Fiber photometry

A bulk optical read-out in which a single implanted optical fiber both delivers excitation light and collects the summed fluorescence of a genetically-encoded indicator from the population of cells around its tip. It trades away single-cell resolution for simplicity, chronic stability and full compatibility with freely-moving animals, yielding a population-averaged activity signal — often from a genetically defined cell type or projection — analogous to a bulk calcium-based field signal.

Because it averages over many cells, fiber photometry cannot resolve individual neurons or their tuning and is low-bandwidth as an interface. Motion, hemodynamic absorption and bleaching create artifacts that must be removed with an isosbestic reference wavelength or a second control channel. It is nonetheless a robust systems-neuroscience workhorse for asking whether a defined population is active during a behavior.