PMC

Discharge rate predictions to broadband noise filtered through acoustical head related directional transfer functions (DTFs). Same as in Figure but for a MNTB neuron with a CF of 15.4 kHz.

Distributions of the fraction of variance, fv (A), and the spatial correlation coefficient (B) for 14 MNTB neurons (tested at 29 overall stimulus levels) for both the first-order linear model (black) and the full-order model (gray). The across-neuron median fv and spatial correlation coefficient for each model is indicated.

Examples of three of 264 RSS stimulus spectra. The RSS stimuli had spectral contrasts chosen from a normal distribution with a standard deviation of 10 dB. Four of 264 stimuli had flat spectra (A), while the remaining stimuli had spectra resembling those in (B,C).

(A) Examples of extracellular voltage waveforms illustrating the pre-potential component in 5 MNTB neurons. (B) Threshold SPL for tones as a function of the characteristic frequency of each neuron. Behavioral audiogram for cat from Heffner and Heffner () (solid line). (C) Frequency tuning bandwidth, Q₁₀ (characteristic frequency divided by the bandwidth 10 dB above threshold), as a function of characteristic frequency. (D) Histogram of spontaneous activity. Data in (B–D) are based on 103 MNTB and 51 GBC neurons.

Discharge rate predictions to broadband noise filtered through acoustical head related directional transfer functions (DTFs). (A) Spatial plot of the spectral levels (dB) for DTF filtered broadband noise stimuli corresponding to the CF of one neuron (10 kHz). (B,C) show the first- and second-order weights, respectively, used for predicting rate responses to DTF stimuli. (D) Spatial plot of empirical discharge rates for the 325 (out of 627) front-hemisphere DTF stimuli and predicted rates with first-order (E) and full-order (F) spectral weight models.

Dependence of first-order weights at BF on overall sound level. For all MNTB (A), GBC (B), and ANF (C) neurons studied (individual lines and symbols), the first-order weight at BF (left-hand ordinate) is plotted as a function of sound level (re: threshold for flat-spectrum stimulus, Figure ). In general, for each neuron the first-order weights were small for low and high sound levels and peaked for moderate sound levels. To summarize this trend, the solid black line shows the across-neuron mean first-order weight as a function of sound level; data for individual neurons was first normalized by the maximum weight before averaging across neurons. (D) Summarizes the across-neuron first-order weights at BF in AN, GBC, and MNTB neurons.

First-order linear weight functions for all 42 MNTB (A) and 21 GBC (B) neurons studied binned according to BF (BF ranges in upper left of each panel). Weight functions were taken from spectral levels in the range of 5–15 dB. Weight function for auditory nerve fibers (AN, C) are replotted from Young and Calhoun (). In each panel, the light gray lines indicate weight functions for individual neurons while the solid line with symbols and error bars indicate the across-neuron mean weight function ±1 standard deviation. Generally, the weight functions in each range of BFs were similar in terms of shape and magnitude.

Encoding of sound spectra by MNTB neuron via RSS method. First order (linear) spectral weight functions for one neuron (CF = 15.4 kHz) at two levels re: threshold (A1 −5 dB, B1 15 dB). Second-order weights for the same two levels (A2,B2). Empirical discharge rates plotted rank-ordered vs. RSS stimuli (black line) along with the predicted rates using the estimated first-order (red line) and full-order (green line) spectral weight models for the same two levels (A3,B3). The fraction of explained variance, fv, is indicated for each model. Scatter plot of modeled rate vs. empirical rate for first-order (black) and full-order (red) models for the same two levels (A4,B4). Solid line is line of equality.

Spectral modulation selectivity of MNTB neurons are sufficient to encode the spectral modulations contained in spatial head related transfer functions. (A) The directional transfer function (DTF) for a sound source located at (0°, 0°). (B) Spectral modulation distribution of the DTF in (A), the ensemble of RSS stimuli, and the ensemble of DTFs used in this study plotted as mean ripple depth (dB) as a function of ripples/octave. DTFs have large ripple depth at low ripples/octave while RSS have constant mean ripple depth up to at least 4 ripples/octave. (C) Spectral modulation selectivity of the population of MNTB neurons in terms of spectral weighting (spikes/s/dB) as a function of ripples/octave. MNTB neurons preferentially resolve low spectral modulations consistent with those provided by the DTFs.

Predictive validation of the spectral weight function model. Fraction of variance, fv, as a function of RSS stimulus level for MNTB (A) and GBC (D) neurons; thin lines indicate data for individual neurons and thick black lines the across-neurons mean. Histogram of the maximum fraction of explained variance Equation (4), fv, in 42 MNTB neurons (109 total RSS levels) (B) and 21 GBC neurons (76 total RSS levels) (E) for both the first-order linear model (black) and the full-order model (gray) containing the second-order non-linear terms. The fv were computed by predicting the discharge rates to arbitrary RSS stimuli that were not used in the estimation of the spectral weight models. The across-neuron median fv for each model is indicated. Histograms of the correlation coefficient, r, relating the predicted spike rate to the empirical rate (e.g., Figure ) are shown in (C,F) for MNTB and GBC neurons, respectively.

(A) BF estimated from the RSS weight function as a function of the CF measured from pure tone frequency-intensity response area. (B) Frequency tuning estimated from the RSS weight functions (RSS Q₁₀, see text for equation) for all neurons and all levels tested as a function of Q₁₀ measured from pure tone frequency-intensity response area (RA Q₁₀). (C) First-order weight function for one neuron (BF = 25.9 kHz) computed over a range of sound levels (−15 to 15 dB). The weight function for each sound level has been normalized by the maximum weight at BF at each particular sound level. For higher sound levels (> −15 dB), the normalized weight function shapes were similar. The bandwidth of the weight functions was computed at one-half the maximum weight at BF (see bar at normalized weight of 0.5). Significant off-BF inhibition (negative weights) can be seen at higher levels (error bars not shown for clarity). (D) Half-height bandwidth for 38 high-BF (>3 kHz) MNTB neurons as a function of sound level re: threshold. Lines and points show data for individual neurons while the solid black line shows the across-neuron mean. (E) Same as in (D), but for GBC neurons. (F) Across-neuron mean spectral selectivity of GBC and MNTB neurons was quite consistent, or level-tolerant, across a wide range levels while the selectivity of AN neurons tended to widen. AN data replotted from Young and Calhoun ().