Fig. 2: Spatial and temporal maps of dendritic spikes (dSpikes).

a Example traces at the soma (orange) and a distal dendrite ( > 300 µm; purple) showing trials without and with a dSpike (*) in response to stimulation at a proximal dendritic branch (D1_stim; 20 ms duration to trigger 2-3 bAPs at 5 Hz). b Distribution of bAP amplitudes at the soma (orange) and at a distal dendrite ( > 300 µm; purple). The amplitude in the soma had a unimodal distribution and the amplitude in the dendrites had a bimodal distribution. Black dotted lines indicate threshold between -dSpike vs. +dSpike. c Counting all bAPs (gray), and bAPs with dSpikes from the same cell (red), as a function of time after optogenetic stimulus onset. Blue bar shows the stimulus timing. d Left: 2 P structural image (gray) overlayed with fluorescence of eYFP (blue) indicating spatial distributions of the optogenetic stimulus. Right: normalized amplitude (∆F/F_ref) map for subthreshold depolarization (stimulus-triggered average from 54 trials). Amplitude heatmaps for the following panels share the same color scale. e Normalized amplitude (∆F/F_ref) map for back-propagating action potentials (bAPs) without dSpikes (-dSpikes; spike-triggered average from 56 spikes). Right: single-trial example of bAPs without a dSpike, sampled along the red line in (d) at the locations indicated by colored arrows. f Corresponding plots for bAPs with dSpikes ( + dSpikes). Normalized amplitude map, spike-triggered average from 8 spikes. Little red flecks at some of the branch tips are within the measurement noise. g Kymographs comparing single-trial instances of bAPs with ( + dSpike) and without (-dSpike) a dendritic spike along the red line in (d). The soma is at the top and the distal apical dendrites are at the bottom. Blue arrowhead indicates place and time of stimulus onset. h Amplitude profiles for each bAP along the red line in (d). Events that rise in the distal region indicate dSpikes. Plots were color-coded by the time of spike peak (at the soma) after stimulus onset.