Figure 9. Intrinsic topographical organization of slow and fast gamma in the subiculum. A, Diagrammatic representation of the whole hippocampus with arrows indicating the axes within which theta, SG, and FG coherence was measured. S, Septal; T, temporal. B, The topography of theta, SG, and FG. Coherence was measured in the proximal–distal (blue), septal–temporal (red), and deep-superficial subicular axes (black). Coherence is plotted as a function of absolute distance. #p < 0.05, *p < 0.01. Note that SG coherence is reduced with distance but is still above the significance threshold (the color above each point indicates the post hoc comparison between the deep-superficial axis and the other color-coded axes). FG coherence drops below the significance level at distances >300 μm. The dashed line is the 95% significance threshold. C, Spike–LFP phase locking between local and distant (1 mm) regions of the subiculum. i, Diagrammatic representation of the experiment in which LFPs from local and distant electrodes were recorded together with spikes next to the local electrode. ii, The phase-locking strength (mean resultant length) is shown and compared between the local LFP electrode and the distant LFP electrode for SG. Filtered LFP traces are shown for SG from the local and distant electrode. All spikes were phase locked to the local SG, and 13 of 21 cells were phase locked to SG in the distant subicular electrode. iii, The same as in ii for FG. Note how the three spikes fire on a similar phase of the local FG but on different phases of FG from the distant electrode because of a slightly slower FG network frequency. D, On the left is a representative histological section showing the location of electrodes located in the proximal and distal subiculum. The mean normalized theta MI plots are shown for the proximal and distal subiculum. The proximal and distal subiculum prefer different ratios of SG and FG, and the distal subiculum has a greater preference for FG whereas the proximal subiculum preferred SG. Black lines are the mean, and gray shading indicates the SE. E, An example phase–amplitude modulation index plot is shown for simultaneously recorded field potentials in the proximal and distal subiculum, demonstrating that the distal subiculum exhibits a higher FG frequency.