The purpose of the proposed project is to characterize the function of the developing auditory system in human infants. Of specific interest is the development of frequency resolution, the ability to respond selectively to one component of a complex sound, an ability that is related to the perception of speech. The proposed experiments will employ a behavioral measure of hearing, the Observer-based Psychoacoustic Procedure developed in this laboratory. Three- to twelve-month-old infants will be tested. The project has four specific aims. (1) To investigate the sources of differences between infants and adults in frequency discrimination by examining the effects of stimulus duration and of masking on the frequency discrimination performance of infants. (2) To describe the course of development of frequency resolution between 3 and 12 months using auditory filter widths and psychophysical tuning curve paradigms. (3) To examine the development of infant temporal resolution in gap detection and temporal modulation transfer function measures. (4) To characterize the Observer-based Psychoacoustic Procedure within a signal detection theory framework and to develop techniques that will optimize the characteristics of the method.