Infants' speech perception is characterized by substantial changes during the first year of life that attune the processing mechanisms to the specific properties of the ambient language. This paper focuses on these developmental changes in vowel perception. More specifically, the emergence and potential cause of perceptual asymmetries in vowel perception are investigated by an experimental study on German 6- and 9-month-olds' discrimination of a vowel contrast that is not phonemic in German. Results show discrimination without any asymmetry in the 6-month-olds but an asymmetrical pattern with better performance when the vowel changes from the less focal to the more focal vowel than vice versa by the 9-month-olds. The results concerning the asymmetries are compatible with the Natural Referent Framework as well as with the Native Language Magnet model. Our results foster two main conclusions. First, bi-directional testing must be mandatory when testing vowel perception. Second, when testing non-native vowel perception, the relation of the stimuli to the native language vowel system has to be considered very carefully as this system impacts the perception of non-native vowels.
Keywords: Vowel perception; asymmetry; infants’ speech perception; perceptual reorganization.