Duration of early vocalizations

The duration of three infants' vocalisations were examined during a six-month longitudinal study. In contrast to most other infant research, this study included in its analysis all vocalisations including those deemed vegetative or those having nonmodal voice quality. All three infants produced vocalisations which decreased in duration in the initial months. However between the 3rd and 5th month a significant increase (p<0.001) in the duration of vocalisations was found (from mean 207ms to 431ms). When vocalisations were analysed using perceptual voice quality categories, all were found to have significant differences (p<0.05) in duration relative to modal voice. Nonmodal voice qualities showed initial decreases in duration before increasing in duration in later months. In contrast those vocalisations produced using modal voice showed a positive linear trend and had the greatest linear rate of change across the study. These findings highlight the importance of including a wide variety of infant vocalisations including those with nonmodal voice quality in infant linguistic developmental studies.