Theories of perceptual development differ in the extent to which initial perceptual sensitivities and language experience influence infants' perception of speech. Extant research focuses largely on infants' ability to distinguish native and non-native speech sound categories. In two experiments, we investigated infants' developing perception of relationships between similar sounds, i.e., vowel harmony patterns, to inform this debate. In Experient 1, we showed that language experience is not necessary to detect vowel harmony
4-month-olds without harmony experience can differentiate harmonic and disharmonic nonce words. We argue that this is due to a universal perceptual grouping bias, wherein similar sounds are perceived as being grouped together despite their temporal distance. Then in Experiment 2, we showed that without relevant language experience, this initial sensitivity to vowel harmony declines by 8-months as infants begin to tune into the sound patterns of their native language. We argue that our results, combined with previous findings, are best explained under perceptual attunement theories. When not reinforced by their language input, infants show a decline in their sensitivity to vowel harmony
but an initial sensitivity to relationships between similar vowels may facilitate infants' learning of vowel harmony patterns in their native language.