After learning about speech perception in class, I was curious about infants' ability to detect phonemes better than older children and adults. I found an article called, "Babies and Phonemes Filtering," that discusses Jusczyk’s Head Turn Experiment, which explains why babies are able to discriminate between the different sound levels and duration, different phonemes and constants of all the languages they are exposed to and why this ability is lost by 12 months as they can then only discriminate phonemes of their native language.
Jusczyk tested two groups of American babies, one group aged 6 months and the other aged 9 months. After recording how long the babies looked at either the right or the left speaker when listening to a word list in either English or Dutch, the results showed that the 9-month-old babies preferred the English list and the 6-month-old babies had no preference. The same word lists were then presented to the babies with only the low frequencies; this resulted in both groups showing no preference. Through this, Jusczyk found that the 9-month-old babies were aware of the phonemes in their own language as they start to use cues to discriminate individual speech sounds of their language.
It is clear that there is a major difference in the auditory sensory abilities of a 6 month old and a 12 month old. These differences include being able to discriminate different prosodic patterns in different languages and being able to differentiate the different phonemes. Children reaching one year of age become better at differentiating phonemes in their native language, but become poor at other languages.
link to article: https://voxy.com/blog/2012/05/babies-phoneme-filtering/