MORPHOLOGICAL AND FUNCTIONAL PRESERVATION OF THE OUTER HAIR-CELLS FROM NOISE TRAUMA BY SOUND CONDITIONING

Guinea pigs were sound conditioned to a low-level, long-term pure tone stimulus (1 kHz, 81 dB SPL, 24 days) before exposure to a traumatic n oise (1 kHz, 105 dB SPL, 72 h). Auditory brainstem response thresholds and distortion product otoacoustic emissions were obtained at selecte d frequencies before sound conditioning and at day 1, 5, 10, and 15 du ring sound conditioning as well as on the final 24th day. Auditory bra instem responses at 1 and 2 kHz were not affected at any time during s ound conditioning. The amplitude of the distortion product otoacoustic emission showed minor alterations (below 10 dB) at selected frequenci es only during the initial stages (day 1, 5, and 10) of sound conditio ning in some, but not all the animals. Distortion product amplitudes w ere similar to control values on the 15th and 24th day of conditioning . Surface preparations of the organ of Corti did not reveal any signif icant hair cell loss induced by sound conditioning. The effect of a tr aumatic exposure (1 kHz, 105 dB SPL, 72 h) on a control group and a so und conditioned group was determined. The distortion product otoacoust ic emission amplitude measured 4 weeks after the cessation of the trau matic exposure revealed significant differences. The amplitude of the distortion product otoacoustic emission for the control group was depr essed at all tested frequencies and at lower frequencies (2.8, 2.1, an d 1.75 kHz) the emissions did not show an increase in response to incr eases in intensity. of the primaries. The sound conditioned group show ed increases in distortion product amplitude with increases in the int ensity of the primaries for all tested frequencies and statistically s ignificant reductions from the pre-exposure values were not found. Sur face preparations from the control group indicated that the traumatic noise exposure affected nearly 100% of the outer hair cells around the 14 mm distance from the round window. The sound conditioned group sho wed a significantly less (50%) outer hair cell loss than the control g roup. The sound conditioned group illustrated an altered pattern of da mage after subsequent noise trauma. There were two distinct regions of outer hair cell loss, one being around the 16 mm distance and the oth er around the 12 mm distances from the round window. These results imp ly that the intrinsic properties of the outer hair cells and/or the or gan of Corti have been altered by sound conditioning.