The ability to discriminate and identify people by their voice is important for social interaction in humans. In early development, learning to discriminate important differences in a number of socially relevant stimuli, such as phonemes and faces, has been shown to follow a common pattern of experience-driven perceptual narrowing, where the discrimination of native stimuli improves, while the discrimination of foreign stimuli worsens. The aim of the present study was to investigate whether similar perceptual narrowing occurs for discriminating individuals by voice. We tested the ability of English-speaking adults and English-learning 6- and 12-month-olds to discriminate either native-species (human) or foreign-species (rhesus monkey [Macaca mulatta]) individuals by their vocalizations. Between 6 and 12 months of age, the ability to discriminate monkey voices decreased significantly and there was a non-significant trend for improved ability to discriminate human voices. The results support the hypothesis of experience-driven perceptual narrowing for voice individuation during the first year after birth.

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Differentiating individuals by their voice is an important social skill for infants to acquire. In a previous study, we demonstrated that the ability to discriminate individuals by voice follows a pattern of perceptual narrowing (Friendly et al., 2013). Specifically, we found that the ability to discriminate between two foreign-species (rhesus monkey) voices decreased significantly between 6 and 12 months of age. Also during this period, there was a trend for the ability to discriminate human voices to increase. Here we investigate the extent to which plasticity remains at 12 months, after perceptual narrowing has occurred. We found that 12-month-olds who received 2 weeks of monkey-voice training were significantly better at discriminating between rhesus monkey voices than untrained 12-month-olds. Furthermore, discrimination was reinstated to a level slightly better than that of untrained 6-month-olds, suggesting that voice-processing abilities remain considerably plastic at the end of the first year.