Cochlear damage resulting from exposure to four different octave bands of noise at three ages.

Three age-groups of CBA/J inbred mice were exposed for 5 min to one of four octave bands of 124-dB noise. The ages corresponded to weaning (20 days), early postpuberty (60 days), and adulthood (180 days). Threshold shifts of the electrocochleographically obtained cochlear nerve action potential were determined 4 days following the noise exposure. Pure-tone bursts of 2-64 kHz were used to elicit these responses. Noise-induced threshold shifts (NITS) were progressively greater in those responses that originated from a more basal (high-frequency) portion of the organ of Corti. The 3--6-kHz noise band did not affect thresholds; 6--12- and 12--24-kHz bands produced a NITS of 25--30 dB; and 24--48-kHz noise produced a NITS of at least 18 dB. The 20- and 60-day-old subjects were more severely affected (25 and 23.5 dB average NITS, respectively) than the 180-day-old mice (9.5 dB NITS). Susceptibility to damage at the apex and middle of the organ of Corti declined as a function of age during exposure. But the base of the organ of Corti remained equally susceptible to damage from 6-12-, 12-24-, or 24-48-kHz noise bands, from weaning age to adulthood. When these data were compared with those from another study, it was concluded that an early sensitive period exists for noise-induced damage to the apex and middle of the organ of Corti but that the base maintains its susceptibility for a much longer age span.