Spatial tuning curves along the chick basilar papilla in normal and sound-exposed ears

Hair cell force generation does not amplify or tune vibrations within the chicken basilar papilla

Frequency tuning within the auditory papilla of most non-mammalian species is electrical, deriving from ion-channel resonance within their sensory hair cells. In contrast, tuning within the mammalian cochlea is mechanical, stemming from active mechanisms within outer hair cells that amplify the basilar membrane travelling wave. Interestingly, hair cells in the avian basilar papilla demonstrate both electrical resonance and force-generation, making it unclear which mechanism creates sharp frequency tuning. Here, we measured sound-induced vibrations within the apical half of the chicken basilar papilla in vivo and found broadly-tuned travelling waves that were not amplified. However, distortion products were found in live but not dead chickens. These findings support the idea that avian hair cells do produce force, but that their effects on vibration are small and do not sharpen tuning. Therefore, frequency tuning within the apical avian basilar papilla is not mechanical, and likely derives from hair cell electrical resonance.

The avian auditory papilla has many similarities to the mammalian cochlea but whether force generation by hair cells amplifies the travelling wave, as it does in mammals, remains unknown. Here the authors show that the chicken basilar papilla does not have a ‘cochlear amplifier' and that sharp frequency tuning does not derive from mechanical vibrations.

The effects of intense sound exposure on phase locking in the chick (Gallus domesticus) cochlear nerve

Little is known about changes that occur to phase locking in the auditory nerve following exposure to intense and damaging levels of sound. The present study evaluated synchronization in the discharge patterns of cochlear nerve units collected from two groups of young chicks (Gallus domesticus), one shortly after removal from an exposure to a 120-dB, 900-Hz pure tone for 48 h and the other from a group of non-exposed control animals. Spontaneous activity, the characteristic frequency (CF), CF threshold and a phase-locked peri-stimulus time histogram were obtained for every unit in each group. Vector strength and temporal dispersion were calculated from these peri-stimulus time histograms, and plotted against the unit's CF. All parameters of unit responses were then compared between control and exposed units. The results in exposed units revealed that CF thresholds were elevated by 30-35 dB whereas spontaneous activity declined by 24%. In both control and exposed units a high degree of synchronization was observed in the low frequencies. The level of synchronization above approximately 0.5 kHz then systematically declined. The vector strengths in units recorded shortly after removal from the exposure were identical to those seen in control chicks. The deterioration in discharge activity of exposed units, seen in CF threshold and spontaneous activity, contrasted with the total absence of any overstimulation effect on synchronization. This suggested that synchronization arises from mechanisms unscathed by the acoustic trauma induced by the exposure.

Loss and recovery of sound-evoked otoacoustic emissions in young chicks following acoustic trauma

Young and adult chickens exhibit substantial inner-ear damage and post-exposure deterioration in cochlear nerve activity following exposure to intense sound. Both the structural and functional losses largely recover in both age groups within 2-4 weeks after exposure. However, some aspects of acoustic trauma differ between the young and adult chicken ear. Overstimulation in the young chick causes considerable post-exposure loss and then recovery of the steady-state endocochlear potential, while in the adult animal there is little post-exposure effect on this potential. Moreover, in adults there is post-exposure loss but little recovery in the distortion product otoacoustic emission (DPOAE). The present study explores the possibility of an age difference in the effects of overstimulation on the DPOAE by examining these emissions in young chicks following exposure to an intense pure tone. Chicks exposed to intense sound were formed into groups at 0 and 12 days of recovery, and these were complemented by two additional groups of age-matched controls. The cubic difference tone emission (the 2f(1)-f(2) DPOAE component) was measured at 9 levels for 13 frequencies in all groups. Shortly after the exposure, the DPOAE reliably declined with the maximum loss at or above the exposure tone frequency. The exposed chicks examined 12 days after exposure showed complete recovery of the DPOAE. It would appear that 12 days of recovery sufficiently repaired inner ear damage to completely restore DPOAE production. This result is different from that in adult chicken and may be related to the greater severity of acoustic damage in the adult ear, a reduced susceptibility of the young ear to acoustic trauma, or the ability of the young animal to more successfully repair the inner ear.