To image both the structure and electrical activity, we developed an integrated OCT/OM system (the schematic diagram is in Fig. 1). The custom-built OM system14 utilized a broadband light-emitting diode light source (SOLA Light Engine, Lumencor, Beaverton, Oregon), which provides high intensity, low noise, and broadband illumination. A filter and dichroic mirror set (Chroma Technology, Bellows Falls, Vermont) was used with an excitation wavelength passband of , an emission wavelength passband of , and a dichroic mirror with cutoff wavelength of 560 nm. A low noise, high dynamic range EMCCD camera (iXon3 860, Andor Technology, South Windsor, Connecticut), and an upright microscope (Axio Scope.A1, Carl Zeiss AG, Oberkochen, Germany) were used for signal recording. Demagnifying optics (0.33X, 0.5X, or 0.63X) were utilized to control the field of view. The OM system records pixels at a maximum frame rate of 500 Hz. Depending on the field of view required by the sample, either a 10X objective lens (, C-Apochromat 10X/0.45 M27, Carl Zeiss AG, Oberkochen, Germany) or a 5X objective lens (, Fluar 5X/0.25 M27, Carl Zeiss AG) was employed. The OM imaging system was placed above an imaging chamber, which uses a coverslip as the bottom to hold the sample.