Patterns of variability in voice onset time: a developmental study of motor speech skills in humans.

This study investigated the developmental patterns of variability in the speech parameter voice onset time (VOT) in 46 children. Five groups of children participated in the study as follows: (i) Group 1, aged 5 years 8 months (n=6); (ii) Group 2, 7 years 10 months (n=10); (iii) Group 3, 9 years 10 months (n=10); (iv) Group 4, 11 years 10 months (n=10); and (v) Group 5, 13 years 2 months (n=10). Coefficient of variation values were examined for the VOT values of both 'voiceless' (/p t k/) and 'voiced' (/b d g/) plosives to determine patterns of variability. Significant effects of age were revealed for both the voiceless and voiced plosives, and levels of variability levelled off for Group 4. The data suggest that although variability in VOT decreases with age, the presence of residual variability may be a prerequisite for the further refinement of motor speech skills.