The relation between index-finger extension and the acoustic quality of cooing in three-month-old infants.

Fourteen full-term, healthy, three-month-old infants were observed during a total of 15 minutes spontaneous face-to-face interaction with their mothers. Facial and manual actions, gaze direction and vocalizations were coded. The infants' cooing vocalizations were categorized into syllabic and vocalic sounds. Index-finger extension occurred frequently in sequence with syllabic sounds, which are speech-like vocalizations, but rarely occurred in sequence with vocalic sounds. No other categories of nonvocal behaviours showed such a relationship. In a subsequent experiment, the infants experienced either conversational turn-taking or random responsiveness from their mothers. In the turn-taking condition, the infants produced a higher ratio of syllabic to vocalic sounds, and a higher frequency of index-finger extension. These results suggest a strong connection between speech and the pointing gesture long before the infant can actually talk.