Extended Figure 5. Neurons active during Aversive encoding selectively participate in burst events offline.
A) Example of a burst event quantified in this figure. The top trace represents the z-scored mean population activity within one of the offline recordings. Three timepoints were chosen (overlaid in circles), the middle representing the peak of a burst event, and the timepoints to its left and right representing t-2sec and t+2sec from the peak, respectively. The bottom three matrices represent binarized spatial footprints depicting the spatial footprints of the cells sufficiently active to participate in a burst (z>2). The matrices represent the timepoints of the three datapoints above it, ordered by time.
B) Representative process of extracting ensemble participations (one mouse example). The left is an example burst period, with the rows in the heatmap representing the activity of the recorded cells during that session, binarized by z>2 and color-coded by whether they were previously active during Aversive encoding (Aversive ensemble, blue) or if they were not previously active (Remaining ensemble, grey). The black trace above represents the z-scored mean population activity during this period, demonstrating a brief burst in activity accompanied by participation by a significant fraction of neurons. On the right is an example non-burst period, where mean population activity remains below threshold.
C) Neuron activities were circularly shuffled 1000 times relative to one another and the mean population activity was re-computed each time. This shuffling method preserved the autocorrelations for each neuron while disrupting the co-firing relationships between neurons. The burst frequency was computed for each of these shuffles to produce a shuffled burst frequency distribution (gray histogram), to which the true burst frequency was compared (blue dotted line). This is an example mouse.
D) The mean burst frequency for the shuffled distribution was computed and compared to the true burst frequency for each mouse. True burst frequencies were greater than shuffled burst frequencies in every mouse (t7 = 6.159, p = 0.000463, N = 8 mice), suggesting that during the offline period, hippocampal CA1 neurons fire in a more coordinated manner than would be expected from shuffled neuronal activities.
E) As in Extended Figure 5C, neuron activities were shuffled, and mean population was re-computed each time. From this population activity trace, the skew of the distribution was computed. If there were distinct periods where many neurons simultaneously fired, we hypothesized that the true distribution of mean population activity would be more skewed with a strong right tail demonstrating large and brief deflections, compared to shuffled neuronal activities. We computed the skew of each shuffled mean population activity, to produce a distribution (gray histogram), to which the true mean population’s skew was compared (blue dotted line). This is an example mouse.
F) The mean skew for the shuffled distribution was computed and compared to the true skew of the mean population activity for each mouse. The true skew was greater than the shuffled skew in every mouse (t7 = 13.36, p = 0.000003, N = 8 mice), supporting the idea that the mean population activity undergoes brief burst-like activations requiring the coordinated activity of groups of neurons.
G) Matrix of burst events for an example mouse, stacked along the y-axis and centered on time t=0 (top), and the average mean population activity around each burst event (bottom).
H) As in Extended Figure 5G but averaged across all mice. Each thin line represents one mouse, and the thick black line represents the mean across mice with the grey ribbon around it representing the standard error (N = 8 mice). There is no periodicity to when these burst events occur.
I) Locomotion of an example mouse during each burst event stacked along the y-axis (top), and the mean locomotion around burst events (bottom). Mice showed a robust and brief slowing down ~1sec before each burst event, before increasing locomotion back up ~2sec later.
J) As in Extended Figure 5I but averaged across all mice. Each thin line represents one mouse, and the thick black line represents the mean across mice with the grey ribbon around it representing the standard error (N = 8 mice). This demonstrates a robust and reliable decrease in locomotion around the onset of burst events.
K) The burst event frequency decreased across the hour (F11,77 = 6.91, p = 5.66e-8, N = 8 mice).
L) A larger fraction of the Aversive ensemble vs the Remaining ensemble participated in each burst event (left) (t7 = 3.68, p = 0.0079, N = 8 mice).