Increasing phonological complexity reveals heightened instability in inter-articulatory coordination in adults who stutter

Abstract

The potential role of phonological complexity in destabilizing the speech motor systems of adults who stutter was explored by assessing the performance of 17 adults who stutter and 17 matched control participants on a nonword repetition task. The nonwords varied in length and phonological complexity. Behavioral results revealed no differences between the stuttering and normally fluent groups on accuracy of nonword repetition. In contrast, dramatic differences between groups were observed in the kinematic data. Indices of the consistency of inter-articulator coordination revealed that adults who stutter were much less consistent in their coordinative patterns over repeated productions. With increasing length and complexity of the nonwords, between-group differences in coordinative consistency were more pronounced. Coordination consistency measures revealed that adults who stutter (but not normally fluent adults) showed within-session practice effects; their coordinative consistency improved in five later compared to five earlier productions. Adults who stutter produced the nonwords at a slower rate, but both groups showed increased rates of production on the later trials, indicating a practice effect for duration for both groups. We conclude that, though the adults who stutter performed behaviorally with the same accuracy as normally fluent adults, the nonword repetition task reveals remarkable differences in the speech motor dynamics underlying fluent speech production in adults who stutter compared to their normally fluent peers. These results support a multifactorial, dynamic model of stuttering in which linguistic complexity and utterance length are factors that contribute to the probability of breakdown of the speech motor system.

Educational objectives: After reading this article, the reader will be able to: (1) summarize the literature on potential language/motor interactions in stuttering, and (2) evaluate to what extent the study findings support the hypothesis that phonologically complex utterances have a destabilizing effect on the speech motor system in individuals who stutter.

Figure 1

Example of one nonword, “mabfieshabe,” produced by an adult who stutters. The plots in the left column are the set of 5 productions from the “early” trials; right column plots are the set of five “later” trials. Top panel: original difference signals before normalization. Note different duration of the various productions of the nonword. Middle panel: plots after time and amplitude normalization. Bottom panel: Standard deviations computed at 2% intervals in relative time. Inset in bottom plots are the lip aperture variability indices for the early and later trials. Note that the LA variability index is reduced for the later set of trials.

Figure 2

Plot of lip aperture variability indices (mean and std. error) for early and later trials for the two groups of speakers. For each nonword and each speaker group, the symbols for the early and later sets of trials are connected by a line.

Figure 3

Plot of duration of the nonwords (mean and std error) for the early and later trials for the two groups of speakers. For each nonword and each speaker group, the symbols for the early and later sets of trials are connected by a line.

Figure 4

Plot of lip aperture variability indices (top) and durations (bottom) for each subject's early 5 and later 5 productions of the nonword “mabshaytaidoib.”