The effect of repeated daily noise exposure on sound-conditioned and unconditioned guinea pigs.

Sound-conditioned and unconditioned guinea pigs were exposed every day for 10 consecutive days to noise exposure resulting in a temporary threshold shift (2767 Hz, 103 dB SPL, 5 min). The cubic distortion product otoacoustic emissions (DPOAE) were followed at a constant L1 intensity of 60 dB SPL at 1.75, 2.8, 3.5, and 4.4 kHz for 90 min post-exposure. Four parameters (area under the curve, duration of loss, maximal loss, and time point for maximal threshold shift) were analyzed to determine the effect of repeated daily noise exposure for each group. The sound-conditioned group (1) was significantly less affected by overstimulation during the initial days of exposure compared to the unconditioned group and (2) gradually became more affected by overstimulation as the daily sessions progressed. The 'training effect' induced by sound conditioning gradually deteriorated after approximately 5 or 6 days of repeated stimulation. However, at day 10 the sound-conditioned group never had emissions that were worse than day 1 overstimulation for the unconditioned group. The unconditioned group, on the other hand, illustrated significantly greater threshold shifts during the initial days of overexposure and then demonstrated a gradual resistance to overstimulation during subsequent days.