Figure 3. Repeated noise exposure induces tinnitus in a subset of experimental animals.

(A) Timeline of the experimental procedures of the chronically-exposed group. Nineteen animals were grouped into sham controls (n=6) and noise-exposed animals(n=13). GPIAS was used as tinnitus assessment and baseline thereof was acquired for four weeks pre-noise exposure. Animals were exposed to the same noise band/sham for two hours twice in sessions conducted four weeks apart, and then assessed for tinnitus eight weeks following the first noise exposure. ABR measurements were performed before and after each noise exposure and GPIAS. Noise-exposed animals were divided into two groups according to GPIAS assessment: noise exposed animals that exhibit no signs of tinnitus (ENT, n=7), and exposed animals that exhibit tinnitus (ET, n=6).

(B) Mean (±SEM) ABR thresholds of animals with tinnitus (ET) and without tinnitus (ENT). ABR thresholds on the ipsilateral side were elevated immediately following noise exposure in both groups, but recovered to baseline levels at 8, 12, 16, 20 kHz 12 weeks after the first noise exposure.

(C) Mean (±SEM) ABR wave I amplitude-intensity functions for ENT and ET animals pre- (baseline) and post-noise exposure (12w) were not significantly different, suggesting no underlying cochlear synaptopathy in both ENT and ET animals after the noise exposure.

(D) Rationale of GPIAS (adapted from Turner et al., 2006). Row 1: Normal animals respond with a robust startle to the presentation of a startle pulse (20ms, 95 dB) embedded in a continuous background sound (65 dB). Row 2: When a silent gap (50ms) is introduced in the background sound, normal animals use the gap to predict the incoming startle pulse and respond with decreased startle amplitude. Row 3: Animals with tinnitus fail to detect the gap due to their tinnitus percept and respond with an uninhibited startle to the pulse presentation. Row 4: The gap is replaced with a prepulse noise (75dB). Both normal hearing and tinnitus animals respond with decreased startle amplitude due to alarm effects of the prepulse noise. Animals with hearing loss fail to detect the prepulse noise and thus respond with an uninhibited startle to the pulse presentation. This assessment tells whether animals’ inhibited responses to gap trials are due to hearing impairment.

(E) Mean (±SEM) normalized startle inhibition ratio (NSIR) was the ratio of the startle amplitudes for the gap (or prepulse inhibition, PPI) trials and those for the no-gap trials. NSIR for gap trials was significantly higher post-exposure(Post) relative to baseline levels (Pre) for ET animals, but not for ENT or control animals. All animals exhibited stable responses to PPI trials both pre and post noise exposure.

(F) Tinnitus indices of animals with tinnitus (ET) were significantly higher than those of controls and no-tinnitus animals (ENT).