Mouse visual neocortex supports multiple stereotyped patterns of microcircuit activity

Abstract

Spiking correlations between neocortical neurons provide insight into the underlying synaptic connectivity that defines cortical microcircuitry. Here, using two-photon calcium fluorescence imaging, we observed the simultaneous dynamics of hundreds of neurons in slices of mouse primary visual cortex (V1). Consistent with a balance of excitation and inhibition, V1 dynamics were characterized by a linear scaling between firing rate and circuit size. Using lagged firing correlations between neurons, we generated functional wiring diagrams to evaluate the topological features of V1 microcircuitry. We found that circuit connectivity exhibited both cyclic graph motifs, indicating recurrent wiring, and acyclic graph motifs, indicating feedforward wiring. After overlaying the functional wiring diagrams onto the imaged field of view, we found properties consistent with Rentian scaling: wiring diagrams were topologically efficient because they minimized wiring with a modular architecture. Within single imaged fields of view, V1 contained multiple discrete circuits that were overlapping and highly interdigitated but were still distinct from one another. The majority of neurons that were shared between circuits displayed peri-event spiking activity whose timing was specific to the active circuit, whereas spike times for a smaller percentage of neurons were invariant to circuit identity. These data provide evidence that V1 microcircuitry exhibits balanced dynamics, is efficiently arranged in anatomical space, and is capable of supporting a diversity of multineuron spike firing patterns from overlapping sets of neurons.

Keywords: circuitry; connectivity; cortex; graphs; two-photon; visual.

Figure 1.

Visual cortex is capable of spontaneous circuit activity. A, Experimental preparation of a 1.1 mm diameter field of view of a V1 slice. B, Identified cells and single imaging frame example of activity from A. Active cells are in red. C, Spike raster of a single circuit event. D, Multiunit average of above raster. E, Single neuron whole-cell patch-clamp example of a visual neuron in an upstate during a circuit event. Action potentials have been truncated for presentation. F, Representative raster (quiescent intervals between events removed) of 14 circuit events observed in a single visual field of view. For each cell (n = 613), a black tick mark indicates a detected spike within a 72.8 ms imaging frame. G, Top, Each data point (red star) represents a single circuit event. Bottom, Each data point (blue x) represents a single circuit event. H, Plots showing cumulative firing across multiple circuit events in rate-matched Poisson (left) and V1 data (right). Each line represents a separate circuit event. Line color is normalized to event durations from short events (cool colors) to long events (hot colors).

Figure 2.

Circuit dynamics and functional graph properties. A, Left, Single example of the degree ratio of various neurons physically arranged in a field of view from V1 data, and Erdős–Rényi random graphs. Functional connectivity is displayed with pial surface toward the upper left (dashed line). Nodes represent cells and are colored by the degree ratio of that node as indicated in the key. Functional edges are drawn between nodes. For graphical display reasons, only edges with weight ≥ 0.2 are shown. All cells shown, even those without drawn edges, have at least 1 edge when weight constraints are removed. Right, Boxplot representing the significant difference (p = 8.1 × 10⁻⁵) in the average node ratio per dataset across all datasets for all edge weights (V1) and Erdős–Rényi random graphs (V1RAND). B, Left, Graphs showing the functional connectivity on a physical layout of neurons with pial surface toward the upper left (dashed line) for a single example dataset and single corresponding Erdős–Rényi and permuted graph. As in the inset: red represents cycle edges; teal represents nodes in cycles; gray represents edges and nodes without cycles. The inset would have a network flow hierarchy value of 2/5, or 0.4. For display reasons, only edges with weight ≥ 0.2 are shown. All cells shown, even those without drawn edges, have at least 1 edge when weight constraints are removed. Right, Boxplot representing the average feedforward value across Erdős–Rényi (V1RAND), data (V1), and permuted (V1PERM) datasets for all edge weights. Areas are significantly different from one another (Kruskal-Wallis: p = 2.6 × 10⁻⁶). C, Single partition Rentian scaling sample embedded into physical V1 slice space. Red dashed box indicates an example Rentian partition. Circles represent neuron centroids: blue if outside partition, red if inside. Gray lines indicate edges that transverse partition. Partition contains 23 nodes with 142 edges. Analysis is overlaid on two-photon slice image. D, Log–log plot of relationship of nodes to edges for multiple sized partitions. Each blue star represents a partition, with the partition in C indicated by an arrow. Red line indicates a linear fit (R² = 0.91, Rentian exponent [slope of line] = 0.82).

Figure 3.

Small clusters define V1. A, Examples of six spatially different circuit activations in the same slice. B, Histogram of circuit event sizes showing small size bias. C, Comparison of number of fuzzy cluster-derived circuit clusters observed in V1 compared with a randomized null hypothesis (p = 6.7 × 10⁻⁷).

Figure 4.

Spatial clusters exhibit different firing trajectories. A, Anatomical spatial representation of clusters. Left, Union of all cells in cluster 1 that are unique to cluster 1 indicated as green-filled cellular contours. Right, Union of all cells in cluster 2 that are unique to cluster 2 indicated as blue-filled cellular contours. Middle, Intersection of cells in cluster 1 and cluster 2 indicated as dark blue-filled cellular contours. B, Same cluster sorting of spike times. Left, Firing of circuit events in cluster 1 (n = 4 events, 511 cells) sorted by mean cluster 1 firing times. Right, Firing of circuit events in cluster 2 (n = 2 events, 353 cells) sorted by mean cluster 2 firing times. Frame duration = 89 ms. C, Alternate cluster spike time sorting. Firing of circuit events in cluster 1 sorted by mean cluster 2 firing times for shared neurons (n = 258). Right, Firing of circuit events in cluster 2 sorted by mean cluster 1 firing times for shared neurons (n = 258).