Intrinsic and extrinsic timing in stutterers' speech: data and implications.
This study examined interarticulator timing amongst stutterers and non-stutterers from the perspective of extrinsic and intrinsic timing models of speech motor control. Timing relationships were examined for an oral closure task across stress, rate, and vowel differences. Sequential aspects of extrinsic timing theories were analyzed by peak velocity sequence profiles of upper lip, lower lip, and jaw, and nonsequential aspects by sequence period (the time period covering all three of the articulator peak velocity onsets times irrespective of sequencing order). Intrinsic timing theory was modeled using phase portrait analyses of the same data. Consistently greater phase angle variability was seen amongst the stutterers, and variability was found to be significantly greater at a faster articulatory rate and when altered stress patterns were required. No group differences were found for peak velocity sequencing, but the (sequence independent) sequence period data somewhat mirrored the phase angle findings. This finding calls into question the notion that phase angle analyses reflect purely intrinsic timing phenomena.