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. 2019 Mar 28;13:119. doi: 10.3389/fncel.2019.00119

Figure 3.

Figure 3

Development of hearing threshold and frequency response area in AVCN units. (A) Representative frequency response areas in BCs (left), and SCs (right) at P12, P13, and P30 (upper, middle and lower panels, respectively). For better comparison, the color scaling is identical for all frequency response areas indicating spike discharge rates between 0 (dark blue) and 420 APs per second (red). Solid black lines indicate the units’ characteristic frequencies (CFs). Note the general increase in firing rates within the excitatory response area. SCs show a stronger reduction in firing activity towards the low-frequency tail compared to BCs. Solid lines (CF), dashed lines (half octave below CF [CF/1.4]), and dotted lines (high-frequency inhibitory sideband [Finh]) depict the positions where the respective rate level functions were calculated. (B) Trend curves based on CF and threshold values of BCs (left) and SCs (right). For each age group, a polynomial-quadratic fit of the threshold values was used to approximate hearing range and sensitivity. Both cell types follow similar maturational pattern exhibiting decreasing response thresholds (effect of cell type p = 0.35, effect of age p < 0.001, interaction cell type × age p = 0.52; two-way ANOVA), and the development of high-frequency hearing between P12 and P14. (C) Maximum firing rates during acoustic stimulation increased from P12 to P30 in both BCs (red) and SCs (blue; effect of cell type p < 0.001, effect of age p < 0.001, interaction cell type × age p = 0.014, two-way ANOVA). Box-plots show medians, the 25 and 75 percentiles, and the interdecile ranges (**p < 0.01; ***p < 0.001). (D) Average rate level functions for P12 to P30 calculated at a CF. (E) The dynamic ranges of rate level functions at CF expand similarly during development for both, BCs and SCs (effect of cell type p = 0.36, effect of age p = 0.005, interaction cell type × age p = 0.26; two-way ANOVA). (F) Average rate level functions obtained at half an octave below CF show bigger dynamic ranges in BCs (effect of cell type p < 0.001, effect of age p = 0.02, interaction cell type × age p = 0.92; two-way ANOVA) and lower firing rates in SCs up to the stimulation level of 60 dB SPL (10–30 dB SPL: effects of cell type p < 0.001, effects of age p < 0.001, interaction cell type × age at 10 dB p = 0.61, at 20 dB p = 0.3, at 30 dB p = 0.71; 40 and 50 dB SPL: effects of cell type p < 0.01, effects of age p < 0.001, interaction cell type × age at 40 dB p = 0.82, at 50 dB p = 0.22; 60 dB SPL: effect of cell type p = 0.02, effect of age p < 0.001, interaction cell type × age 0.76; two-way ANOVA). (G) The average rate level functions at the Finh. Note the stronger effect of inhibition in SCs compared to BCs (10 dB SPL: p = 0.009; 20 dB SPL: p = 0.003; 30–80 dB SPL: p < 0.001, two-way ANOVA).