Fig. 1.

Spatiotemporal dynamics of reverberatory activity in neuronal networks. A Ca²⁺ imaging of a complete neuronal network. Green pseudocolor shows saturated Ca²⁺ fluorescence of all cells in the network activated by high K⁺ treatment at the end of the experiment. Red pseudocolor shows an average of background-subtracted Ca²⁺ images (ΔF) acquired within 100 ms after the stimulus, indicating neurons that were initially activated during this period. Scale bar, 200 µm. B Network reverberation evoked by single-pulse stimuli to a voltage-clamped neuron, indicated by an arrowhead in A. Stimulation pulses (100 mV, 1 ms) were given every 30 s at the time indicated by the black arrowhead. Color codes for the amplitude of polysynaptic currents (nA). C Polysynaptic current trace recorded from the reverberatory network (trial 1 in B). Scale bars: 1 s, 300 pA. D An enlarged segment of the trace in B to show seven periodic reverberatory events (upper) and a corresponding number of firing neurons (lower). Scale bars: 100 ms, 300 pA, 20 neurons. E Normalized activation signal (color-coded) of all 67 neurons in the network during the period shown in D. Similar firing patterns are visualized. F Example traces showing the inference of spike timing. In another network, during a reverberatory episode evoked by single-pulse stimulation (upper), a short segment of simultaneously recorded Ca²⁺ fluorescence signals (ΔF/F₀) from four neurons (middle; colored dots), which are fitted locally (colored curves). Precise spike timings were determined from fitting results (lower). Short vertical bars below the traces show the corresponding timing of detected spikes. Scale bars: 500 ms, 0.5 nA (upper); 50 ms, 0.5 ΔF/F₀ (middle). G All 27 consecutive reverberatory events from the reverberatory episode corresponding to the top panel shown in F sliced to 50-ms sections. Color codes for the normalized convolved spike trains of all neurons. H Similarity indexes (SI) of all reverberatory event pairs during the reverberatory episode. I Schematic showing how shuffling and jittering were applied. Spikes were shuffled or jittered before convolution was applied.