Speech Therapy and Theories of Speech Motor Control: Part I

Edy Strand recently published a detailed description of her Dynamic Temporal and Tactile Cueing treatment strategy. As she says this is a hugely valuable paper because it provides a complete description of a treatment designed for severe speech sound disorders, especially Childhood Apraxia of Speech, and more importantly, it summarizes in one place the theoretical foundation for the treatment. I think that, on the whole, this is an efficacious treatment although there are some procedures, derived directly from the outdated theoretical underpinnings, that are questionable however, and therefore I am going to devote several blogs to more recent theory and basic science research on the development of speech motor control and apraxia of speech. In this first blog, I review Schema Theory, even though this theory is just not right! But it has a long history and remains currently popular across almost all clinically-oriented papers on motor speech disorders.

The theory that is referenced in Edy Strand’s paper is Richard Schmidt’s “Schema Theory of Discrete Motor Skill Learning,” published in Psychological Review in 1975 and subsequently brought to speech-language pathology by Ray Kent and others as a useful framework for thinking about speech therapy. The important idea underlying this theory is that motor skills are made up of brief, discrete motor acts that are executed all-at-once as open-loop generalized motor programs, adapted with specific response specifications (called parameters) for the current conditions. The theory assumes “open-loop” control because sensory feedback is often too slow to impact movement after it has started. According to this theory feedback is processed after the movement is over and incorporated into the schema for the future execution of the generalized motor program. I have used golf as an example before; even though I haven’t played much in years let’s do it again: if we are adopting this theory we would think of practice sessions as developing different generalized motor programs for each type of shot, a long drive, a short 7-iron shot, the up-and-down pitch onto the green, and the putt into the hole. Which shot you choose depends upon your recall schema: what is your target and which type of shot is likely to achieve it? I personally recall that when close to the green my pitch is better than my chip (whereas my husband has the opposite preference). How you address the ball depends upon the initial conditions (flat ground, hill, tall grass etc.). The motor control parameters (also known as response specifications) depend upon the distance to the target (how high to lift the club, speed of follow through, force applied and so on). Based on the initial conditions and the desired outcome, I launch the shot with my wedge, expecting a certain “feel” as I hit the ball based on past experience with the sensory consequences of hitting this shot; I can always “recognize” a good hit even before I see the ball land (often I just turn my back on the ball, I don’t even want to see it land!). But in any case, the actual outcome is important for updating the “recall” schema; specifically, if I have actually achieved my target, I add all this information, the initial conditions, the response specifications, the recognition schema and the recall schema to my memory. The generalized motor program is an abstraction across all these remembered practice trials, permitting correct specification of the response parameters in future shots. Furthermore, I should be able to adapt the generalized motor program to similar shots, even if the ball is a little further or closer to the green for example.

When applied to CAS, in which current research suggests unreliable or degraded somatosensory feedback, the use of this model focuses attention on the child’s processing of initial conditions, inaccurate planning or programming of the movement due to poor selection of response specifications, and/or poor recognition schema (not knowing when the movement “feels right”). Therefore, certain procedures are recommended. DTTC providers use manual or gestural cues to shape the child’s articulators into the “initial position” and encourage the child to “hold” the position momentarily so as to fully process those initial conditions before launching the movement. During the initial stages of therapy, the SLP uses a slow rate and co-production so that the child is getting extra feedback during the practice trial, presumably with the goal of stabilizing the recognition schema. Imitative models support the child’s knowledge of the target which, when combined with copious knowledge of results feedback should support the development of recall schema. And finally, a great deal of practice with an errorless approach ensures that the child lays down many memory traces of correctly executed motor programs.

The recommendations that are provided make a certain amount of sense given the context of schema theory (even though there is in fact no evidence for the specific efficacy any one of these particular procedures). The problem is that it is not clear that schema theory is a reasonable foundation for modern speech therapy practice.

First, citing Richard Schmidt himself, he cautioned in 2003 that “schema theory was intended to be an account of discrete actions. Hence, continuous actions, such as steering a car or juggling, which are both of longer duration (allowing time for response-produced feedback to have a role) and more based on the performer’s interactions with the environment were outside the area for schema theory…long-duration actions might be based on interplay between open-loop subactions and feedback-based corrections… . Interestingly, tasks such as juggling seem appropriate for analysis in terms of the dynamical systems perspective” (p. 367). I would argue that our understanding of, not only juggling, but speech motor control has benefited immensely from the dynamical systems perspective and I will come back to that in the next blog. If juggling is considered too complex and continuous to be explained by schema theory, probably speech is not a good fit either.

Second, modern theories of speech motor control have shown that on-line correction of motor action even over short durations occurs despite the limitations of feedback control. The explanation lies in the continuous operation of feedforward control mechanisms. More on feedforward control in another blog.

References

Rvachew, S., & Brosseau-Lapré, F. (2012). Developmental Phonological Disorders: Foundations of Clinical Practice. San Diego, CA: Plural Publishing.

Schmidt, R. A. (1975). A schema theory of discrete motor skill learning. Psychological Review, 82(4), 225-260. doi:10.1037/h0076770

Schmidt, R. A. (2003). Motor schema theory after 27 years: Reflections and implications for a new theory. Research Quarterly for Exercise and Sport, 74(4), 366-375.

Strand Edythe, A. (2019, Early View). Dynamic Temporal and Tactile Cueing: A Treatment Strategy for Childhood Apraxia of Speech. American Journal of Speech-Language Pathology. doi:10.1044/2019_AJSLP-19-0005