Genetically-encoded calcium indicator (GCaMP)

A fluorescent protein sensor — calmodulin fused to a circularly-permuted GFP — whose brightness rises when intracellular calcium increases, serving as an indirect, temporally filtered proxy for spiking. GCaMP is the workhorse of optical read-out because it is bright, genetically targetable to defined cell types, and stable for months of chronic imaging. The frontier lies in faster and more linear variants (the jGCaMP8 series) whose rise times approach the millisecond scale, improving spike-timing inference.

The fundamental limitation is that calcium is a low-pass, nonlinear surrogate for membrane potential: even the fastest indicators have decay times of tens to hundreds of milliseconds, so they blur high-frequency firing and report neither subthreshold voltage nor precise spike times. Recovering spike trains from calcium traces is an active statistical problem, and any reported activity reflects the indicator's kinetics as much as the neuron's.