Melodic and articulatory development is delayed in deaf infants aged 2-4 months

There is a paucity of knowledge about the early vocal development of infants with profound sensorineural hearing loss. In a search for potential differences in cooing vocalisations between hearing impaired (n = 12) and healthy infants (n = 12) aged 2-4 months, more than 2300 sounds were quantitatively analysed. Using objective criteria, laryngeal activity was analysed. Vocalisation melodies (time function of fundamental frequency) were recorded and grouped into simple (single arc) or complex (multiple arc) melody pattern. Occurrence of supra-laryngeal (articulatory) activity was also analysed using frequency spectrograms. Statistical analysis employed multilevel mixed-effects modified Poisson regression models, with cooing vocalisations nested within infants. Infant age, but not sex, was significantly associated with increasing complexity in both vocal activities over the observation period. Constrained auditory feedback affects both laryngeal and articulatory activity. The group of hearing impaired infants produced significantly less melodic complexity and their cooing showed fewer articulatory activity. Recognition of a delay in these early developmental processes will considerably improve our understanding of deviations in early preparatory processes for language acquisition in hearing impaired infants. Most importantly, it highlights the need to address the lack of objective, detailed studies of pre-lingual sound production in the context of increasingly earlier hearing aid provision.