Support for Speech Perception Interventions in Speech Therapy

I am writing a third blog on this strange experimental protocol in which the talker produces a syllable repeatedly and the talker’s speech output is altered in a systematic fashion so that the talker hears him or herself say something that does not correspond to their own articulatory gestures. I am fascinated by these experiments because they are a window onto feedback control which is essential for a successful speech therapy outcome. Initially in traditional speech therapy the SLP is providing a lot of external feedback about the child’s articulatory gestures (knowledge of performance feedback) and the correctness of the child’s speech output (knowledge of results feedback). But given that the SLP cannot follow the child around outside the clinic room, eventually the child must learn to use self-generated feedback for speech motor learning to occur. Can children use auditory feedback to change their own speech?

In a previous blog, On Birds and Speech Therapy, I discussed interesting work from Queen’s University  suggesting that toddlers do not use feedback control like adults do during speech motor learning.  These researchers found that adults will compensate for perturbations of their own speech by adjusting their articulation to get the desired auditory feedback. In contrast, very young children do not compensate in this way. I suggested that this may be because toddlers do not perceive speech with the same degree of precision as adults. This hypothesis was supported by another study in which speakers of French and English did not show the same compensation effect to a perturbation that made their vowels sound like a French vowel. The English talkers did not respond to a perturbation to which they were not perceptually sensitive (see Feedback Control and Speech Therapy Revisited).

Recently, I was delighted to find another study involving children provides even stronger confirmation that perceptual representations play a key role in the child’s ability to use feedback for speech motor learning. Shiller and Rochon (2014)  randomly assigned 5- to 7-year-old children with typical speech to two training conditions: the control group received speech perception training for the /b/-/d/ contrast; the experimental group received speech perception training for the /ɛ/-/æ/ contrast. Prior to and subsequent to this training both groups experienced the perturbation experiment: both groups repeated said “Beb” while their own speech was altered to sound more like “Bab”. Prior to perceptual training, both groups showed a small compensation for this perturbation in the feedback of their own speech. After speech perception training the experimental group showed twice as much compensation as before whereas the control group showed no change in the amount of compensation.  The results show that children can indeed use feedback for speech motor adaptation; furthermore, this ability improves as perceptual boundaries between phoneme categories become better defined —with age or with training.

The conclusions of the study are very gratifying. Citing my own work on the importance of speech perception training as a strategy to facilitate speech production learning by children with speech sound disorders, the authors conclude:

“The results of the present study complement this work nicely, demonstrating that improvements in children’s auditory perceptual abilities do not simply improve motor performance, but also alter the capacity for auditory-feedback based speech motor learning—a process that is central to the clinical treatment of speech production disorders.” (p. 1314)

No surprise that I like this study a lot!

Feedback Control and Speech Therapy Revisited

In August 2012 I posted a comment about MacDonald, E. N., Johnson, E. K., Forsyth, J., Plante, P., & Munhall, K. G. (2012). Children’s development of self-regulation in speech production. Current Biology, 22, 113-117. (see On Birds and Speech Therapy). In this paper the authors reported that toddlers did not compensate for perturbations of their own vowel formants and they concluded that toddlers “do not monitor their own voice when speaking in the same way as adults do”. I was skeptical of this claim since it is hard to imagine how children learn to talk at all if they do not have access to feedback control mechanisms. I suggested that perceptual explanations would make more sense and now there is published evidence that this is indeed the case, interestingly from a paper including Munhall as author, specifically, Mitsuya, T., Samson, F., Ménard, L., & Munhall, K. (2013). Language dependent vowel representation in speech production. Journal of the Acoustical Society of America, 133, 2993-3003.

The paper is fascinating because it shows that English and French talkers to not show the same compensation effect when participating in this experimental paradigm and when the vowels involve French rounded vowel categories (i.e., English talkers do not change their own speech to compensate to a perturbation that makes their own speech sound more like a French vowel whereas French speakers do). Furthermore, the amount of compensation that a talker produces is related to the talker’s underlying phonological representation of the vowel space, as represented in acoustic-phonetic terms. In this study, when the English listeners did not respond to the particular perturbation of their vowel formants that was used, the researchers did not conclude that English people are incapable of using feedback control mechanisms! Rather they concluded that “the function of error reduction itself appears to be language universal, while detection of error is language specific.” However, the use of feedback for error reduction is dependent upon the talker’s perception of the feedback which in turn is related to the listener’s phonological representations (previously this was not clear because the research participants are not always consciously aware of the way that the experimenters are manipulating their speech).

Obviously the same logic should be applied to the toddlers’ apparent failure to use feedback control in a similar experimental manipulation in which the toddler’s speech was changed from one English vowel to sound a little bit more like another English vowel. In fact, a perceptually motivated interpretation is favoured in Mitsuya et al.; when referring back to McDonald et al. they say “a stable phonemic representation is required for error detection and correction in speech, and sometime between 2 and 4 yr of age such a representation emerges and stabilizes.” This is not the interpretation that made the headline in Science Daily but it is the conclusion that makes more sense to me.

What are the implications for speech therapy? The research clearly supports my view that it is essential to ensure that your clients with speech sound errors have stable perceptual and phonological representations – this is a critical component of a treatment program aimed at establishing speech motor control and speech articulation accuracy As Mitsuya et al suggest, the acoustic target for speech is not just the phonetic category itself but the target category in relation to its neighbors. The treatment approach that I have always advocated is focused on phonemic perception: the important procedures include presenting the child with a large population of variable exemplars of the target category. These exemplars should identify the centre of the category, highlighting the important cues and the prototypical characteristics, while also allowing the child to explore the edges of the category so that the child can experience it in relation to similar but contrasting categories. Thus SAILS  presents the child with a task in which highly variable stimuli are judged to be the TARGET or NOT THE TARGET and some of the stimuli are rather ambiguous. SLPs do not always like the fact that not all of the stimuli are prototypical exemplars of the target category but in fact this amount of variability is important for the establishment of phonological representations. Mitsuya et al.’s paper is important because it reinforces the point that stable acoustic-phonetic representations for speech targets are essential for the use of feedback control in speech motor learning.