Speech Therapy and Speech Motor Control: Part 2

Speech Therapy and Theories of Speech Motor Control: Part 2

In Part 1 of this blog series I described the theoretical basis of Dynamic Temporal and Tactile Cueing as recently published by Edy Strand. Specifically, the treatment is founded on Schmidt’s Schema Theory in which generalized motor programs are learned. During speech production the child must select the right program and apply the correct parameters before implementing it all at once. If the parameters are selected incorrectly, a speech error will occur. It is rather like making toast. If you forget to reset your settings after toasting bagels, your Wonderbread will come out black! The problem as stated by Schmidt is that by the time you realize that your toast settings are wrong and your motor gestures are off track, it’s too late— the toast is burned and you have said “Trat! Doast!” Learning occurs by “trial and error” — after much experience with your toaster you learn the settings (parameters) for getting the right amount of toastiness for different items. Learning to operate your toaster is similar to acquiring one “generalized motor program.” Speech motor learning is assumed to operate this way because sensory feedback is too slow to support on-line adjustments to the parameters in a direct way. I used a different analogy in the previous blog — once you have committed to swinging your golf club, you tend to follow through.

The problem with this model of speech motor control is that we know for certain that real time modification of vocal tract movements occurs in response to somatosensory and auditory feedback. Strangely we have known since the early eighties that the speech system is highly sensitive to error on-line; therefore, I don’t know why this idea of open-loop control persists. The proof comes from studies in which (typically) an adult is asked to repeatedly produce a particular syllable or disyllable and then experiences a perturbation in sensory feedback (either somatosensory feedback or auditory feedback). An early example of this paradigm involved productions of “aba”: during 15% of trials a mechanism placed an unexpected load on the talker’s lower lip. Here is where it gets interesting: the research participants corrected for this perturbation in the articulatory trajectory of the bottom lip very rapidly with compensatory actions of the top and the bottom lip (the bottom lip would need to exert greater upward force and the top lip would need to produce greater downward extent in order to produce the labial closure and the expected transitions into and out of the consonantal closure). Decades of experiments have followed involving many other perturbations in the domain of articulatory gestures, somatosensory (skin) sensations, and auditory feedback. For example, while the research participants are repeatedly saying “bed” you can trick their ear into thinking they are saying “bad” which leads to compensatory adjustments in articulation to get the expected auditory percept.

This kind of dynamic compensation across the entire vocal tract is made possible by an “internal model” — a neural model that simulates the behavior of a sensorimotor system in relation to its environment. The internal model can generate a prediction of the sensory consequences of implementing a motor plan via simulation. For speech, future outputs in the somatosensory and auditory domains are simulated; furthermore, the simulator takes into account delayed sensory feedback, noise in the perceptual system and other variables so that when feedback arrives it can be compared with the prediction and provide reliable error messages. Continuous tracking of the vocal tract state is thus permitted and forms the basis for ongoing planning of movements as speech unfolds. If an unexpected event occurs, as in the perturbation experiments that I have described, error corrections are dynamic across the entire system; therefore, if the predicted trajectory of acoustic formant transitions from the [a] into the [b] closure is not occurring, lower lip, upper lip, jaw and tongue movements can all be harnessed to produce the desired outcome.

As Houde and Nagarajan (2011) explain, “speech motor control is not an example of pure feedback control or feedforward control” (p. 11). The acquisition of speech motor control is dependent upon the development of the internal model of vocal tract function as well as detailed knowledge of auditory targets. This understanding has implications for the treatment of childhood apraxia of speech. I will explore these implications further in the next and final blog in this series.

Readings

Abbs, J. H., & Gracco, V. L. (1983). Sensorimotor actions in the control of multi-movement speech gestures. Trends in Neurosciences, 6, 391-395.

Houde, J. F., & Jordan, M. I. (2002). Sensorimotor adaptation of speech I: Compensation and adaptation. Journal of Speech, Language & Hearing Research, 45(2), 295-310.

Houde, J. F., & Nagarajan, S. S. (2011). Speech production as state feedback control. Frontiers in Human Neuroscience, 5, doi: 10.3389/fnhum.2011.00082.

Tourville, J. A., Reilly, K. J., & Guenther, F. H. (2008). Neural mechanisms underlying auditory feedback control of speech. NeuroImage, 39, 1429-1443.

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  1. Speech Therapy and Speech Motor Control: Part 3 | Developmental Phonological Disorders

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