” 3 timeseach and “My physician wrote out a prescription” once) as fast/slow as possible.Word durations and interword intervals were measured. Linear mixed-effectsmodels were used to assess differences related to MSD subtype and stimulicomplexity on bidirectional rate modulation capacity as indexed by word durationand interword interval. Articulatory accuracy was also judged and compared.Results: Patients with prosodic AOS demonstrated a reduced ability to go faster;while they performed similarly to patients with spastic dysarthria when counting,patients with spastic dysarthria were able to increase rate similar to controls duringsentence repetition; patients with prosodic AOS could not and madeincreased articulatory errors attempting to increase rate. AOS patients mademore articulatory errors relative to other groups, regardless of condition; however,their percentage of errors reduced with an intentionally slowed speaking rate.Conclusions: The findings suggest comparative rate modulation abilities in conjunctionwith their impact on articulatory accuracy may support differential diagnosisbetween healthy and abnormal speech and among subtypes of MSDs(i.e., type of dysarthria or AOS). Findings need to be validated in a larger, morerepresentative cohort encompassing several types of MSDs.