For example, when babies born to native-English-speaking parents played three times a week during that window with a native-Mandarin-speaking tutor, at 12 months, they had progressed in their ability to recognize both English and Mandarin sounds, rather than starting to retrench in the non-native language. By contrast, children exposed only to audio or video recordings of native speakers showed no change in their language trajectory. Brain-imaging of the same children backed up the results of test-based measures of language specialization.