Figure 5. Theta cycle skipping in CA1 and PFC.

(A) Spike rasters of example cells during CA1 theta oscillations. Top: a non-skipping (firing on adjacent cycles) CA1 cell; Middle: a cycle-skipping (firing on every other cycles) CA1 cell; Bottom: a cycle-skipping PFC cell. Black and green lines: broadband and theta-filtered LFPs from CA1 reference tetrode, respectively. (B) Auto-correlograms (ACGs) of three example single cells in CA1 (left) and PFC (right). Each plot is of data from a single type of maze travel (outbound or inbound; travel type denoted). For each plot, cycle skipping index (CSI) is denoted on the upper left corner (CSI < 0: firing on adjacent cycles; CSI > 0: cycle skipping), and cell number with maze travel type (IN or OUT) matched to (A) is denoted on the upper right corner. Red line: low-passed (1–10 Hz) ACG to measure CSI (see Materials and methods). Note that cells on the bottom two rows exhibit cycle skipping, with CSI > 0. (C) ACGs of all theta-modulated cells in CA1 (left) and PFC (right) ordered by their CSIs (high to low from top to bottom). Red arrowheads indicate division between cells with CSI > 0 (above) vs. <0 (below). Each row represents a single cell with one type of maze pass (outbound or inbound). Only cells with theta-modulated ACGs are shown (see Materials and methods). Note that the proportion of theta-cycle skipping cells in CA1 is consistent with that reported in previous studies (Kay et al., 2020). (D) CSI of theta-modulated cells didn’t differ significantly on outbound versus inbound trajectories for each region (n.s., p’s > 0.99 for CA1 and PFC), but was larger in PFC than CA1 (****p<1e-4, Kruskal-Wallis tests with Dunn’s post hoc). Data are presented as mean and SEMs.