The development of speech processing skills in children with and without speech difficulties.

Children with developmental speech disorder of no known aetiology constitute a heterogeneous group, both in their presenting difficulties, which can include additional language and speech perception difficulties, and in the developmental course of the disorder. This thesis examines this heterogeneity from a developmental and psycholinguistic perspective. Using a longitudinal design, speech processing and language skills are explored over three years in a group of children with speech difficulties (n=47) and an age- and nonverbal IQ-matched longitudinal control group (n=47), mean age 4;06 - 6;07. Other measures were of developmental history, family history, psychosocial status and therapy input. Key areas of investigation were: the proportion of children whose speech later resolves; uncovering the 'resolving' and 'persisting' profile; the role of input processing in speech development, in particular, the role of accent variability; and the occurrence of dissociable speech processing patterns on matched word/nonword repetition and on speech input tasks. Group characteristics were examined through an analysis of patterns of dissociation on tasks across the group and an examination of patterns of association on speech and language measures (in comparison to the control group) in order to establish the developmental relationships between different aspects of speech processing. Thus concurrent and longitudinal relationships were examined using descriptive statistics, prospective and retrospective subgroup analyses and multiple regression analyses. A 'persisting' speech profile was identified as a pervasive speech processing and language difficulty and/or more severe speech output problems. A 'resolved' profile was confined to early, moderate, specific speech difficulties. Apart from nonword repetition, there was no evidence that speech outcome was related to different rates of speech or language development. Using evidence from normal and atypical development, an interactive view of speech development is outlined. Despite the need to understand development as interactive, speech output performance is argued to be the main factor mediating and constraining change between the ages of 4-6 in children with speech difficulties. An emerging discrepancy between word and nonword repetition, with nonword repetition not improving at similar rates to word repetition in some children with persisting speech difficulties, is cited as additional evidence that speech output, in particular, motor programming deficit, is the core characteristic of a persisting speech disorder.