After the training, the infants were presented with new stimulus of either children or females producing the phonemes with either rising or falling intonation. This experiment was repeated with more difficult contrasts. Similar studies were also performed on infants using consonants. Despite these differences, speakers of a language easily classify a given consonant in one category no matter what situation it is spoken in. The infants were able to categorize these differences as well.
Newborn infants perceive the phonetic differences between the sounds of all the world's languages.
Adults however only perceive differences in speech sounds that are important in the language that they speak. Speech production mirrors this pattern. All infants babble essentially the same sounds, while adults have a difficult time producing sounds that are not a part of their language and thus have an accent. These limits on perception and production develop at a relatively young age. When children reach 1 year, they no longer distinguish all of the phonetic variation that they once heard.