Calcium imaging is a scientific technique usually carried out in research which is designed to show the calcium (Ca²⁺) status of a cell, tissue or medium. Calcium imaging techniques take advantage of so-called calcium indicators, fluorescent molecules that can respond to the binding of Ca²⁺ ions by changing their fluorescence properties. Two main classes of calcium indicators exist: chemical indicators and genetically encoded calcium indicators (GECI). Calcium imaging can be used to optically probe intracellular calcium in living animals. This technique has allowed studies of Ca²⁺ signalling in a wide variety of cell types and neuronal activity in hundreds of neurons and glial cells within neuronal circuits .