[MUSIC PLAYING] NARRATOR: Infants are born with the flexibility to learn any language. The obscure Indian dialect spoken here is helping psychologist Janet Werker discover when babies lose that potential, when, in fact, they begin to master a single language. 

JANET WERKER (VOICEOVER): The Thompson, or [NON-ENGLISH], language is useful to me because it has a set of consonants that are not used in English and that English speakers cannot discriminate between. 

- [NON-ENGLISH SPEECH] 

NARRATOR: Werker records these consonant sounds to pinpoint the critical period of development, the point when babies exposed to English can no longer discriminate among Indian sounds. 

- So first off, what I would like you to do is help me think of some words that are used in the Thompson language that differ in only a single sound. 

- Like [NON-ENGLISH] and [NON-ENGLISH]. 

JANET WERKER: Oh boy, those sound the same to me. Now, can you say those again? 

- [NON-ENGLISH], that's frying. 

- OK. 

- And [NON-ENGLISH], to secure, maybe secure your pack or whatever. 

JANET WERKER: OK. Can you say this word, this one I've written down here, which means to fry? And then pronounce the first consonant and vowel three times very slowly. 

- [NON-ENGLISH]. 

- [NON-ENGLISH] And then the other word is [NON-ENGLISH]. That's to secure. [NON-ENGLISH]. 

MAN: [NON-ENGLISH]. 

[BELLS RINGING] 

JANET WERKER (VOICEOVER): We teach the baby to turn her head when the sound changes. In testing, we can then tell if the baby can hear the sound change because she turns her head in anticipation of the toys coming on. The toys then serve as a reward for correct head turn. 

NARRATOR: This 8-month-old baby has no trouble anticipating the appearance of the toys. She turns when she hears the sound change. This baby is a year old. For him, the difference in sounds no longer exists. He only turns after the toys have appeared. 

JANET WERKER (VOICEOVER): There's a reorganization in speech perception across the first year of life so that the young infant is born ready to learn any human language and experience then functions to narrow that universal set of abilities, that universal discriminatory ability to be specific to the particular language that the baby is learning. 

- What do you say? [NON-ENGLISH] 

NARRATOR: Exposure to the environment commits some neurons to forming useful networks. We become better at one language as we lose the ability to perceive others.