Elsevier/North-Holland Biomedical Press ELECTROPHYSIOLOGICAL STUDY OF TH E MA TU RA TIO N OF AUDITORY RESPONSES FROM THE IN N ER EAR OF THE CHICK

Three electrophysiological functions of the chick basilar papilla were studied during development by recording the compound action potential (AP) at the round window. The autidory thresholds showed a continuous maturation between the fifteenth day of incubation (E 15) and the first post-hatching day (P 1), when they attained adult values. Responses matured first to low frequencies and later to high frequency stimuli. The input-output (intensity-amplitude) functions matured regularly and never demonstrated the classical two slopes seen in mammals. The tuning properties, studied by tone-on-tone masking of the AP, achieved mature values before the thresholds: the Q10s reached adult values at El7 for a 500-Hz probe tone and at El9 for a 1000-Hz probe tone. The fact that a low-to-high frequency developmental trend was found with the embryonic middle ear cleared of fluids further suggests that this property of auditory ontogeny may be a function of changes in the transduction properties of the cochlea.