Article Figures & Data
Figures
- Figure 1.
Stimulus paradigm. A, Probability distribution of sound levels in a dynamic stimulus with an HPR centered at 72 dB SPL. The level distribution spans a 75 dB range (from 26 to 100 dB SPL) in 1 dB steps and contains a 12-dB-wide HPR (from 66 to 78 dB SPL) in which levels occur with 80% probability. The ordinate shows the number of occurrences of each level for a 5 min dynamic stimulus in which each level within the HPR occurs 380 times and each level outside the HPR occurs 20 times. B, Example sequence of sound levels drawn from the HPR distribution in A. Each level step in the dynamic stimulus lasts 50 ms with no silent interval between consecutive steps. Only 15.5 s of the level sequence are shown; the complete stimulus lasts about 5 min. C, Probability density of sound levels for a sample of natural speech (black stems) is similar to that of the HPR stimulus (gray shade). The root mean square amplitudes were measured over successive 50 ms intervals (the duration of each level step in the HPR stimuli) for a corpus of 280 sentences (460 s) pronounced in quiet by a male speaker. The speech sound levels cover a range of 72 dB, which is close to the 75 dB range in the HPR stimuli. The mean level of the speech sample is 52 dB relative to the minimum (dB re Min.) level; the SD is 10 dB, which is close to the 11–16 dB SD for the HPR stimuli (depending on the HPR mean level).
- Figure 2.
Rate responses of an AN fiber to pure tones at its CF (550 Hz) for the baseline level distribution and for four distributions with HPRs centered at 36, 48, 60, and 72 dB SPL. The spontaneous rate was 61 spike/s. A, Rate-level functions. Dots, Measured data; lines, fitted curves using the five-parameter model; black, baseline rate-level function; colors, rate-level functions for HPRs marked by same-color horizontal bars along the horizontal axis; large filled circles, HPR mean levels. B, Maximum firing rate, normalized by the baseline maximum rate (198 spike/s), decreases with increasing HPR mean level with a slope of −0.57%/dB. Solid line, Least-squares fit. C, Normalized rate-level functions. Vertical dashed lines mark the level midpoints (L50). D, Threshold (Lth), L50, and saturation level (Lsat) all grow nearly linearly with increasing HPR mean level. Solid lines, Least-squares fits; dashed line, identity (y = x).
- Figure 3.
Dynamic range adaptation is distinct from classic firing rate adaptation. A, Pure dynamic range adaptation would only involve a horizontal shift of the rate-level function (ΔL) with no change in maximum firing rate. Black, Unadapted rate-level function; red, adapted rate-level function with pure dynamic range adaptation. B, Classic firing rate adaptation is characterized by constant rate decrement (ΔR) across the entire range of sound levels with no change in operating point. Blue, Adapted rate-level function with firing rate adaptation. C, Mixed adaptation (magenta) includes both a rate decrement (blue dashed line; ΔR) and a horizontal shift (ΔL). The shift component ΔL was isolated by normalizing the rate-level function (blue dashed line). D, Apparent dynamic range adaptation may occur in some neurons when a constant rate decrement (ΔR) would cause firing rates to become negative at low sound levels. Blue dashed line, Theoretical adapted rate-level function with constant firing rate decrement; blue solid line: actual adapted rate-level function with the negative firing rates converted to zero. The rate decrement can create an apparent horizontal shift (ΔL*) of the normalized rate-level function (black dashed line).
- Figure 4.
Rate-level functions of a medium-SR AN fiber in response to broadband noise stimuli for level distributions with four level distributions (HPR mean levels were 48, 60, 72, and 84 dB SPL). The layout is the same as in Figure 2. The CF was1300 Hz. The spontaneous rate was 5 spike/s, and the baseline maximum rate was 457 spike/s.
- Figure 5.
Characteristics of dynamic range adaptation for the population of AN fibers using CF-tone and broadband noise (BBN) stimuli. A, B, Rate of growth in level midpoint L50 with HPR mean level (L50 slope) as a function of CF (A) and spontaneous rate (B). C, L50 slopes as a function of the average HPR mean levels relative to each fiber's threshold (re Th.). Solid line, least-squares fit. D, Rates of growth in statistical saturation level Lsat versus the L50 slope. Dashed line, Identity (y = x).
- Figure 6.
Characteristics of firing rate decrement across the AN population. A, The slope of decrease in maximum rate (expressed as a percentage of the maximum rate in the baseline condition) with HPR mean level does not depend on CF. B, Slopes of maximum firing rate with HPR mean level show a weak but significant negative correlation with slopes of dynamic range shift. BBN, Broadband noise.
- Figure 7.
Precision of level coding for broadband noise stimuli in a low-SR AN fiber (CF, 1180 Hz) is dependent on level distribution. A, Mean firing rate as a function of stimulus level for four level distributions (HPR mean level was 48, 60, 72, and 84 dB SPL). B, Slopes of the curves fit to the mean rate data in A. C, SDs of the firing rates across stimulus trials versus stimulus level. Dots, Measured data; lines, BARS-fitted curves. D, The sensitivity index δ′ is the mean rate slope divided by the rate SD. E, L50 and the sound level of maximum δ′ (Lδ′max) both increase nearly linearly with HPR mean level. Solid and dashed lines, Least-squares fits; dotted line, identity (y = x). F, Neural JNDs at the HPR mean level (filled circles) and minimum JNDs (open circles) as a function of HPR mean level.
- Figure 8.
Effect of HPR mean level on the precision of level coding by single AN fibers for tone and broadband noise (BBN) stimuli. A, Scatter plot of rate of growth in level of maximum sensitivity Lδ′max versus rate of growth in level midpoint L50. Dashed line, Identity (y = x). B, Optimally coded HPR mean level Loc (the HPR mean level that coincides with the fiber's level of maximum sensitivity Lδ′max) is strongly correlated with baseline threshold across the AN fiber population.
- Figure 9.
Mean FI of population of AN fibers for broadband noise stimuli with different HPRs. A, Mean FI versus stimulus level (solid lines) and corresponding noise floor (dotted lines; see Materials and Methods) for five different HPR mean levels (36, 48, 60, 72, and 84 dB SPL). The legend shows the number of fibers from which the mean FI was estimated. This number varies with HPR mean level because not every fiber could be studied with every HPR level. B, The level of the peak mean FI increases with HPR mean level for both AN and IC neuron populations. Open circles, AN; filled triangles, IC [Dean et al. (2005), their Fig. 2h]; black solid line, least-squares fit to the AN data; gray solid line, least-squares fit to the IC data; dotted line, identity (y = x). For the IC data, two different data points are shown at the highest HPR mean level (75 dB SPL) because the FI curve was bimodal; these points were not included in the linear fit. The optimally coded HPR mean level Loc is the HPR mean level that coincides with the peak level of FI curve. Loc is 60 dB SPL for the AN fiber population and is hard to determine for the IC because of the ambiguous data at 75 dB SPL. C, Minimum neural JND in level (open circles, minimum JND) and JND at the HPR mean level (filled circles, data; black line, polynomial fit) versus HPR mean level. Red, JND curve corresponding to the red FI curve in A, for which the HPR is centered at 60 dB SPL; gray dotted line, criterion for robust level coding, which is 2 dB above the grand minimum JND across all the HPR measurements. The horizontal dashed lines with arrows denote the ranges of robust level coding with dynamic range adaptation artificially held at a fixed set point (red) and naturally determined by the level distribution (black), respectively.
- Figure 10.
Comparison of dynamic range adaptation to broadband noise between IC and AN neurons. A–C, Rate-level functions of three IC neurons for four different HPR mean levels. These neurons were chosen to illustrate the diversity of rate-level function shapes in the IC. Dots, Raw data; lines, BARS-fitted curves. D, Comparison of L50 slopes for the AN and IC neuron populations. The median and 25–75% quartiles of the L50 slopes are 0.24 dB/dB, 0.21–0.30 dB/dB for the AN population and 0.37 dB/dB, 0.29–0.52 dB/dB for the IC population.
