DESCRIPTION: The broad long term goal of the proposed research is to better understand how the ear and hearing develop normally. Understanding the ontogeny of audition and mechanisms controlling it will help provide the basis for identifying factors that can affect development and produce abnormal growth and function. The Specific Aim of the proposed work is to elucidate the ontogeny of the cochlear frequency-place code and frequency selectivity patterns from the time of their emergence to seven days post-hatch. The work will be conducted in the context of testing the following working hypotheses in the chicken (Gallus domesticus): Hypothesis 1) The cochlear apex responds to sound first during ontogeny whereas the base begins to respond later. The cochlear frequency-place code and frequency selectivity mature first in apical regions whereas these features mature last in basal regions of the cochlea. Hypothesis 2) The cochlear tonotopic map is stable throughout development in all regions (i.e. frequency place code does not shift), but it is not complete prior to hatching (i.e. does not represent all frequencies of the adult). Corollary A) Maturation of cochlear frequency selectivity and the frequency-place code is accompanied by changes in CF thresholds, not in shifting CFs. Hypotheses 1 and 2 will be tested directly by measuring the cochlear tonotopic maps for the hatchling at P7 and for embryonic groups at ages E11-E20, E15-17 and E12-14. Spontaneous activity and frequency tuning curves for individual primary afferents will be measured. Cochlear sensitivity, and frequency selectivity will be expressed in terms of CFs, threshold at the CF, slopes of the tuning curve flanks and Q10dB. Impaled neurons will be labeled with biocytin-BRP conjugates and their dendritic terminals in the papilla identified. The longitudinal as well as transverse position of terminals on hair cells in the papilla will be measured.