This has been revealed by recent research by the University of Washington, where the brain was observed (non-invasive magnetoencephalography). Each time a baby hears speech (syllables), regions that are used for listening and motor speech preparation are activated at the same time.
So the baby's brain is already "back to speech" after 7 months, trying to figure out which motor movements are needed. This emphasizes the importance of talking to babies at every opportunity - even if they are not yet responding.
Against this background, the supposedly incoherent babbling of babies can be seen, which is also used for language preparation. And the babies have already built up a great vocabulary in this learning phase, we learn when they actually begin to speak "verbally". This usually starts in 12. to 17. Month.