This is called the maintenance-loss model . The central assumption of this model when applied to speech perception is that the neonate is equipped with sensitivity to all possible phonemes of all languages. The role of experience is to maintain sensitivity to only those ponemes that the infants hear. Sometimes, with the appropriate lab tests, continuing adult sensitivity to nonnative phonemic distinctions can be found. In other words, evidence points to there being a decline in ability, followed by recovery.
Some nonnative contrasts are easy for adults to discriminate, even with no training at all.
Phones taht are similar to those used in the native language will be susceptible to reorganization, but phones that are completely unlike the ponetic repertoire of the language of input will not be assimilated to native-language pones and will remain discriminable. The central assumptin of this Functional Reorganization Hypothesis is that children's changing sensitivities reflect not absolute, hardware changes in the auditory system, but rather the needs of the developing linguistic community. From this view, young infants respond to any detectable phonetic variation in speech. Advantage of this is that it accounts for adult ability to discriminate, under proper conditions, phonemes from other languages. 2-5 year olds have most difficulty discriminating words that differ in only a single phoneme, which seems contrary to this approach.