Slow speech enhances younger but not older infants' perception of vocal emotion

Infants attend more to infant-directed speech (IDS) than to adult-directed speech (ADS), but infants also prefer speech judged to be high in positive emotion over less emotional speech regardless of whether it is IDS or ADS. Emotion in voices is often conveyed by absolute pitch, pitch contours, and tempo (or duration). The purpose of our study was to explore how perceived emotion in speech is enhanced or attenuated by duration. We tested 18- and 32-week-old infants for attention to IDS that was either high or low in emotion (as judged by adults) and at two different durations (normal vs. slow). The results showed that 18-week-olds attended more to slow IDS (with affect constant), attended more to high affect (with duration constant), and showed equal attention when affect and duration were juxtaposed. In contrast, 32-week-olds showed greater attention to normal IDS regardless of its emotional level. Slower IDS may enhance younger infants' perception of vocal emotion but does not increase attention in older infants perhaps because they no longer rely on this acoustic cue for emotion. We suggest future studies to help tease apart these interpretations