(A) Analysis of the onset kinetics of large-amplitude dendritic events. A1. Left, experimental configuration showing dendritic whole-cell recording with presynaptic stimulation of the PP. Right, traces of dendritically recorded voltage in response to a single burst of presynaptic TBS from two different neurons with large dSpikes in control (same as the traces in Figure 2A, black). Purple arrows indicate a large dSpike in the early phase of each burst response (purple), which had a more gradual onset (insets, purple trace) and a lower apparent voltage threshold. In contrast, a bAP in the late phase of the burst response (red) had a sharp kink at its onset (inset, red arrow and trace) and a higher apparent voltage threshold. A2. Phase plots (dV/dt vs V) of the events color-coded in A1. Insets show a linear fit (dotted line) to the initial phase of the large dSpikes (purple) and the bAP (red); ‘initial phase slope’ was measured as the slope of the fit. This measure represents the apparent voltage sensitivity of membrane current at the onset of the spike (see ‘Materials and methods’). (B) Distinguishing between bAPs and large dSpikes on the basis of apparent voltage threshold and initial phase slope. B1. Scatter plot of initial phase slope vs apparent voltage threshold of large-amplitude dendritic events. Gray areas illustrate gaps in the distribution of both measures. The events in the bottom-left quadrant (the red circle and the black square, same as the ones indicated by the purple arrows in the top and the bottom traces in A1, respectively; see also Figure 2A) were identified as large dSpikes. The events in the top-right quadrant (such as the one indicated by ‘c’) were identified as bAPs. The events in the other two quadrants (indicated by ‘a’, ‘b’, ‘d’, ‘e’, and ‘f’) are ambiguous cases: those in the top-left quadrant (‘a’ and ‘b’) are most likely bAPs, because they have a large initial phase slope and are initiated in the falling phase of the dendritically recorded EPSPs; those in the bottom-right quadrant (‘d’, ‘e’, and ‘f’) are likely large dSpikes, because they have a small initial phase slope, and their broad width suggests a larger contribution from Cav and/or NMDAR channels. B2. Traces of the events indicated by letters in B1, shown together with the corresponding responses in 20 nM TTX.