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.