Speech errors commonly observed in children with cleft lip and palate are posterior placement of oral targets, Gabreels (2001). Several authors have offered explanations by showing physiological evidence of velopharyngeal incompetence. But most recently there has been growing interest in phonological explanations of speech disorder in this population. It has been viewed that “marking the locus of phoneme boundary” is one of the ways to describe relations between acoustic variations and phonetic perception. VOT voice onset continuum is one of the strongest phonetic cues for consonant discrimination and refers to the time interval between the release of the articulatory occlusion and the onset of vocal-fold vibration.
The aim of the present study was to measure the VOT continuum and mark the occurrence of phoneme boundary in speech of children with cleft lip and palate. The study aimed to compare the locus of phoneme boundary between two cleft lip and palate group and control group.
Speech sample was obtained from two groups. Group 1 - individuals with cleft lip and palate who exhibited error in placement of oral targets, Group 2 – typically developing children who were age and gender matched. Further the samples collected were synthesised using VOT continuum technique for voiced and voiceless stop consonants namely bilabial sounds (/p/, /b/), palatal sounds (/t/, /d/) and velar sounds (/k/, /g/). VOT truncation at +/- 10msec, +/- 30 msec , +/- 50msec were done. The synthesized stimulus was later presented to 15 Speech Language Pathologist and untrained listeners. The participants had to indicate the percentage of phoneme crossing. According to Repp and Liberman (1987), phoneme locus can be identified at 50% of zero crossing line. Using this approach, the group of phonemic boundaries were calculated and subjected to statistical analysis.
The results indicated that trained listeners could identify the boundary at 90% for CLP anterior placement and 95% of stimuli for typically developing group. But however, the phoneme boundary varied to 75% for posterior placement of speech in CLP indicating the phoneme boundary is divergent in CLP group. The present study suggests that marking the phonetic variations in cleft palate speech can aid in better understanding of speech perception cues in cleft lip and palate individuals.