This thesis presents thirteen research papers published between 1987-97, and a summary and discussion of their contribution to the field of developmental speech disorders. The publications collectively constitute a body of work with two overarching themes. The first is methodological: all the publications report articulatory data relating to tongue movements recorded using the instrumental technique of electropalatography (EPG). The second is the clinical orientation of the research: the EPG data are interpreted throughout for the purpose of informing the theory and practice of speech pathology. The majority of the publications are original, experimental studies of lingual articulation in children with developmental speech disorders. At the same time the publications cover a broad range of theoretical and clinical issues relating to lingual articulation including: articulation in normal speakers, the clinical applications of EPG, data analysis procedures, articulation in second language learners, and the effect of oral surgery on articulation.
The contribution of the publications to the field of developmental speech disorders of unknown origin, also known as phonological impairment or functional articulation disorder, is summarised and discussed. In total, EPG data from fourteen children are reported. The collective results from the publications do not support the cognitive/linguistic explanation of developmental speech disorders. Instead, the EPG findings are marshalled to build the case that specific deficits in speech motor control can account for many of the diverse speech error characteristics identified by perceptual analysis in previous studies.
Some of the children studied had speech motor deficits that were relatively discrete, involving, for example, an apparently isolated difficulty with tongue tiplblade groove formation for sibilant targets. Articulatory difficulties of the 'discrete' or specific type are consistent with traditional views of functional lingual articulation in developmental speech disorders articulation disorder. EPG studies of tongue control in normal adults provided insights into a different type of speech motor control deficit observed in the speech of many of the children studied. Unlike the children with discrete articulatory difficulties, others produced abnormal EPG patterns for a wide range of lingual targets. These abnormal gestures were characterised by broad, undifferentiated tongue-palate contact, accompanied by variable approach and release phases. These 'widespread', undifferentiated gestures are interpreted as constituting a previously undescribed form of speech motor deficit, resulting from a difficulty in controlling the tongue tip/blade system independently of the tongue body. Undifferentiated gestures were found to result in variable percepts depending on the target and the timing of the particular gesture, and may manifest as perceptually acceptable productions, phonological substitutions or phonetic distortions.
It is suggested that discrete and widespread speech motor deficits reflect different stages along a developmental or severity continuum, rather than distinct subgroups with different underlying deficits. The children studied all manifested speech motor control deficits of varying degrees along this continuum. It is argued that it is the unique anatomical properties of the tongue, combined with the high level of spatial and temporal accuracy required for tongue tiplblade and tongue body co-ordination, that put lingual control specifically at risk in young children. The EPG findings question the validity of assumptions made about the presence/absence of speech motor control deficits, when such assumptions are based entirely on non-instrumental assessment procedures.
A novel account of the sequence of acquisition of alveolar stop articulation in children with normal speech development is proposed, based on the EPG data from the children with developmental speech disorders. It is suggested that broad, undifferentiated gestures may occur in young normal children, and that adult-like lingual control develops gradually through the processes of differentiation and integration. Finally, the EPG fmdings are discussed in relation to two recent theoretical frameworks, that of psycho linguistic models and a dynamic systems approach to speech acquisition
