We directly compared cellular fluorescence changes and spiking using loose-seal, cell-attached recordings. The contrast of the visual stimulus was adjusted online to maintain a moderate spike rate. GCaMP6s produced large fluorescence transients even in response to single action potentials (> 6 times larger than for GCaMP5K, Fig. 3 and Supplementary Video 1), yielding high detection rates for single spikes (99 ± 0.2%; at 1% false-positive rate, n=9 cells, 250 spikes). GCaMP6f and GCaMP6m showed slightly lower spike detection efficiencies, but with faster kinetics (Fig. 3, Supplementary Table 3). Individual spikes within a burst resulted in step-wise fluorescence increases (Fig. 3b), which were resolvable if they were separated by an interval on the order of the rise time of the sensor or more (100-150 ms, GCaMP6s; 75-100 ms, GCaMP6m; 50-75 ms, GCaMP6f; Fig. 3f, Supplementary Fig. 10, Supplementary Table 3). These data show that GCaMP6s can detect activity with near 100% action potential detection in pyramidal cells.
