Article Figures & Data
Figures
- Figure 1.
ITD tuning to a pure-tone sound stimulus of best frequency. A–D, A low-BF neuron (A–C, 200 Hz from a juvenile chicken, P5; D, 400 Hz from a young adult chicken, P23). F–J, A middle-high-BF neuron (F–H, 2400 Hz from a juvenile chicken, P4; I, 1800 Hz, P27; J, 2000 Hz, P26). A, F, Raster plots. The gray and white bands indicate a set of five repeats of ITD. The horizontal bars indicate sound presentation here and the subsequent PSTHs. B, G, PSTHs at the best (top) and the worst ITDs (bottom) obtained at 98 dB SPL (B) and 68 dB SPL (G). Bin width, 1 ms. C, D, H, I, J, ITD-tuning curves. ITD-tuning curve of best contrast is indicated by a solid line with error bars of SEM, and the horizontal broken line and the shade indicate the mean ± SEM of spontaneous firings. Stimulus SPL and the rate threshold are indicated. E, Firing rates of the best ITD (solid lines) and the worst ITD (broken lines) as a function of SPL, from five units presented in this figure. The symbols identify units; the open symbols are units from juvenile chickens, and the filled symbols are units from young adult chickens. Contra., Contralateral; Ipsi., ipsilateral.
- Figure 2.
Peak–trough contrast and the firing rate at the worst ITD. A, B, Peak–trough contrasts as a function of stimulus intensity relative to the rate threshold. A is for low-BF, and B is for middle-high-BF units, normalized by the maximum firing rate after subtraction of the spontaneous firing rate. The connected symbols indicate the same unit. The filled symbols indicate the maximum SPL applied to the unit. The broken lines connecting diamonds indicate units from young adult chickens. The colors indicate units shown in Figure 1, C (A, blue solid line), D (A, blue broken line), H (B, blue solid line), I (B, blue broken line), and J (B, red broken line). C, Peak–trough contrasts evaluated at the maximum SPL for units in A and B after normalization (filled symbols in A, B). y = −0.38 × log(BF) + 3.13; r = 0.87; **p < 0.01; n = 21 (14 from juvenile and 7 from young adult). The different symbols indicate different SPLs. The data from young adult chickens were plotted by diamonds hereafter. D, Spontaneous firing rate (filled columns), and firing rates at the worst ITD after normalization by the spontaneous firing rate (open columns) at SPL of the best peak–trough contrast. Error bars indicate SEM.
- Figure 3.
Distribution of SON projection and VGAT immunoreactivity. A–C, Projections from SON to NL visualized by RFP. D, Dorsal; M, medial. D–F, VGAT immunoreactivity. A, D, Low-BF; B, E, middle-BF; C, F, high-BF region. BF region was determined in reference to Rubel and Parks (1975). G–H, VGAT-positive particles and cells within a region of interest (ROI) (43.6 × 53.4 μm). I, Number of VGAT-positive particles per cell. Mean VGAT-positive particle size was 1.31 ± 0.05 μm2. Numbers in parentheses are the numbers of regions of interest in which particles were counted in G–I. Data in Figures 3 and 4 were obtained from juvenile chickens. Error bars indicate SEM. **p < 0.01.
- Figure 4.
Immunostaining with VGAT and RFP projection from SON. A–D, SON projections to NL at high magnification. The asterisks indicate cell somas. The broken line in A demarcates the area of cell somas from dendrites. A, B, Low-BF; C, middle-BF; D, high-BF region; E, VGAT; F, merged image; G, SON projection. The double-labeled particles are indicated by filled arrowheads and RFP-positive but VGAT-negative particles by open arrowheads; these may be an axon and a cut end of it.
- Figure 5.
Unit responses after SON lesion in the juvenile chicken. A–C, Low-BF units (400 Hz). E–G, Middle-high-BF units (2200 Hz). A, PSTHs at the best ITD. B, PSTHs at the worst ITD. C, The ITD-tuning curves. E–G correspond to A–C. The error bars are SD in C (n = 2 repeats). D, H, Normalized peak–trough contrasts correspond to Figure 2, A and B. The maximum SPL applied to each unit is indicated by open symbols. I, Suppression of spike firing rate at the best and the worst ITD. Some units in the control low BF showed almost no spikes at the worst ITDs and were evaluated only at the best ITDs. J, The normalized peak–trough contrast plotted against BF for units after SON lesion, evaluated at the maximum SPL applied to the unit (D, H, open symbols). The plots for the control units (dots) are the same as in Figure 2C. Different symbols indicate SPL. The thick line is for the SON lesion; y = −0.25 × log(BF) + 2.06; r = 0.88; *p < 0.05; **p < 0.01; n = 13; and the thin line is for the control. Error bars indicate SEM.
- Figure 6.
Rate threshold and vector strength. A, Rate threshold of units in control and after SON lesion. In low-BF neurons, the rate threshold was reduced significantly after SON lesion. The numbers in parentheses are the numbers of units, and ** indicates the statistical significance of p < 0.01 (t test). The diamonds indicate the mean from young adult chickens. Rate thresholds were not different in the control between the young adult and the juvenile chickens: p = 0.28 for low-BF and p = 0.23 for middle-high-BF units. B, VS from all units recorded. VSs were calculated from unit activities at the best ITD at the maximum SPL applied in each unit when phase-locking was significant (Rayleigh test; p < 0.05) and were not different between the control and the units after SON lesion (t test; p > 0.15 for all BF range). Bin width for statistics was set as follows: <300, 400, 600–800, 1000–1600, and >2000 Hz. Error bars indicate SEM.
- Figure 7.
Responsiveness to ITD and the effects of SON lesion. A, ITD-tuning curves of best contrast for four units shown in Figures 1, C and H, and 5, C and G, for ITD of ±500 μs. The firing rate was normalized by the maximum firing rate of each unit. The broken lines are for middle-high-BF units (BF ≥ 1 kHz), and the solid lines are for low-BF units (BF < 1 kHz). Controls are in blue, and after SON lesions are in red in A and B. The shaded area indicates ±100 μs ITD to measure the responsiveness. Contra., Contralateral; Ipsi., ipsilateral. B, Responsiveness to ITD from units in Figures 2, A and B, and 5, D and H (27 units from the juvenile chickens for both control and SON lesion and 7 units from the young adult chickens for control). The triangles are from the units in A. The vertical dashed line at 1 kHz indicates the boundary between the low and middle-high BF. C, Mean responsiveness to ITD for units in B. The diamonds indicate the mean from young adult chickens. *p < 0.05. Error bars indicate SEM.
- Figure 8.
Sound-phase dependency of low-BF unit activity. A, B, Period histograms (left), ITD-tuning curve, vector strength, and the best phase (right, from the top to the bottom) for a control in A and after SON lesion in B. A and B are the units shown in Figures 1C (98 dB SPL; BF, 200 Hz) and 5C (38 dB SPL; BF, 400 Hz), respectively, at the SPLs of best peak–trough contrast. A period histogram was calculated at each ITD (indicated in each plot) in reference to the phase of contralateral sound. At the bottom (A, B, left), all unit activities were plotted regardless of ITD. Vector strength was calculated when the phase-locked firing was significant (Rayleigh test; p < 0.05). Contra., Contralateral; Ipsi., ipsilateral.
- Figure 9.
Simulation of EPSPs, period histograms, and ITD-tuning curves. A, Period histograms with inhibition (left as a control, shaded column) and without inhibition (right as after SON lesion, open column) were calculated for a unit of BF 500 Hz. At the top, bilateral stimuli were applied in-phase, without ITD. Overlaid EPSPs were calculated at the soma. The histograms at the bottom show all the firing activities regardless of ITD. The averaged conductance for the two EPSCs was 0.09 nS for the calculations. This conductance was selected to mimic the quasi-maximum sound intensity level we applied in the experiments. SON lesion was mimicked by setting the inhibitory conductance to zero, and making the ipsilateral EPSP larger than the contralateral EPSP by a factor of 2; contralateral, 0.06 nS, versus ipsilateral, 0.12 nS. B, Amplitude (top) and half-peak width of ipsilateral EPSP (bottom) were calculated at the soma from a series of 40 simulations. C, ITD-tuning curves were simulated for the control (blue) and for the SON lesion (red) by the same set of parameters in A and plotted as a function of interaural phase difference. Averaged EPSP conductance was indicated. A horizontal broken line indicates the spontaneous firing rate. D, Normalized peak–trough contrast as a function of averaged conductance of bilateral excitatory synaptic inputs, when inhibition was included (●), and inhibition was excluded (open symbols). Different open symbols indicate different levels of imbalance of EPSP conductance, 1:1 for contralateral (contra) to ipsilateral (ipsi), ○; 1:2, ▵; 1:3, □.
Additional Files
Supplemental Data
Files in this Data Supplement:
- supplemental material - Supplemental Legend
- supplemental material - Supplemental Figure 1
- supplemental material - Supplemental Figure 2
- supplemental material - Supplemental Figure 3
