Development is a progressive process that results in the growth and proliferation of motor, perceptual and cognitive skills. A growing body of evidence shows, however, that seemingly paradoxical regressive processes also contribute to perceptual development and to the emergence of specialization. This evidence shows that unisensory perceptual sensitivity in early infancy is so broadly tuned that young infants respond to, and discriminate, native sensory inputs (e.g. speech sounds in their own language and faces from their own species and race) as well as non-native sensory inputs (e.g. speech sounds from other languages and faces from other species and other races). In contrast, older infants only respond to native inputs. For example, younger but not older infants discriminate monkey, human, and other-race faces, native and foreign speech contrasts, and musical rhythms from different cultures. Here, we review new findings indicating that perceptual narrowing is not just a unisensory developmental process, but a general, pan-sensory one. These new data reveal that young infants can perceive non-native (monkey) faces and vocalizations as well as nonnative speech gestures and vocalizations as coherent multisensory events, and that this broad multisensory perceptual tuning is present at birth. These data also reveal that this broad tuning narrows by the end of the first year of life, leaving older infants only with the ability to perceive the multisensory coherence of native sensory inputs. Together, these findings suggest that perceptual narrowing is a pan-sensory process, force us to reconsider the traditional progressive theories of multisensory development, and open up several new evolutionary questions.
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