subtraction

The passage around this formula

Analysis. What the microscope actually outputs is a time series of fractional fluorescence change, conventionally reported as $\frac{\Delta F}{F} = \frac{F(t) - F_0}{F_0}$. where $F_0$ is a baseline fluorescence estimated from a quiet period. Spikes are then inferred from this signal, not observed directly — calcium influx is slower than an action potential and low firing rates can fall below detection threshold, so a $\Delta$ F/F trace is a lossy, nonlinear encoding of spiking, not a direct spike train. Any claim about “single-spike resolution” from calcium imaging alone should be read skeptically unless the paper validates it against a simultaneous electrode recording.

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