Long before babies begin to learn specific words, they engage in an important sound-sorting process that enables them to distinguish the sounds they hear in words. For example, by 6 months, most babies are able to categorize vowel sounds such as the difference between “ee” and “ah.” This understanding is one of the essential first steps in a baby’s language acquisition.
Gauthier, B., Shi, R., & Xu, Y. (2007). Learning phonetic categories by tracking movements. Cognition, 103, 80
Moore, D. S., Spence, M. J., & Katz, G. S. (1997). Six month-olds