Pure tone masking patterns in adult chickens before and after recovery from acoustic trauma

Three female adult chickens were trained to detect pure tones using a positive reinforcement technique and an adaptive threshold tracking procedure. Masked thresholds for tone bursts were measured in the presence of pure-tone maskers at 500, 1000, and 2000 Hz. Masker levels of 40-, 60-, and 70-dB sound-pressure level (SPL) were employed. Masking was maximal at the frequency of the masker. Tone-on-tone masking patterns were symmetrical at low masker levels, but at higher masker levels, more masking was evident at frequencies above the masker than below the masker. Distinct low threshold notches were frequently observed above and below the peak in the masking pattern. After the normal masking patterns had been measured, the chickens were exposed to a 525-Hz traumatizing tone at 120 dB SPL for 48 h and then allowed to recover. After thresholds had returned to their pre-exposure levels, tone-on-tone masking patterns were remeasured. The post-exposure masking patterns were virtually identical to the pre-exposure masking patterns.