Figure 8. Correlated, high frequency synaptic activity underlies temporally sparse, sequential action potential patterns in HVC.

A, Top: sonogram of a single motif. Bottom: Examples of precisely timed membrane potential traces from different HVC PNs and a cell outside of HVC (HVC shelf) during singing; membrane potential records from two motif renditions are shown (light versus dark color), with different colors corresponding to different cells (HVC_RA (3) and HVC_RA (6) corresponds to cell #3 and #6 in Figure 8B). Occurrences of correlated synaptic onset timings are highlighted with vertical dashed lines. B, Motif-aligned sequential action potentials (middle) and synaptic onsets (bottom) in six identified HVC_RA neurons recorded from the same zebra finch; neurons with at least ten motif renditions are selected, with different colors depicting different neurons. C, Synaptic event timings between individual HVC projection neurons are all correlated (significantly non-zero); HVC_RA - HVC_RA pairs (red; Correlation Coefficient (CC) = 0.17 ± 0.04; n = 25 pairs, P < 0.001, t-test compared with shuffled data); HVC_X - HVC_X pairs (blue; CC = 0.11 ± 0.01; n = 177 pairs, P < 0.001); HVC_RA – HVC_X pairs (purple; CC = 0.14 ± 0.02; n = 137 pairs, t-test, P < 0.001). No significant difference was detected between different PN pairs (P > 0.15). No significant correlation in synaptic activity was detected between cell pairs involving HVC_RA cells and cells outside of HVC (Shelf, gray; CC = -0.02 ± 0.03; n = 15 pairs, P = 0.32). Dashed lines: 3SD from shuffled data. Error bars: SE. D, Averaged synaptic event rates obtained from the same bird showed striking similarity of input timings between two the projection neuron classes (HVC_RA and HVC_X cells). E, The temporal structure of synaptic input timings are highly similar in HVC_RA and HVC_X populations (purple; HVC_RA – HVC_X groups; n = 4 birds, CC = 0.41 ± 0.13, P < 0.001) but not in HVC_RA and Shelf populations (gray; HVC_RA – HVC shelf groups; n = 4 birds, CC = 0.01 ± 0.07, P = 0.95). Dashed lines: 3SD generated from shuffled data. F, Left: the magnitude-squared coherence spectrum calculated from pairs of HVC neurons (same color codes as in C) showed a broad peak ~ 50 Hz, which was not evident in HVC_RA - HVC shelf neuron pairs. Right: grouped average of magnitude-squared coherence calculated within HVC projection neurons (red) and between HVC projection neurons and HVC shelf neurons (black). G,H, Synaptic events precede sound onsets but not offsets in both HVC_RA and HVC_X cell types. dPSP peak at -37.6 ± 0.48 ms (n=21 HVC_RA), -33.1 ± 0.15 ms (n=95 HVC_X). Dashed lines are 2 SD of dPSP rate. A subset of HVC_X cell data was presented in [Hamaguchi and Mooney 2014].