On Monkeys and Speech Therapy

A few months back Science Daily published yet another article about the possible evolutionary origins of speech (see Monkey Lip Smacks Provide New Insights into the Evolution of Human Speech, May 31, 2012: http://www.sciencedaily.com/releases/2012/05/120531135641.htm). Speculating about the evolutionary origins of speech and language is an academic parlour game of some interest to me but like any other sport I find it more entertaining to watch than participate. However, as with other sports, the game sometimes spills over into real life and causes some damage to innocent bystanders and thus I find it necessary to comment in this case.  

 The Science Daily article is based on a study by Ghazanfar and colleagues that used x-ray movies to observe the functional coordination of vocal tract structures during the production of lip smacks and chewing in adult monkeys (http://www.sciencedirect.com/science/article/pii/S0960982212004757). Another study that reported the rhythmic structure of lip smacks and chewing in infant, juvenile and adult monkeys is also relevant (http://onlinelibrary.wiley.com/doi/10.1111/j.1467-7687.2012.01149.x/abstract). The authors are following from the frame/content theory put forward by MacNeilage (1998: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=29997). MacNeilage emphasizes the syllable as the “an organizational superstructure for the distribution of consonants and vowels” that “evolved from ingestive cyclicities (e.g., chewing).” Then he goes further and suggests that since “ontogeny recapitulates phylogeny in the realm of human motor function”, speech must arise from ingestive cyclicities in developmental time as well. This is where the parlour game gets dangerous. I don’t think that it is any accident that shortly after this time a whole host of speech therapy approaches, books, kits, videos, workshops and websites devoted to “oral motor therapies” sprang up with the express purpose of providing “a stable foundation for speech by first addressing instability in the jaw, lips and tongue” (http://speech-language-pathology-audiology.advanceweb.com/article/oral-motor.aspx).  The explicit rationale for these approaches is that “motor skills in feeding and non-speech movements act as prerequisites to speech clarity. Feeding and non-speech activities are targeted prior to speech production tasks to ensure adequate muscle functioning is available”. It has taken a decade of kinematic and electromyographic studies in infants and young children to both prove and transmit the message that chewing and speech are not related to each other. As Francoise and I describe in detail in our book (http://www.pluralpublishing.com/publication_dpd.htm) the muscle activation patterns for these two functions are completely different with reciprocal activation of agonist and antagonist mandibular muscle groups during chewing versus coactivation of these muscle groups during speech. More importantly, as shown by Steeve et al (2008) (http://www.ncbi.nlm.nih.gov/pubmed/18664699?dopt=Abstract) muscle activation patterns for chewing and babble are both uncoordinated in young infants and thus it is not true that speech emerges from a previously established “ingestive cyclicity”. Rather, speech and nonspeech oral behaviors involve distinct coordinative structures that develop along divergent but parallel paths. Clinical research is now emerging on the foundation of this basic research with some small sample studies showing that nonspeech oral motor exercises are not efficacious(http://www.uwo.ca/fhs/csd/ebp/reviews/2011-12/Peter.pdf).

Now, back to Science Daily. The thing is that MacNeilage (1998) also proposed that “an evolutionary route from ingestive cyclicities to speech is suggested by the existence of a putative intermediate form present in many other higher primates, namely, visuofacial communicative cyclicities such as lipsmacks, tonguesmacks, and teeth chatters.” The hypothesis of these intermediate forms must explain why the adherents of this theory are not at all concerned about a decade of research showing that speech and chewing in humans are not functionally or developmentally related in any fashion. In fact, the study trumpeted by Science Daily makes the point that the functional coordination of vocal tract structures is distinct during chewing versus lip smacks. Furthermore this research team claims that chewing and lip smacks develop along divergent paths in the monkey, with chewing achieving a slow stable rhythm at a young age whereas lipsmacks require a longer period to achieve stability at a faster rhythm. Notwithstanding the whole “ontogeny recapitulates phylogeny” thing, this is taken as evidence for the frame-content theory because speech in the infant shows a similar developmental trajectory, beginning with a slow a variable rhythm and finishing with a fast and stable rhythm. The fact that silent jaw wags, proposed by MacNeilage as a human equivalent of lipsmacks, are actually slow and not fast, doesn’t seem to bother them. In terms of clinical implications, the fact remains that the coordinative structures for communicative and ingestive behaviors develop along divergent paths in monkeys and in humans (for further evidence see Shephard et al. http://www.jneurosci.org/content/32/18/6105.abstract). Practice in one domain does not generalize to the achievement of motor control in the other domain.

I must admit that I found MacNeilage’s argument hard to follow the first time – it is even less clear now. But as I say, speculating on events that occurred two to six million years ago is a game best left to those who play it often. For myself, my concern is for children whose speech therapists believe unwisely that chewing (or lipsmacking) is a prerequisite for speech development. The notion that some level of oral-motor maturation is required for speech therapy is persistent and leads to two harmful practices – waiting too long to implement therapy to improve speech production accuracy or preceding speech therapy with useless exercises directed at jaw stability, tongue strength and the like. Throughout our book Françoise and I stress that “maturation of articulatory and neurophysiological structures and developmental changes in sensory feedback systems are not the key explanatory factors in speech development.” Rather than viewing structure as a limit on function, we believe that it is the child’s drive to function like other members of the human community that motivates practice, and practice itself causes the development of speech motor control.

What’s in a name?

Françoise and I are starting to get feedback on our book and we are beginning to hear that in the Québec community we are considered to be somewhat “out there” (in the words of one reader, “those two think differently”). Having spent 928 pages expounding on the many ways in which we think differently on the topic of developmental phonological disorders we are pleased that readers are noticing this. But we are surprised to find that the evidence of our “different thinking” is apparently in the title: readers are suspicious of the content of the book because our use of the use of the term “developmental phonological disorders” diverges from ASHA’s term “speech sound disorders”. Given that the preferred descriptor for children with inaccurate or unintelligible speech in Québec is “troubles phonologiques” this is somewhat puzzling but to be sure there are a myriad of terms in use and plenty of room for confusion. In fact we do not eschew the term “speech sound disorder” but see it as a broader cover term that encompasses all of the categories of speech problems that may be observed in children while “developmental phonological disorder” is a more specific subcategory described in the Speech Disorders Classification System (Shriberg et al., 1997).

Nonexaustive list of historical and current terms used to describe children who have poor speech accuracy (all combinations of items across the three columns are possible)









Speech sound








In reference to the 1997 Speech Disorders Classification System the term Developmental Phonological Disorder refers to children with Speech Delay (persisting substitution and deletion errors in children younger than 9 years of age), or Developmental Apraxia of Speech, or Residual Speech Errors (an update to this nosological framework is described in Shriberg et al. 2010 and in our book). Nondevelopmental Speech Disorders and Speech Differences are excluded from the category. I retain the term Developmental Phonological Disorders from the 1997 framework because, as we demonstrate in our book, children in all these categories (Speech Delay, Apraxia, Residual Errors) face developmental challenges in the acquisition and integration of knowledge at the acoustic-phonetic, articulatory-phonetic and phonological levels of representation. Furthermore and most importantly, a majority across all three subcategories have specific underlying problems with phonological processing.

Speaking for myself, I am profoundly uninterested in any arguments about what to call this particular population of children. I would not criticize anyone else’s choice anymore than I would argue about whether the four footed friend currently sleeping on the dining table should be called “that darn cat” or “le maudit chat”. Ultimately, speech-language pathologists (or speech therapists or orthophistes) are constrained by local rules and customs. In the U.S. the terms “developmental” and “delay” are both avoided because the insurance companies apparently won’t support the treatment of developmental delays regardless of whether this makes any sense at all. Some battles are not worth fighting. We can let the insurance companies tell us what to call things but we shouldn’t let insurance companies (or psychologists or psychiatrists or neurologists or anyone else) tell our profession how to think about the children that we treat. It is important to always be clear about the characteristics of the children to whom we are referring. For example, it is quite common for reviewers to suggest that my research not be funded because, after all, speech delay is not really important given that children with speech delay have good outcomes unlike children with language impairment. This is a misreading of the long term follow-up studies in which the sub-population with “speech disorder” is usually defined to cover residual errors, voice and fluency cases. In those same studies, the selection of cases called “language impaired” will include children with concomitant speech sound errors or a past history of speech delay.  And as Anne Tyler has shown in her lab and we have shown in ours, children with poor speech intelligibility inevitably have difficulties with finite verb morphology, long considered to be a primary marker for specific language impairment. In other words, speech sound disorders cannot be less important than language impairment because many times we are talking about the same kids with the same underlying neurodevelopmental disorder of phonological processing.


Beitchman, J. H., Wilson, B., Brownlie, E. B., Walters, H., & Lancee, W. (1996). Long-term consistency in speech/language profiles: I. Developmental and academic outcomes. Journal of the American Academy of Child and Adolescent Psychiatry, 35. 804-814. http://www.jaacap.com/article/S0890-8567(09)60818-2/abstract

Haskill, A. M., & Tyler, A. A. (2007). A comparison of linguistic profiles in subgroups of children with specific langauge impairment. American Journal of Speech-Language Pathology, 16, 209-221. http://www.ncbi.nlm.nih.gov/pubmed/17666547?dopt=Abstract

Mortimer, J., & Rvachew, S. (2010). A longitudinal investigation of morpho-syntax in children with Speech Sound Disorders. Journal of Communication Disorders, 43, 61-76. http://www.sciencedirect.com/science/article/pii/S0021992409000823

Shriberg, L. D., Austin, D., Lewis, B. A., McSweeny, J. L., & Wilson, D. L. (1997). The Speech Disorders Classification System (SDCS): Extensions and lifespan reference data. Journal of Speech, Language, and Hearing Research, 40(4), 723-740. http://www.ncbi.nlm.nih.gov/pubmed/9263939?dopt=Abstract

Shriberg, L. D., Fourakis, M., Hall, S. D., Karlsson, H. B., Lohmeier, H. L., McSweeny, J. L., et al. (2010). Extensions to the Speech Disorders Classification System (SDSC). Clinical Linguistics & Phonetics, 24, 795-824.http://informahealthcare.com/doi/full/10.3109/02699206.2010.503006

Welcome to Developmental Phonological Disorders Blog

Welcome to my blog which is intended to contain two kinds of posts:  occasional postings on topics related to phonological development and disorders or any topics that seem of interest to me and relevance to the speech-language pathology community; other posts, categorized as “teaching”,  will be specifically related to the book “Rvachew, S. & Brosseau-Lapre, F. (2012). Developmental Phonological Disorders: Foundations of Clinical Practice. Plural Publishing.”; these posts will be a sort of diary of my experiences teaching from the book for the first time in the coming academic year – I invite other instructors to comment and engage with me on the topic of teaching students about phonological (or speech sound) disorders in this space. A seperate page will contain a running list of errors in the book as we find them as well as downloads of figures and other resources that can be used in teaching.