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.