Contributions of the middle ear to the development of function in the cochlea.

The contribution of middle ear immaturities to the development of cochlear microphonic potential (CM) responses was studied throughout the ontogeny of auditory function in the Mongolian gerbil. CM produced by direct mechanical stimulation of the stapes was compared with CM generated by acoustic stimulation of the intact ear at various postnatal ages. The results indicated that during development acoustically generated CM reflects middle ear as well as inner ear maturational factors. With direct stapes driving, CM was first elicited at 10 days after birth (DAB), two days earlier than with acoustic stimulation. Controlling for the contributions of middle ear immaturity, maturation of the inner ear accounted for approximately a 75 dB improvement in CM thresholds between 10 and 18 DAB. Comparisons between the results obtained with the acoustic and stapes driving stimulation protocols suggested that the major maturational changes in the middle ear conduction apparatus occurred between 14 and 16 DAB. This period was associated with the final stages of resorption of middle ear mesenchyme and ossicular ossification. Before 16 DAB, acoustically evoked CM thresholds reflect approximately a 25 dB loss in sensitivity due to middle ear immaturity.