Figure 3.
Interspike interval characteristics of anteroventral neurons. A, Two-dimensional log-scale ISI scatter plots for HD unit. The abscissa indicates the interval to the previous action potential, and the ordinate indicates the interval to the next one. The points below the dashed horizontal red line represent the action potentials within the 4 ms range that defines bursting mode. The ISI plot on the right presents the action potentials from the central cluster (marked with red). Bottom, Sample recordings of the same head direction unit for 1 s duration (left) and 500 ms duration (right; marked with red). B, C, ISI plots for all spikes (left) and for the main clusters (right) of two HD-by-theta units. The central cluster is marked in red, with two further clusters (purple and green). Sample recordings of the same cell with 1 s duration (bottom, left) and 500 ms duration (bottom, right). Note that the spikes from the green ISI cluster in B take first position in the spike trains, while the spikes from the purple ISI cluster are positioned last. The intermediate spikes correspond to the points from the central red ISI cluster. D, Comparison between the relative number of spike trains between HD-by-theta and HD cell groups (left; t test, p < 0.01). Right, The ratio between additional green/purple clusters and central red cluster (ISI ratio) for the anteroventral HD-by-theta and HD cell groups (right; t test, p < 0.001). Error bars indicate SEM. **p < 0.01; ***p < 0.001. E, Three examples for all spike ISI histograms of HD units. F–H, ISI histograms of HD-by-theta units for all spikes (F), for the main ISI clusters (G), and when filtered for compact spike trains (H).