This project examines vowel perception and plasticity during second-language (L2) learning by adults. The study will evaluate whether individuals learn to 'perceptually switch' between their L1 (first-language) and L2 vowel systems, and assess the role of fine-grained phonetic detail in the learning process. Study 1 will use a new method to generate phonetically detailed L1 (first-language) and L2 perceptual vowel maps for native speakers of German, Spanish, and French. Study 2 will train L2 learners to identify English vowels and examine how their L1 and L2 vowel spaces change over time. Study 3 will develop a new phonetic training technique to specifically promote the learning of fine-grained phonetic information. The research will substantially contribute to our scientific understanding of phonetic perception and plasticity, introduce methodological innovations, and help guide the development of new computer-based phonetic training methods.