Hippocampal conjunctive and complementary CA1 populations relate sensory events to movement

Abstract

Hippocampal CA1 neurons respond to sensory stimuli during enforced immobility, movement, and their transitions in a new conveyor belt task. Head-fixed mice were exposed to light flashes or air streams while at rest, spontaneously moving, or running a fixed distance. Two-photon calcium imaging of CA1 neurons revealed that 62% of 3341 imaged cells were active during one or more of 20 sensorimotor events. Of these active cells, 17% were active for any given sensorimotor event, with a higher proportion during locomotion. The study found two types of cells: Conjunctive cells that were active across multiple events, and complementary cells that were active only during individual events, encoding novel sensorimotor events or their delayed repetitions. The configuration of these cells across changing sensorimotor events may signify the role of hippocampus in functional networks integrating sensory information with ongoing movement making it suitable for movement guidance.

Keywords: Health sciences; Neuroscience.

Graphical abstract

Figure 1

The experimental paradigm for studying hippocampal CA1 neuronal responses to sensory stimulation during immobility and locomotion (A) Experimental setup. A head-fixed mouse on a linear conveyor belt receives a mild air stream on the back to motivate running. (B) Experiment timeline and list of 7 configurations that mice experienced while calcium imaging was performed. Inset shows schematics of light and air trial structures. Light was ON for 0.2 s. AOn, AOff, and Arb indicate air onset, air offset, and arbitrary events. During NB condition, air was ON until the animal had covered 150 cm and in Configuration 4, light was applied at 110 cm from AOn as shown by vertical red line marker. (C) Average speed overall trials by animals around the stimulus and arbitrary (Arb) events during Configurations 3, 4, and 5. Magenta lines indicate occurrences of events. (D) Average distribution of speeds from 5 mice at events mentioned in C. The numbers indicate overall mean ± SEM speed. (E) Average image (left) of the calcium imaging time series from a representative animal, and arbitrarily color-coded regions of interest indicating identified cell bodies (right) in a ∼400 × 400 μm imaging window. (F) Normalized calcium traces (ΔF/F₀), from 100 representative cells from a representative animal for a recording of ∼25 min, overlayed with indicators of light and air stimuli (vertical-colored lines). Thick red lines indicate application of the Brake. Cells are arranged by time of peak firing. The speed signal is shown at the bottom (cyan color). The thick blue vertical line at far right is speed scale and indicates 30 cm/s. Note: some cells shown within red and blue ovals fire only during Brake and No-Brake conditions respectively. Thick black lines in (C) and (D) indicate mean over animals whereas shaded regions represent SEM.

Figure 2

More cells responsive during the No-Brake compared to the Brake condition for AOn, AOff, and Arb events (A) Raster plots and peri-event histograms of representative excited (Exc) and inhibited (Inh) cells showing increased and decreased firing, respectively, in response to AOn (t = 0, magenta line). The blue lines show the means across trials. (B) Graphs in the top two rows show trial-averaged animal speed (Sp) and quantification of brain motion (MC) for Brake versus No-Brake conditions for a representative animal. For the same animal, the third and fourth rows show heatmaps of rate vectors of cells responsive to AOn and corresponding population vector correlation maps respectively. The bottom row shows an average population correlation heatmap from 5 animals. (C–E) Results of two-way RM-ANOVA for assessing responsivity (Resp), response fidelity (RF), and mutual information Zscore (zMI) respectively, across brake conditions (B versus NB) and events (Aon, AOff, and Arb). (F–H) Results of three-way RM-ANOVA for assessing Resp, RF, and zMI respectively, across brake conditions, events, and cell types (Exc versus Inh). For all statistical tests, n = 5 mice. Error bars = SEM. ∗p<0.05, ∗∗p<0.01, ∗∗∗p<0.001.

Figure 3

Larger separation of cell subpopulations across the Brake (B) and the No-Brake (NB) conditions for AOn, AOff, and Arb events (A) Results of a two-way RM-ANOVA for assessing the percent of cells across population types (Comp1, Comp2, and conjunctive) and events (AOn, AOff, and Arb). (B) Results of a three-way RM-ANOVA assessing the percent of cells across cell types (Exc versus Inh) in addition to factors in (A). (C) Overall responsivity, conjunction, and complementation across B and NB conditions from pooled cells across event and cell types. The percent labels in the Venn diagram indicate the total percentages of responsive cells for Brake and No-Brake conditions. (D and E) Average and SEM (insets) heatmaps (over 5 animals) of the percentage of conjunction and complementation of Exc and Inh cell groups active in Brake and No-Brake conditions around AOn, AOff, and Arb events. Numbers in diagonal pixels represent the percentages of responsive cells for the selected row or column. (F–H) Dendrograms resulting from agglomerative hierarchical clustering of the average conjunction and complementation heatmaps/matrices. CC indicates cophenetic correlation. For all statistical tests, n = 5 mice. Error bars = SEM. ∗p<0.05, ∗∗p<0.01, ∗∗∗p<0.001.

Figure 4

Larger separation of cell subpopulations across the Brake and No-Brake conditions that responded to a light stimulus (A) Raster plots and peri-event histograms of representative Exc and Inh cells. The magenta line at t = 0 indicates light onset. The blue lines show means over trials. (B) Graphs in the top two rows show trial-averaged animal speed (Sp) and quantification of brain motion (MC) for Brake versus No-Brake conditions for a representative animal. For the same animal, the third and fourth rows show heatmaps of rate vectors of cells responsive to light and corresponding population vector correlation maps respectively. The bottom row shows the average population correlation heatmap from 5 animals. (C–E) Results of one-way RM-ANOVA assessing responsiveness, response fidelity, and mutual information Zscore of Exc and Inh cells for light stimulus in Brake condition (B-L). (F–H) Results of two-way RM-ANOVA assessing response variables for Exc and Inh cells in the No-Brake conditions for light stimulus combined with air (NB-AL) and identical Arb event where only air was present (NB-A). (I) Overall responsivity, conjunction, and complementation for B-L, NB-AL, and NB-A cell populations. The percent labels in the Venn diagram indicate the total percentages of responsive cells. (J) Results of three-way RM-ANOVA for assessing the percent of cells across population types (Comp1, Comp2, and Conj), cell types (Exc versus Inh) and event pairs e.g., B-L::NB-AL. For all statistical tests, n = 5 mice. Error bars = SEM. ∗p<0.05, ∗∗p<0.01, ∗∗∗p<0.001.

Figure 5

Separate subgroups of cells are active for distinct sensorimotor events (A and B) Average and SEM (insets) heatmaps (over 5 mice) of the percentage of conjunction and complementation of cell groups active in the Brake and No-Brake conditions around sensory air, light, and Arb events. (C–E) Dendrograms resulting from agglomerative hierarchical clustering of the average conjunction and complementation heatmaps/matrices. (F–H) Dendrograms resulting from agglomerative hierarchical clustering of the average conjunction and complementation heatmaps/matrices after excluding arbitrary events. CC indicates cophenetic correlation.

Figure 6

Larger differences in cell subpopulations active around spontaneous locomotion onset (MOn) and offset (MOff) events compared to other sensorimotor events (A) Raster plots and perievent histograms of representative Exc and Inh cells to MOn. The magenta line at t = 0 indicates motion onset. The blue lines show means over trials. (B) Graphs in the top two rows show trial-averaged animal speed (Sp) and quantification of brain motion (MC) for MOn versus MOff conditions for a representative animal. For the same animal, the third and fourth rows show heatmaps of rate vectors of cells responsive to MOn (and MOff) and corresponding population vector correlation maps respectively. The bottom row shows an average population correlation heatmap from 5 animals. (C–E) Results of two-way RM-ANOVA for assessing responsivity, response fidelity, and mutual information Zscore for MOn versus MOff and Exc versus Inh cells. (F) Overall responsivity, conjunction, and complementation for Brake, No-Brake, and Motion conditions. The percent labels in the Venn diagram indicate the total percentages of responsive cells. (G) Results of two-way RM-ANOVA for assessing the percent of cells across population types (Comp1, Conj, and Comp2) and condition pairs e.g., Brake and Motion (B-M). (H and I) Dendrograms resulting from agglomerative hierarchical clustering of the average conjunction and complementation heatmaps/matrices when MOn and MOff events were included. CC indicates cophenetic correlation. For all statistical tests, n = 5 mice. Error bars = SEM. ∗p<0.05, ∗∗p<0.01.

Figure 7

Adjacent trial-wise dynamics of cellular responses and conjunction/complementation in cellular populations (A) Percent of active cells (firing rate >0) over trials for 20 types of cellular responses to different sensorimotor events exhibited across the 7 behavioral configurations. (B) Percent of complementary and conjunctive cells for pairs of adjacent trials. Pairs 1–9 correspond to Trials 1–2, 2–3, 3–4, etc. up to 9–10. (C) Result of two-way RM-ANOVA assessing responsivity across 20 events and 10 trials. (D–F) Results of two-way RM-ANOVA assessing the percent of conjunction and complementation across 20 events and 9 trial-pairs. For all statistical tests, n = 5 mice. Error bars = SEM. ∗p<0.05, ∗∗p<0.01.

Figure 8

Trial-pair-wise dynamics of conjunction and complementation of cellular populations for all 20 sensorimotor events (A) Heatmap of the average percentage of conjunctive cells (n = 5 mice) between trials from all 20 events. The 200 × 200 pixels (off diagonal) belong to trial-pairs from all 20 events, as shown by the x and y labels. Each big box enclosed by thin black lines shows 10 × 10 trial pairs of the given event. For example, the color value of the pixel in row 1, column 2 indicates the percentage of conjunctive cells for trial 1 and 2 of presentation of the light stimulus. (B) Heatmap of the average percentages of complementary cells (n = 5 mice) between trial pairs (like A). Here, for example, the color value of the pixel in row 1, column 2 indicates the percentage of cells present in trial 1 and not present in trial 2. (C–E) Dendrograms resulting from agglomerative hierarchical clustering of the average conjunction and complementation heatmaps/matrices. (F–H) Dendrograms resulting from agglomerative hierarchical clustering of the average conjunction and complementation heatmaps/matrices after excluding arbitrary events. CC indicates cophenetic correlation.