DEVELOPMENT OF HEARING IN NEONATAL RATS - AIR AND BONE CONDUCTED ABR THRESHOLDS

While the human full-term neonate can hear at birth, in the rat the on set of auditory function as monitored by recording auditory nerve-brai nstem evoked responses (ABR) has been reported to begin on post-natal day (PND) 12-14 and reaches adult thresholds at about 22 days. In orde r to determine the factors involved in this late onset and then rapid threshold improvement in rats, the ABR to both air conducted (AC) and bone-conducted (BC) auditory stimulation was determined in neonatal ra ts. ABR to maximal intensity BC stimuli (55 dB above adult rat ABR thr eshold - 55 dB HL) could be recorded from PND 7-8 while AC responses to 80 dB HL stimuli, only from PND 11. The air-bone gap (a measure of conductive immaturities only) disappeared on PND 15. This shows that there are both conductive (external and middle ear -Air-bone gap) and sensori-neural (inner ear - BC threshold) immaturities in the neonatal rat; the conductive factors are resolved by PND 15 while the sensori- neural continue after that. With respect to conductive factors, it see ms that the state of the ear canal is not important while the chief co nductive factors involved probably include mesenchyme resorption and/o r ossicular ossification. The chief sensori-neural factor may be the d evelopment of the endocochlear potential, It is likely that the human fetus in-utero undergoes similar stages of development.