Article Figures & Data
Figures
- Figure 1.
Progressive age-related cochlear synaptopathy occurs before hair cell loss. A, Schematic cross-section showing three of the ∼20 auditory nerve fibers (ANFs) making synaptic contact with an IHC. Presynaptic ribbons and postsynaptic receptor patches are also schematized. The x–y–z axis shows the viewing angle for the confocal x–y projections shown for example IHCs in (B), where immunostaining reveals the juxtaposition of presynaptic ribbons (red) and postsynaptic receptor patches (green). C, Mean ± SEM percentage survival of cochlear synapses (green line), IHCs (gray solid line), and OHCs (gray dashed line) relative to 16-week-old animals at two cochlear locations and five age groups: 16 weeks (n = 9), 32 weeks (n = 11), 64 weeks (n = 14), 108 weeks (n = 9), and 128 weeks (n = 8). Ages of individual animals were within 5% of each target age.
- Figure 2.
Synaptopathy is reflected in neural ABR wave 1 amplitudes. A–C, Preneural, OHC-based DPOAE measures. A, Mean ± SEM change in DPOAE thresholds relative to 16-week-old animals at two cochlear locations. B, DPOAE growth functions for the different age groups. C, Scatterplots showing the correlation between DPOAE thresholds and mean number of OHCs per row per imaging region of interest for all animals tested. D–F, Neural, ABR-based measures. D, Mean ± SEM change in ABR wave 1 thresholds relative to 16-week-old animals at two cochlear locations. E, ABR wave 1 growth functions for the different age groups. F, Scatterplots showing the correlation between ABR wave 1 amplitudes at 30 dB SL and mean number of cochlear synapses for all animals tested. Age ranges and group sizes in all panels similar to Figure 1. The oldest age group (128 weeks) in F had fewer animals that had responses at 30 dB SL due to elevated hearing thresholds.
- Figure 3.
Ouabain differentiates early neural and hair cell-based responses. Shown are mean ± SEM ABR wave 1 thresholds (A) and amplitudes at 30 kHz (B) measured before and after the application of 10 mm ouabain (n = 5) or saline (n = 3) to the round window of the cochlea. Group sizes apply to all panels. C, D, Mean ± SEM DPOAE thresholds and amplitudes at 30 kHz, respectively, for the two groups before and after treatment.
- Figure 4.
Ouabain differentiates EFRs generated from early neural- and hair cell-based sources. A, Grand-averaged magnitude spectra for the EFRs at 1024 and 4096 Hz AM before and after treatment with ouabain or saline. Black triangles indicate location of AM frequency. B, Individual EFR amplitudes at 1024 and 4096 Hz AM before and after treatment with ouabain or saline. C, Mean ± SEM percentage decrease in EFR amplitudes elicited by AM frequencies between 768 and 4096 Hz. D, Comparison of EFRs at 4096 Hz AM recorded using mastoid electrodes and round window electrodes in five animals. Inset shows the grand-averaged magnitude spectrum (bold) with individual spectra shown behind. All EFRs elicited from the 30 kHz cochlear region at 80 dB SPL.
- Figure 5.
Temporal processing deficits occur at the cochlear synapse and are correlated with the degree of synaptopathy. A, B, Mean ± SEM EFR amplitudes at 80 dB SPL elicited to AM frequencies between 1024 and 4096 Hz at 12 kHz and 30 kHz frequency regions. Insets in A and B shows magnitude spectra of EFRs at 1024 and 4096 Hz AM with peaks at the respective modulation frequencies against the surrounding noise floor. C, D, Correlation between synapses/IHC and the EFR ratio calculated as the absolute value of log10(EFR1024)/log10(EFR4096) at the two cochlear frequency regions shown in A and B. Age ranges and group sizes were 16 weeks (n = 9), 32 weeks (n = 11), 64 weeks (n = 12), 108 weeks (n = 9), and 128 weeks (n = 7).
- Figure 6.
The dynamic range for encoding sound level decreases with age. A, B, Mean ± SEM EFR amplitudes at 1024 Hz AM measured as a function of sound level across the various age groups at 12 and 30 kHz frequency regions. Age ranges and group sizes are similar to Figure 1. Dashed lines indicate responses below the noise floor. C, D, Mean ± SEM EFR amplitudes at equal SLs of 0 dB (threshold) to 30 dB at the two cochlear frequency regions shown in A and B. The oldest age group (128 weeks) had fewer animals that had responses at 30 dB SL due to elevated hearing thresholds, with only two animals at 128 weeks reaching 30 dB SL at 12 kHz (data point not shown). E, F, Correlation between EFR amplitudes from C and D at 30 dB SL and the number of remaining synapses/IHC across all the age groups. Age ranges and group sizes are similar to Figure 5.
- Figure 7.
Dynamic range for early neural coding of modulation depth decreases with age. A, B, Mean ± SEM EFR amplitudes at 1024 Hz AM measured as a function of AM depth across the various age groups at 12 and 30 kHz frequency regions. Sound level of presentation was 30 dB SL at all age groups except 128 weeks, where the sound level was fixed at 90 dB SPL. Dashed lines indicate responses below the noise floor. Age ranges and group sizes were 16 weeks (n = 8), 32 weeks (n = 6), 64 weeks (n = 9), 108 weeks (n = 6), and 128 weeks (n = 6).
- Figure 8.
Responses from the central auditory pathway show signs of compensatory gain with age. Mean ± SEM EFR amplitudes at AM frequencies from 16 to 4096 Hz in octave steps across the various age groups at the 12 kHz (A) and 30 kHz (B) frequency regions. Sound level of presentation was 80 dB SPL. Inset shows mean ± SEM ABR wave 5: wave 1 ratios across age. Age ranges and group sizes are similar to Figure 1.
Tables
Source Sum Sq df Mean Sq F Prob>F 12 kHz Age 2.74 4 0.68 147.01 1.8 × 10−87 Level 26.72 12 2.23 478.6 3.1 × 10−297 Age*level 1.51 48 0.031 6.75 2.1 × 10−32 Error 2.78 598 0.005 Total 35.111 662 30 kHz Age 4.9 4 1.22 261.8 7.5 × 10−130 Level 14.22 12 1.18 254.3 1.1 × 10−225 Age*level 1.52 48 0.031 6.81 9.8 × 10−33 Error 2.79 598 0.005 Total 24.3 662 Test of main effects of age and sound level, as well as their interaction, on DPOAE amplitudes at two cochlear frequencies using two-way ANOVAs. Sum Sq, The sum of squares due to each source of variability; df, degrees of freedom associated with each source; mean Sq, mean squares for each source; F, F-statistic; Prob>F, p-value associated with each source.
Source Sum Sq df Mean Sq F Prob>F 12 kHz Age 0.35 2 0.17 357.09 6.2 × 10−99 Level 0.45 8 0.06 115.83 3.6 × 10−111 Age*level 0.1 43 0.002 4.98 8.3 × 10−20 Error 0.26 530 0.0005 Total 4.55 590 30 kHz Age 0.15 2 0.07 137.29 3.3 × 10−48 Level 0.16 6 0.03 51.73 1.9 × 10−49 Age*level 0.12 40 0.003 5.53 2.3 × 10−21 Error 0.26 498 0.0005 Total 2.33 555 Shown are results of test of main effects of age and sound level, as well as their interaction, on ABR wave 1 amplitudes at two cochlear frequencies using two-way ANOVAs.
Sum Sq, The sum of squares due to each source of variability; df, degrees of freedom associated with each source; mean Sq, mean squares for each source; F, F-statistic; Prob>F, p-value associated with each source.
Source Sum Sq df Mean Sq F Prob>F 12 kHz Age 149.5 4 37.38 283.43 1.8 × 10−116 Level 257.55 8 32.19 244.09 1.5 × 10−150 Age*level 37.12 32 1.16 8.8 3.1 × 10−30 Error 53.84 409 0.13 Total 500.76 453 30 kHz Age 173.57 4 43.4 184.55 4.9 × 10−90 Level 246.63 8 30.82 131.12 1.2 × 10−107 Age*level 26.95 32 0.84 3.58 1. × 10−09 Error 95.7 407 0.24 Total 558.7 451 Shown are results of test of main effects of age and sound level, as well as their interaction, on EFR amplitudes elicited to 1024 Hz AM frequency at two cochlear frequencies using two-way ANOVAs.
Sum Sq, The sum of squares due to each source of variability; df, degrees of freedom associated with each source; mean Sq, mean squares for each source; F, F-statistic; Prob>F, p-value associated with each source.
Source Sum Sq df Mean Sq F Prob>F 12 kHz Age 8.8 4 2.2 24.94 2.1 × 10−16 Level (SL) 31.49 3 10.5 118.9 2.6 × 10−42 Age*level 2.17 12 0.18 2.05 2.3 × 10−02 Error 15.71 178 0.08 Total 58.7 197 30 kHz Age 3.6 4 0.91 14.19 4.6 × 10−10 Level (SL) 40.04 3 13.34 208.12 8.5 × 10−58 Age*level 2.67 12 0.22 3.47 1.3 × 10−04 Error 11.35 177 0.06 Total 60.23 196 Shown are results of test of the main effects of age and sound level relative to threshold (sensation level), as well as their interaction, on EFR amplitudes elicited to 1024 Hz AM frequency at two cochlear frequencies using two-way ANOVAs.
Sum Sq, The sum of squares due to each source of variability; df, degrees of freedom associated with each source; mean Sq, mean squares for each source; F, F-statistic; Prob>F, p-value associated with each source.
Source Sum Sq df Mean Sq F Prob>F 12 kHz Age 4.9 4 1.21 11.32 3 × 10−08 Depth 93.65 5 18.73 174.65 2.8 × 10−68 Age*depth 1.086 20 0.054 0.51 9.6 × 10−01 Error 19.95 186 0.11 Total 122.40 215 30 kHz Age 14.65 4 3.66 34.38 5.9 × 10−21 Depth 105.54 5 20.51 192.51 1.3 × 10−67 Age*depth 2.31 20 0.12 1.08 3.7 × 10−01 Error 17.90 168 0.11 Total 143.40 197 Shown are results of test of main effects of age and depth of AM, as well as their interaction, on EFR amplitudes elicited to 1024 Hz AM frequency at two cochlear frequencies using two-way ANOVAs.
Sum Sq, The sum of squares due to each source of variability; df, degrees of freedom associated with each source; mean Sq, mean squares for each source; F, F-statistic; Prob>F, p-value associated with each source.
Source Sum Sq df Mean Sq F Prob>F 12 kHz Age 36.4 4 9.1 50.27 3.6 × 10−32 AM frequency 336.05 7 48 265.15 1.6 × 10−122 Age*AM frequency 14.62 28 0.52 2.88 4.4 × 10−06 Error 52.87 292 0.18 Total 481.6 331 30 kHz Age 19.25 4 4.81 38.42 8.5 × 10−26 AM frequency 262.5 7 37.5 299.3 1.1 × 10−128 Age*AM frequency 10.12 28 0.36 2.89 4.4 × 10−06 Error 36.23 290 0.12 Total 347.55 329 Shown are results of test of main effects of age and frequency of AM, as well as their interaction, on EFR amplitudes at two cochlear frequencies using two-way ANOVAs.
Sum Sq, The sum of squares due to each source of variability; df, degrees of freedom associated with each source; mean Sq, mean squares for each source; F, F-statistic; Prob>F, p-value associated with each source.
