Probing Deep Brain Circuitry: New Advances in in Vivo Calcium Measurement Strategies. 
The study of neuronal ensembles in awake and behaving animals is a critical question in contemporary neuroscience research.  Through the examination of calcium fluctuations, which are correlated with neuronal activity, we are able to better understand complex neural circuits.  Recently, the development of technologies including two-photon microscopy, miniature microscopes, and fiber photometry has allowed us to examine calcium activity in behaving subjects over time.  Visualizing changes in intracellular calcium in vivo has been accomplished utilizing GCaMP, a genetically encoded calcium indicator.  GCaMP allows researchers to tag cell-type specific neurons with engineered fluorescent proteins that alter their levels of fluorescence in response to changes in intracellular calcium concentration.  Even with the evolution of GCaMP, in vivo calcium imaging had yet to overcome the limitation of light scattering, which occurs when imaging from neural tissue in deep brain regions.  Currently, researchers have created in vivo methods to bypass this problem; this Review will delve into three of these state of the art techniques: (1) two-photon calcium imaging, (2) single photon calcium imaging, and (3) fiber photometry.  Here we discuss the advantages and disadvantages of the three techniques.  Continued advances in these imaging techniques will provide researchers with unparalleled access to the inner workings of the brain.