Research indicates that infants lose the capacity to perceive distinctions in nonnative sounds as they become sensitive to the speech sounds of their native language (i.e., by 10- to 12-months of age). However, investigations into the decline in nonnative phonetic perception have neglected to examine the role of non-linguistic information. Exaggerated prosodic intonation and facial input are prominent in the infants' language-learning environment, and both have been shown to ease the task of speech perception. The current investigation was designed to examine the impact of infant-directed (ID) speech and facial input on infants' ability to discriminate phonemes that do not contrast in their native language. Specifically, 11-month-old infants were tested for discrimination of both a native phoneme contrast and a nonnative phoneme contrast across four conditions, including an auditory manipulation (ID speech vs. AD speech) and a visual manipulation (Face vs. Geometric Form). The results indicated that infants could discriminate the native phonemes across any of the four conditions. Furthermore, the infants could discriminate the nonnative phonemes if they had enhanced auditory and visual information available to them (i.e., if they were presented in ID speech with a synchronous facial display), and if the nonnative discrimination task was the infants' first test session. These results suggest that infants do not lose the capacity to discriminate nonnative phonemes by the end of the first postnatal year, but that they rely on certain language-relevant and non-linguistic sources of information to discriminate nonnative sounds. / Ph. D.