Although not the focus of her article, phonological development in young children with speech sound disorders of various types is highly germane to Stoel-Gammon's discussion (this issue) for at least two primary reasons. Most obvious is that typical processes and milestones of phonological development are the standards and benchmarks against which we measure disorder and delay. Factors that impact children without disorders may suggest underlying causes or co-occurring symptoms of speech sound deficits, prognostic indicators of improvement, appropriate remediation strategies or some combination of these. Equally important is the fact that studying children with disorders can help us to verify and, in some cases, even unpack relationships among factors that are so closely interwoven in children who develop their phonologies at the typically very rapid rate that their individual influences cannot be discerned. Childhood Apraxia of Speech (CAS) is a particularly interesting case in point because, while it is universally accepted to be a motor speech disorder, symptoms include deficits in speech perception and often in literacy-related skills as well.

Childhood Apraxia of Speech is a speech sound disorder the source of which is presumed to reside in the speech motor programming and planning centers of the brain. The primary speech symptoms include: ‘inconsistent errors on consonants and vowels in repeated productions of syllables or words, lengthened and disrupted coarticulatory transitions between sounds and syllables, and inappropriate prosody, especially in the realization of lexical or phrasal stress’ (ASHA, 2007: 5). The impression that the listener receives of the speech of a person with moderate to severe CAS is of effort and struggle. However, expressive language and phonological awareness symptoms are also common. These symptoms are likely to persist longer in children with CAS than in children with other speech–language delays/disorders (Lewis, Freebairn, Hansen, Iyengar & Taylor, Reference Lewis, Freebairn, Hansen, Iyengar and Taylor2004).

Stoel-Gammon provides ample evidence that production skills are related to lexical development in children who are typically developing (TD); children with simpler phonological systems have smaller lexicons and vice versa. As she notes, these findings extend to children with unusually small lexicons (late talkers) or large lexicons (precocious talkers). However, in all of these cases the underlying determinant of both the lexicon size and the maturity of the phonological system could be some shared cognitive–linguistic ability. Thus, the relationship that she posits between articulatory ability and lexicon is not rigorously supported by the studies that she cites.

However, in CAS there is a clear-cut motor disorder; both prelinguistic and older children with this syndrome vocalize less, and with less complexity, due to an inability to plan complex articulatory gestures. The fact that these children have small productive vocabularies is no proof of a relationship between speech skill and lexicon size, however, since their small production vocabularies likely result, at least in large part, from their inability to accurately and reliably produce recognizable words. There is some evidence for cognitive–linguistic as well as motor planning components of the disorder, though, in that they typically demonstrate other, higher-order linguistic deficits that depend upon fully-formed phonological representations: reduced perception and production of vowels (Maassen, Groenen & Crul, Reference Maassen, Groenen and Crul2003), syllables (Marquardt, Sussman, Snow & Jacks, Reference Marquardt, Sussman, Snow and Jacks2002), rhymes (Marion, Sussman & Marquardt, Reference Marion, Sussman and Marquardt1993) and phoneme sequences – especially in nonwords (Bridgeman & Snowling, Reference Bridgeman and Snowling1988); deficits in word attack; and difficulties with spelling that do not necessarily relate to the child's current speech errors (Snowling & Stackhouse, Reference Snowling and Stackhouse1983; Lewis et al., Reference Lewis, Freebairn, Hansen, Iyengar and Taylor2004). The nature of these deficits observed in children with CAS suggests impoverished phonemic representations (Marquardt, Jacks & Davis, Reference Marquardt, Jacks and Davis2004). Thus, based on the evidence presented by Stoel-Gammon, it may be hypothesized that the small vocabularies of children with CAS, which result from motor difficulties, have an upstream impact on the cognitive–linguistic aspects of phonology.

In addition to – or perhaps as a result of – its relationship with phonological maturity, lexicon size is also strongly related to phonological awareness (Walley, Metsala & Garlock, Reference Walley, Metsala and Garlock2003). Zaretsky, Velleman & Curro (Reference Zaretsky, Velleman and Curro2010) extended this finding to motor speech disorders in a case study of one child with severe CAS. They documented further deficits in phonological memory (as determined via non-word repetition, with scoring adjusted for her typical speech sound patterns) as well as in phonological awareness (PA), impacting the participant's ability to carry out phonological manipulation tasks (e.g. subtests of the Lindamood Auditory Conceptualization Test-3; Lindamood & Lindamood, Reference Lindamood and Lindamood2005). Despite these phonological processing difficulties, the girl was able to acquire some splinter literacy skills that made it possible for her to learn to read many real words and even sentences. Unfortunately, phonological memory testing has not been systematically carried out with this population; nor have studies of the effects of lexical frequency and neighborhood density on aspects of phonological production or processing or on lexical development been reported for speech-sound-disordered children.

There is no known connection between CAS as a motor speech disorder and these other aspects of linguistic development except the spiraling effects of decreased production hypothesized by others and summarized by Stoel-Gammon: articulatory difficulties result in fewer vocalizations, which reduce opportunities for both practice and feedback, which in turn decrease the child's skill, motivation and lexical learning. Decreased lexical learning, in turn, may impact the development of other aspects of phonology, including higher-order literacy-related skills, and possibly other components of the linguistic system as well.

The story is not as simple as the above may imply, however. There is inconclusive evidence as to whether children with other motor speech or physiologically based speech sound disorders demonstrate deficits in higher-order phonological skills. Bishop (Reference Bishop1985) reports on seven teenagers with dysarthria whose spelling skills, even of nonwords, are within normal limits. In a later meta-analysis, however, Williams & Koppenhaver (Reference Williams and Koppenhaver2008) suggest that while children who use Augmentative and Alternative Communication (AAC) devices (and who are therefore presumably non-vocal communicators) do demonstrate a positive relationship between phonological awareness and reading, they appear to have more difficulty applying PA to literacy tasks. They also struggle with spelling; interestingly, hearing someone else produce the word (which they themselves cannot do) makes a significant difference in their spelling accuracy. However, children who use AAC devices are typically physically limited, and may have smaller vocabularies resulting from reduced life experiences; this clouds the picture.

Stackhouse (Reference Stackhouse1982) found no differences between the reading, spelling and nonsense-word matching skills of ten children with cleft palate and matched typically developing peers; the ten children with CAS, who comprised the third group of participants, were impaired on all of these tasks. Rice (Reference Rice1998) also demonstrated that children with cleft palate have PA skills within normal limits for their ages, although she did report somewhat ambiguous findings about the relationship between their articulatory skills and their PA skills. Of course, as Rice points out, children with cleft palate do not necessarily vocalize less than TD children; thus, despite their distorted speech production they may still benefit from vocal practice, parental responses and lexicons that are within normal limits (confirmed within her study). Thus, it may not be the accuracy of the child's productive lexicon, but its sheer size, that has the reported impacts on PA and other higher-order phonological skills.

In children with non-physiological speech sound disorders (including CAS but not dysarthria), the factors that increase the likelihood of PA difficulties include how atypical the child's phonological patterns are (Leitao, Hogben & Fletcher, Reference Leitao, Hogben and Fletcher1997; Dodd, Reference Dodd2005), whether input (i.e. perception) as well as output phonological difficulties are present (Stackhouse & Snowling, Reference Stackhouse and Snowling1992) and whether the speech sound disorder persists to the early school years (Bishop & Adams, Reference Bishop and Adams1990; Carroll & Snowling, Reference Carroll and Snowling2004; Bird, Bishop & Freeman, Reference Bird, Bishop and Freeman1995; Nathan, Stackhouse, Goulandris & Snowling, Reference Nathan, Stackhouse, Goulandris and Snowling2004). All of these factors impact negatively on those with CAS.

Thus, in children with speech sound disorders, as well as in children who are typically developing, there remains much we do not yet know about interactions between early speech sound development and other linguistic abilities. Further research on these questions in both populations, across multiple languages, are necessary to increase our understanding of the complex relationships among articulatory, phonological, lexical, literacy and other linguistic skills. Children with CAS, a disorder with motoric origins and cognitive–linguistic consequences, will be an important population to include in such studies.