The sequential development of jaw and lip control for speech

Abstract

Vertical displacements of the upper lip, lower lip, and jaw during speech were recorded for groups of 1-, 2-, and 6-year-olds and adults to examine if control over these articulators develops sequentially. All movement traces were amplitudeand time-normalized. The developmental course of upper lip, lower lip, and jaw control was examined by quantifying age-related changes in the similarity of each articulator's movement patterns to those produced by adult subjects and by same-age peers. In addition, differences in token-to-token stability of articulatory movement were assessed among the different age groups. The experimental findings revealed that 1- and 2-year-old children's jaw movements were significantly more adult-like than their upper and lower lip movements, which were more variable. In contrast, upper and lower lip movement patterns became more adult-like with maturation. These findings suggest that the earliest stages of speech motor development are constrained by the nonuniform development of articulatory control, with the jaw preceding the lips. The observed developmental patterns suggest that the properties of the oral motor control system significantly influence the pattern of speech sound acquisition.

Figure 1

A 2-year-old subject fitted with calibration and movement markers for upper lip, lower lip, and jaw.

Figure 2

The treated kinematic traces from upper lip (UL), lower lip (LL), and jaw (J) produced by an adult subject saying baba. For ease of interpretation, each signal has been centered about its mean and the UL signal has been inverted.

Figure 3

The left panel displays 10 kinematic records based on 10 repetitions of baba for the upper lip, lower lip, and jaw. The right panel shows alignment of the signals following the spatial and temporal normalization procedure. The high degree of token-to-token stability exhibited by this subject was typical for adult subjects.

Figure 4

In the across-age comparison, each subject's kinematic traces were correlated with an average trajectory computed on the adult subjects' traces. Data points in this figure represent the average coefficient values across subjects within each age group. All coefficients were transformed using the Fisher's z before averaging and analysis. In this figure, the inverse transform was applied to each average for display purposes. The inset displays the kinematic traces from a 1-year-old subject for the upper lip (UL), lower lip (LL), and jaw (J) and the average adult trajectory that each trace was correlated with. A separate average trajectory was computed for each adult subject by removing his or her data from the computation. Error bars represent standard error among subjects in each age group.

Figure 5

In the within-age comparison, each subject's kinematic traces were correlated with an average trajectory computed on the traces produced by subjects in the same age group. Data points in this figure represent the average coefficient values across subjects within each age group. All coefficients were transformed using the Fisher's z before averaging and analysis. In this figure, the inverse transform was applied to each average for display purposes. The figure inset displays the kinematic traces from a 2-year-old subject and the average 2-year-old trajectory that the traces were correlated with. A separate average trajectory was computed for each subject by removing his or her data from the computation. Error bars represent standard error among subjects in each age group.

Figure 6

In the within-subject comparison, each subject's kinematic traces were correlated with an average trajectory that was based on his or her individual traces. Data points in this figure represent the average coefficient values across subjects within each age group. All coefficients were transformed using the Fisher's z before averaging and analysis. In this figure, the inverse transform was applied to each average for display purposes. The figure inset displays the kinematic traces produced by a 2-year-old subject and the computed average that the traces were correlated with. Error bars represent standard error among subjects in each age group.