Early developmental changes in infants' vocal responses in interactions with caregivers.

Objectives: The study aimed to explore the developmental trajectory of infants' vocal responses and the temporal characteristics of vocal interactions between infants and caregivers in natural home environments, focusing specifically on Korean infants aged 3, 6, 9, and 12 months.

Methods: One-day home recordings were collected using a LENA recorder at ages 3, 6, 9, and 12 months for each infant. In-depth analyses of the LENA recordings were conducted on 20 5-minute segments that showed the highest conversational turn counts, as determined by the LENA automated analysis. The temporal characteristics of infant vocalizations during interactions with caregivers were examined through infant vocal response, vocal response latency, and overlapping vocalizations.

Results: The results indicated that the proportion of infant vocal responses following adult speech differed across age groups. Specifically, the highest proportion was observed at 3 months, with a noticeable decrease at 6 and 9 months, and a slight increase at 12 months. Infant vocal response latency increased with age, indicating longer response delays as infants got older. The proportion of overlapping vocalizations was consistent among 3, 6, and 9-month-olds but decreased significantly at 12 months.

Conclusion: These findings highlight distinct changes in infant vocal interactions during the first year of life. Although the analysis was cross-sectional, the observed patterns suggest developmental changes in the timing and structure of vocal response as infants age. This study indicates that infants adapt their vocal interactions in response to caregivers' speech, which may reflect a gradual learning of conversational coordination.