Developmental differences in infant attention to the spectral properties of infant-directed speech.

Across several independent studies, infants from a few days to 9 months of age have shown preferences for infant-directed (ID) over adult-directed (AD) speech. Moreover, 4-month-olds have been shown to prefer sine-wave analogs of the fundamental frequency of ID speech, suggesting that exaggerated pitch contours are prepotent stimuli for infants. The possibility of similar preferences by 1-month-olds was examined in a series of experiments, using a fixation-based preference procedure. Results from the first 2 experiments showed that 1-month-olds did not prefer the lower-frequency pitch characteristics of ID speech, even though 1-month-olds were able to discriminate low-pass filtered ID and AD speech. Since low-pass filtering may have distorted the fundamental frequency characteristics of ID speech, 1-month-olds were also tested with sine-wave analogs of the fundamental frequencies of the ID utterances. Infants in this third experiment also showed no preference for ID pitch contours. In the fourth experiment, 1-month-olds preferred a natural recording of ID speech over a version which preserved only its lower frequency prosodic features. From these results, it is argued that, although young infants are similar to older infants in their attraction to ID speech, their preferences depend on a wider range of acoustic features (e.g., spectral structure). It is suggested that exaggerated pitch contours which characterize ID speech may become salient communicative signals for infants through language-rich, interactive experiences with caretakers and increased perceptual acuity over the first months after birth.