Accuracy and variability of isochronous rhythmic timing across motor systems in stuttering versus nonstuttering individuals.

Ten adults who stutter and ten adults who do not stutter completed speech, orofacial nonspeech, and finger isochronous rhythmic timing tasks in a synchronization-continuation paradigm with auditory stimuli and with 450 ms, 650 ms, and 850 ms interstimulus onset intervals. Responses consisted of bilabial contact in the syllable /pa/ during a speech task, bilabial contact in an orofacial nonspeech task, and thumb-index finger contact in a finger movement task. Effector movements were transduced, and time points associated with minima in the derived lip or finger aperture signals were automatically extracted. Multiple analyses of timing accuracy and variability were completed for both the synchronization and continuation phases, including decomposition of total timing variance into central clock and motor implementation variance according to the Wing-Kristofferson model. The combined results from descriptive comparisons, statistical significance testing, and effect size computations suggest that the stuttering and nonstuttering participants showed highly similar levels of both timing accuracy and timing variability. This was true (a) for all three motor tasks, (b) at all movement rates, and (c) for synchronization as well as continuation movements. As one component of a systematic approach to investigating the role, if any, of timing difficulties in stuttering, these findings extend growing evidence that stuttering individuals do not differ from nonstuttering individuals in the ability to generate temporal movement patterns with a simple isochronous rhythm. We present some hypotheses about the implications that may follow from brain imaging and clinical neurological studies that have investigated the neural substrates recruited by this particular experimental task.