Enhanced bimodal distributions facilitate the learning of second language vowels

This study addresses the questions of whether listening to a bimodal distribution of vowels improves adult learners' categorization of a difficult L2 vowel contrast and whether enhancing the acoustic differences between the vowels in the distribution yields better categorization performance. Spanish learners of Dutch were trained on a natural bimodal or an enhanced bimodal distribution of the Dutch vowels /a/ and /a:/, with the average productions of the vowels or more extreme values as the endpoints respectively. Categorization improved for learners who listened to the enhanced distribution, which suggests that adults profit from input with properties similar to infant-directed speech.