Six-month-old infants use cross-modal synchrony to identify novel communicative signals

Humans adopt non-linguistic signals, from smoke signals to Morse code, to communicate. This communicative flexibility emerges early: embedding novel sine-wave tones in a social, communicative exchange permits 6-month-olds to imbue them with communicative status, and to use them in subsequent learning. Here, to specify the mechanism(s) that undergird this capacity, we introduced infants to a novel signal-sine-wave tone sequences-in brief videotaped vignettes with non-human agents, systematically manipulating the socio-communicative cues in each vignette. Next, we asked whether infants interpreted new tone sequences as communicative in a fundamental cognitive task: object categorization. Infants successfully interpreted tones as communicative if they were produced in dialogues with one agent speaking (Study 1) or both agents producing tones (Study 2), or in monologues involving only a single agent (Study 3). What was essential was cross-modal temporal synchrony between an agent's movements and the tones: when this synchrony was disrupted (Study 4), infants failed in the subsequent task. This synchrony, we propose, licensed an inference that the tones, generated by an agent, were candidate communicative signals. Infants' early capacity to use synchrony to identify new communicative signals and recruit them in subsequent learning has implications for theory and for interventions to support infants facing communicative challenges.