An investigation of the cognitive-linguistic profile of children with phonological delay and phonological disorder

© 2019 Rebecca Emma WaringAll children make errors as they learn to talk, and while the vast majority of children spontaneously attain error-free speech within a predictable timeframe, approximately 6% to 10% of children struggle to achieve intelligible speech without intervention for no apparent anatomical, physiological, or psychological reason (American Psychiatric Association, 2013). While much is now known about the types of speech sound errors these young children make (McLeod & Baker, 2017), little is known about why they make these errors. One explanation for these errors is that some children fail to deduce the correct rules governing how speech sounds combine legally to form words. This PhD thesis investigates the cognitive–linguistic profile of two widely recognised subgroups of children with speech sound disorders (SSD) of unknown origin: phonological delay, that is, children who make typical speech errors but are delayed based on the norms for their chronological age, and phonological disorder, that is, children who make atypical speech errors that are heard in less than 10% of children at any age. Over a series of five empirical studies, three core executive function processes were investigated in the same cohort of monolingual English-speaking preschool children with phonological delay, phonological disorder, and typically developing speech. Children were tested on a series of standardised and commonly employed receptive vocabulary, phonological short-term memory and phonological working memory tasks, inhibitory control tasks, and rule abstraction and cognitive (set) shift tasks. Results indicated that children with phonological delay and phonological disorder had separate and distinct cognitive–linguistic profiles. Specifically, children with phonological disorder had phonological short-term memory skills (i.e., ability to mentally ‘hold’ verbal information briefly) similar to children with typical speech development, while children with phonological delay did not. Further, both SSD subgroups demonstrated significantly reduced phonological working memory performance (i.e., ability to mentally ‘hold’ and ‘manipulate’ verbal information) compared to children with typical speech development. However, for children with phonological delay, phonological working memory was capped by their ‘holding’ performance rather than a specific difficulty mentally ‘manipulating’ verbal information, while children with phonological disorder (who have intact ‘holding’ performance) had specific difficulties mentally ‘manipulating’ verbal information. Finally, children with phonological disorder also demonstrated significant domain-general rule abstraction and cognitive (set) shift deficits compared to their peers with typical speech development and phonological delay. All children had receptive vocabularies within the typical range. These findings have both clinical relevance and implications for theories of typical and impaired phonological development. Specifically, the findings highlight (1) the importance of differential diagnosis and recognising SSD subgroups in SSD research and clinical practice, (2) the importance of understanding the role of both domain-general cognitive processes and linguistic processes to phonological acquisition and explaining SSD subtypes, (3) how and why deficits in rule abstraction and cognitive (set) shift may contribute to atypical phonological development and atypical speech errors, and (4) the need for third-generation intervention approaches, that is, innovative and tailored treatment techniques that concurrently address speech and literacy difficulties by targeting underlying deficits