Figure 6.
Distribution of threshold changes and their relationship to EPSPs. A, Left, Histogram of peak threshold drops (0.5 mV bins) and decay time constants (1 ms bins) from all cells (n = 32). Three outliers with decay time constants of 20.1, 30.2, and 47.7 ms are not shown. Right, Histogram of peak threshold rises (0.2 mV bins, n = 32) and decay time constants (20 ms bins, n = 30, excluding 2 cells with no detectable threshold rise). A drop in threshold invariably preceded any threshold rise. B, Peak threshold drop as a function of EPSP size, as measured by amplitude (left) or peak depolarization rate (middle), shown with linear regression p value and coefficient of determination. Red lines show linear fits. Right, Relationship between EPSP amplitude and peak slope. C, Timing relationship between EPSPs and threshold drops. Left, Example of EPSP-associated threshold drop, as in Figure 5B (different cell). Middle, Early phase of threshold drop plotted on an inverted scale with the EPSP slope (green) and voltage trajectory (blue), both derived from an average over trials that did not contain short-latency (<20 ms) spikes. Right, Histogram of time differences (0.5 ms bins) between EPSP peak slope and peak threshold drop for all cells (n = 32, like the 0.8 ms difference shown in the example).