Neural activity causes rapid changes in intracellular free calcium1-4. Calcium imaging experiments have relied on this principle to track the activity of neuronal populations5,6 and to probe excitation of small neurons and neuronal microcompartments2,7-10. Genetically-encoded protein sensors can be targeted to specific cell types2,9,11,12 for non-invasive imaging of identified neurons and neuronal compartments8,13-15 over chronic timescales6.
