Investigating the use of a feedforward motor strategy in the speech of children who stutter
Rick Arenas, M.A.
Doctoral Candidate, University of Iowa
There are two primary strategies that people use when performing any sort of motor task, including speech: feedback and feedforward. Feedback relies on moment-to-moment sensory awareness of system output so that ongoing operations can be altered to obtain a specific motor goal. The feedforward strategy uses internal representations to pre-plan and anticipate the necessary motor sequence to achieve a motor goal. It has been speculated that these two strategies are used simultaneously in any motor movement.
In the case of speech behaviors, once they are acquired, a feedforward strategy is assumed to be primarily used, and feedback is only employed when there is a discrepancy between the anticipated sensory output and the actual sensory feedback.
One way to investigate the use of a feedforward strategy is to look at anticipatory adjustments in the motor output. Goffman, Smith, Heisler, & Ho (2008) found evidence of feedforward strategy in speech by examining the breadth of coarticulation across an entire utterance. Coarticulation refers to the influence of surrounding phonemes on the production of a given phoneme.
Using an Optotrack system to measure anterior-posterior lip movement, Goffman et al., compared lip rounding across an entire utterance in sentences that differed only by a single vowel (e.g. The mom has a beet in the box., The mom has a boot in the box.), as produced by eight year old children and young adults. One of the vowels requires lip rounding ("oo" in boot) and the other does not ("ee" in beet). When comparing lip rounding between the two sentences they found that in the 'boot' sentence there were adjustments in the lip rounding that preceded the "oo" sound. In both age groups, these adjustments were actually present at the beginning of the utterance, suggesting that the motor plan for the entire utterance was generated prior to the initiation of speech, and hence providing evidence of a feedforward motor strategy in speech.
Max, Guenther, Gracco, Ghosh & Wallace (2004) hypothesized that disfluencies in people who stutter (PWS) may result from an over-reliance on a feedback motor strategy. They speculated that PWS use feedback more frequently than individuals who don't stutter because they have an inability to consistently generate an adequate feedforward model when initiating speech. For my dissertation project, I am going to investigate the use of feedforward speech motor strategies in the speech of children who stutter (CWS), using the same paradigm that Goffman et.al developed. Specifically, I am going to compare anticipatory lip rounding adjustments between normally fluent children and CWS. If it is true that people who stutter rely less on a feedforward strategy when compared to fluent speakers, I hypothesize that fluent children will show more pronounced anticipatory lip rounding adjustments, and these adjustments will be evident sooner in the utterance.
In addition, because we know that speech disfluencies are more frequent in longer and more complex sentences, I will use several sets of sentences that vary in length and syntactic complexity. If between-group differences are present, I can conclude that these differences will become greater as a function of incremental changes in these two variables.
The results of this study will help to gain a better understanding of the motor strategies used by children who stutter. The hope is that this information can lead to a better understanding of how and why some treatment approaches work and perhaps help improve stuttering treatment. I look forward to sharing the results once the study is completed.
Max, L., Guenther, F.H., Gracco, V.L., Ghosh, S.S., and Wallace, M.E. (2004). Unstable or insufficiently activated internal models and feedback-biased motor control as sources of dysfluency: A theoretical model of stuttering. Contemporary Issues in Communication Science and Disorders, 31, pp. 105-122.
Goffman, L., Smith, A., Heisler, L. and Ho, M. (2008). The breadth of coarticulatory units in children and adults. Journal of Speech, Language and Hearing Research, 51, 1424 1437.