Hearing loss and tinnitus
As detailed in the Method section, the HL was assessed by individual ABR threshold comparison. HL group effects were analyzed using a two-factorial ANOVA with the factors ear (plugged control, trauma, sham trauma) and frequency (1 kHz, 2 kHz, 4 kHz, 8 kHz, 16 kHz). The results of the factor ear (F(2, 148)=12.74, p<0.001) revealed a significant higher mean HL (± standard error) in the trauma ears (9.75 dB ± 1.26 dB) compared to the plugged control ears (2.01 dB ± 1.17 dB, Tukey post-hoc test: p<0.001) and the sham trauma ears (-0.86 dB ± 2.69 dB, Tukey post-hoc test: p=0.001). Plugged control and sham trauma ears did not show a significant difference in HL (Tukey post-hoc test: p=0.59). Neither in the factor frequency (F(4, 148)=0.10, p=0.98) nor in the interaction of both factors (F(8, 148)=0.27, p=0.98) we found a significant effect on the HL. In other words, the traumatized ears showed a general significant increase in hearing thresholds after the trauma while neither plugged control nor sham trauma ears were affected by the noise exposure.
Of the nine animals that received a monaural acoustic noise trauma, four developed behavioral signs of a possible tinnitus percept in at least one tested frequency (1 kHz, 2 kHz, 4 kHz, 8 kHz, 16 kHz; T group). This was indicated by a significant decrease in effect size in the GPIAS paradigm 13 d after the trauma (t-tests, p<0.05). Three animals developed a possible tinnitus percept at one frequency (2 kHz (p=0.028 and p=0.007) or 4 kHz (p=0.004)), one animal at two frequencies (4 kHz (p=0.01) and 8 kHz (p=0.001)) and one at three frequencies (2 kHz (p=0.016) and 8 kHz (p=0.001) and 16 kHz (p<0.001)). The other five trauma animals did not show any significant decrease in effect size (NT group). As a side note, we found a strong tendency for a difference in the mean HL of the trauma ears of NT (14.2 ± 3.6 dB) and T animals (7.3 ± 1.6 dB) in a one-factorial ANOVA (F(1, 58)=4.0, p=0.05). None of the three sham noise trauma animals did show a significant decrease in effect size of the behavioral paradigm (sham control group) nor a difference in HL relative to the control ears of NT or T animals (F(2, 102)=2.35, p=0.11).