Figure 7.

The firing-rate-level function of the inferior colliculus (IC) neurons elicited by noise-bursts (50 ms, 0 to 100 dB SPL) recorded from 2, 6, and 12-month-old mice. For the 2 month old group (blue circle), the averaged firing rates maintained at a very low level to sound stimuli between 0 and 20 dB SPL, increased almost linearly from 30 to 60 dB SPL and saturated at above 60 dB and reached to 194 ± 15 Hz at 100 dB SPL. For the 6 month group (red square), the firing rates was low at 0–40 dB SPL and showed a rapid increase at above 50 dB SPL and reached to 334 ± 12 Hz at 100 dB SPL (n = 132). The mean discharge rate of the G-12M group) was low at 60–90 dB SPL (black triangle, n=151) and was slightly higher than the 2 month old group at 100 dB SPL. The averaged firing rates were significant different at 70, 80, 90 and 100 dB SPL (grey shade area) between the three groups (two-way ANOVA, F= 225.8, df=11, 780, P<0.0001). The rate-level functions were fitted with logarithm curves (dashed lines). The slope of the logarithm function was 0.04, 0.067 and 0.086, and the EC50 (the sound intensity that induced halfway response between the baseline to the maximum response) was 44, 60 and 95 dB SPL for the G-2M, G-6M and G-12M groups, respectively.