Awake hippocampal reactivations project onto orthogonal neuronal assemblies

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Science  16 Sep 2016:
Vol. 353, Issue 6305, pp. 1280-1283
DOI: 10.1126/science.aaf3319

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  1. Fig. 1 Imaging hippocampal dynamics in the CA1 region of awake mice alternating between running and rest.

    (A) Recurring sequences of neuronal activation during mouse runs. Mouse speed (top row), heat map raster plot of single cell fluorescence signal (middle), and summed activity of cells (over a 200-ms window). Red dashed line indicates significance threshold for synchronous activity detection (five cells in this example); ripple band power (120 to 220 Hz, bottom row) is displayed over time. (B) Contour map of the neurons imaged in (A) during immobility. Cells shown in red were active during one synchronous calcium event [SCE, gray box from (A)]; all other active cells are shown in gray. Scale bar: 100 μm. (C) Sharp wave–associated ripple (SWR) oscillation co-occurring with the event outlined in (A) (black: raw LFP trace, power spectrogram). (D) Co-occurrence rate of contralateral ripples with SCE and vice versa measured during immobility. (E) Raster plot (left) of cell activation during 21 successive SCEs. Neurons were ordered according to their activation onset during the RUN sequence. Identified replay events (ordered reactivation of neurons) are indicated within a 500-ms window (red: forward replay; blue: backward replay). Right: Fraction of SCEs identified as forward or backward replay across all imaging sessions.

  2. Fig. 2 SCEs segregated into functional cell assemblies.

    (A) Raster plot of all SCEs, within one representative imaging session, sorted by cell assemblies. SCEs displaying ordered sequential replay are indicated in red. (B) Contour map of the imaged cells; scale bar: 100 μm. Filled colored contours indicate different cell assemblies; filled gray contours indicate other active neurons. (C) Number of cell assemblies recruited during each SCE, pooled across all sessions where RUN sequences could be observed. (D) Top: Representative contour maps depicting two assemblies (filled in red and blue, respectively) from one day to the next. Bottom left: Contour map of the cells that remained in the same assembly across consecutive days days; scale bar: 100 μm. Bottom right: Box plot depicting the probability that a given cell pair is active on two consecutive days for cell pairs that do or do not belong to the same assembly on the first day. (E) Temporal dynamics of cell assemblies. Shortly after the activation of an assembly (t = 0), the probability that it was reactivated within a 4-s time interval (black) was lower than for a different assembly (gray); occurrences are normalized; the expected value for a random activation is 1.

  3. Fig. 3 Discrete segments of RUN sequences were reactivated during SCEs.

    (A) Raster plot displaying neuronal activation over time for a representative imaging session. Onsets of calcium transients were color-coded according to their cell assembly affiliation (labeled A to D). SCEs significantly recruiting one or two assemblies were labeled with the respective letter. RUN sequences were labeled ABCD. (B) Probability distribution pooled across sessions of the number of RUN sequence segments reactivated within SCEs. (C) Raster plot of all SCEs sorted by cell assemblies. Cells were color-coded according to their activation order within RUN sequences. (D) Probability distribution for “multiple-assembly SCE”: (i) to reactivate RUN sequence segments (multiple segments); (ii) to activate “nonreplay” assemblies (no segment); or (iii) a combination of a RUN sequence segment and a “nonreplay” assembly (single segment). Experimental probabilities were tested against random data.

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