Development of the cochlear amplifier.

The development of the cochlear amplifier was examined in gerbils aged 14 days after birth (dab) to adult, for stimulus frequencies from 1 to 48 kHz. Distortion product otoacoustic emissions (DPOAEs) were employed to determine the characteristics of active emissions associated with cochlear amplifier operation. DPOAEs were also used to determine the characteristics of "passive" emissions remaining when the cochlear amplifier operation was interrupted by acute furosmide intoxication. The input-output functions of the passive emissions, and of the active emissions at low stimulus levels, could be approximated by parallel straight lines. The horizontal distance between these parallel lines, i.e., the increase in stimulus level required to obtain a passive emission amplitude equal to the active emission, is an estimate of the gain of the cochlear amplifier. Further, the lowest stimulus level at which active and passive emissions become approximately equal defines a passive threshold level. At 14 dab, the cochlear amplifier gain was already at adult levels for the midfrequencies (4-8 kHz), but no emissions were detected at the extremes (at 1 kHz, and 24 kHz and above). During the period over which the endocochlear potential (EP) is known to increase most sharply (14 to 18 dab), the gain at all frequencies increased. At low frequencies there was little or no gain in the youngest age group, but matured by 23 dab. The gain at the middle frequencies subsequently decreased, resulting by 30 dab in a gain that was remarkably flat across frequency from 1 to 32 kHz. The passive thresholds generally improved with age at all frequencies, but most dramatically at the high frequencies. Results are consistent with the view that the elements of the cochlear amplifier are functional in the base of the cochlea at all ages, but that auditory function is primarily limited by the lower passive base cutoff frequency at younger ages. The proposed increase in passive base cutoff frequency with age accounts for the known place code shift.