Figure 2 (A-D) Exemplar STCs in response to SGN illumination with a single μLED (A), a block of four neighboring μLEDs (B) and all μLEDs of an oCI inserted via the round window in a CatCh-transduced gerbil (C) and with all µLEDs in a wildtype gerbil (WT; D). μLEDs were spaced by 250 μm.  (E) Acoustic auditory brainstem responses (average of 1000 stimuli at a stimulation rate of 10 Hz) upon click stimulation from 20-100 dB SPL before (left) and after deafening (right; star: threshold). (F) Optically evoked auditory brainstem responses before (left) and after deafening (right), elicited by ~35 mW pulses of 1 ms duration, delivered by a laser-coupled fiber (1000 stimuli presented at 10 Hz), recorded from the same animal. Optogenetic activation of the auditory system was still possible after deafening. Differences in amplitude and waveform before and after deafening are likely due to different positioning of recording electrodes and the optical fiber.  (G-H) Exemplar STCs in response to SGN illumination with all µLEDs in a CatCh-transduced, kanamycin-deafened gerbil before (G) and after (H) sacrifice.  (I) Maximum strength of ICC responses (mean ± standard deviation) evoked by oCI stimulation with 1 (indiv.; n = 129/77 µLEDs in N = 9/3 hearing/deaf gerbils), 4 (block; n = 62/27 blocks in N = 9/3 hearing/deaf gerbils), and 16 (all; n = 24/9 oCIs in N = 9/3 hearing/deaf gerbils) active µLEDs, and with a laser-coupled optical fiber (~35 mW; n = 9/3 fiber stimulations in N = 9/3 hearing/deaf gerbils) in CatCh-transduced gerbils (One-way ANOVA (F-value: 73.25, 4 degrees of freedom, p = 5.19*10-45) and post-hoc multiple comparison test; indiv. vs. block: p = 9.9*10-9; block vs. all: p = 0.0096; all vs. fiber: p = 0.0016; all CatCh vs. all WT: p = 9.9*10-9;**: p < 0.01, ***: p < 0.001, n.s. = non-significant). oCI stimulation with all µLEDs has also been performed in WT animals (n = 7/7 oCIs, N = 2/2 hearing/deaf gerbils) and in sacrificed CatCh-transduced animals (post-mortem; n = 5/1 oCIs in N = 5/1 hearing/deaf gerbils). Brown dots indicate data recorded from deafened animals. (J) Number of active electrodes (d´ > 1; mean ± standard deviation) upon oCI stimulation with 1 (n = 129/77 µLEDs in N = 9/3 hearing/deaf gerbils), 4 (n = 62/27 blocks in N = 9/3 hearing/deaf gerbils), and 16 µLEDs (n = 24/9 oCIs in N = 9/3 hearing/deaf gerbils), and with optical fiber stimulation (n = 9/3 fiber stimulations in N = 9/3 hearing/deaf gerbils) in CatCh-transduced gerbils (One-way ANOVA (F-value: 30.18, 4 degrees of freedom, p = 1.42*10-21) and post-hoc multiple comparison test; indiv. vs. block: p = 6.8*10-5; block vs. all: p = 0.006; all vs. fiber: p = 0.25; all CatCh vs. all WT: p = 9.9*10-9;**: p < 0.01, ***: p < 0.001, n.s. = non-significant). Responses to oCI stimulation with all µLEDs were also observed in hearing WT animals (n = 3 oCIs in 2 gerbils). No active electrodes were found in response to 4/7 oCI stimulations in 2/2 hearing/deaf wildtype gerbils). (K) Thresholds (d´ = 1, measured at the best electrode (BE)) for ICC activation evoked with 16 active µLEDs on an oCI (n = 24/9 in N = 9/3 hearing/deaf gerbils) and with a laser-coupled optical fiber (n = 9/3 in N = 9/3 hearing/deaf gerbils; p = 0.012, Wilcoxon rank sum test). Central horizontal line indicates the median, lower, upper horizontal lines indicate the minimum and maximum values, and box edges indicate the 25th and 75th percentile, respectively.