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This study examined the effect of speaking contexts on articulatory kinematics in habitual and clear speech conditions. Ten male and 10 female participants (ages 18"“29) completed speaking tasks in three contexts and two conditions. The contexts were word, phrase, and passage, with both mid-sentence and end of-sentence stimuli in the phrase and passage contexts. The two conditions were habitual and clear speech. Participants had sensors attached to the mid-tongue, jaw, lower lip, and upper lip, and an electromagnetic articulograph tracked their movements. Three tokens for each stimulus were analyzed for duration, displacement, and velocity. Articulatory coordination was measured through absolute and percent jaw contribution, and displacement correlations. Statistical analysis revealed significant changes across both conditions and contexts. Generally, the articulator movements were larger for clear versus habitual speech and decreased progressively in size from word to phrase to passage. Duration significantly increased in the clear speech condition and decreased from word to phrase to passage, which likely underlies the changes seen in the other measures. Percent jaw contribution to lower lip movement was significantly higher in the clear speech condition, percent jaw contribution to tongue movement was significantly higher for the passage compared to the other contexts, and jaw and lower lip correlations with the tongue were higher in the clear condition and lower in the passage context. Incidental rate variation and motor equivalence across speakers limit the degree to which we can interpret these results in terms of articulatory coordination. Overall, this study demonstrates significant changes in speech kinematics across contexts in both clear and habitual conditions, indicating that researchers should exercise caution when generalizing findings from studies using short, contrived stimuli.

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.

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