The goal of our research is identify the processes by which human infants with no prior linguistic knowledge and highly limited cognitive skills acquire the ability to understand and manipulate highly complex language systems in a short time and without explicit instruction. The talk will present results from studies that investigated how Japanese infants learn certain characteristics of Japanese phonology, knowledge of which is considered prerequisite for the acquisition of abstract, symbolic properties of language. One distinctive characteristics of Japanese phonology, for example, is duration-based vowel distinction, which can be used for both lexical differentiation (obasan vs obaasan) and for phrasal/prosodic differentiation (dakara vs dakaraaa). How do babies learn that lexical and prosodic information systems are different, and how do they determine whether a given long or short vowel is being used lexically or prosodically? Our studies compare babies' behavioral responses with speech input provided by their environment, computational acquisition models, and brain imaging studies. The talk will discuss results from these and related studies, highlighting the unique opportunities that Japanese language properties provide to disentangle fundamental questions pertaining to acquisition.