Plasticity in adult and child speech production

This study examined the ability of nine adult and nine children (aged 6–9 years) to speak with thin (0.5 mm) experimental prostheses in the mouth. One prosthesis conformed closely to the upper teeth and hard palate; the other substantially altered oral cavity geometry by reducing by one-half the volume under the palatal vault. Degree of adaptation was estimated by having listeners rate the adequacy of words (pea, tea, sea, she) produced in two lists read before insertion of the prostheses (baseline), immediately after insertion, 4 and 12 min after insertion, and twice after removal. The children were less able than adults to adapt to modest vocal tract changes, but more able to adapt to changes requiring reparameterization that went beyond the normal bounds of articulatory variation. Ratings of the children's but not the adults' production of sea decreased significantly compared to baseline immediately after insertion of the conforming prosthesis. However, the children showed somewhat greater success in adapting to the cavity-altering prosthesis, the presence of which caused a sharp drop in listeners' ratings. Their sea, but not the adults', received significantly higher ratings when produced 12 min than immediately after insertion. These findings suggest that the developing coordinative structures of children are less flexible but more plastic than those of adults. [Work supported by NIH grant NS20572.]