Abstract: An apparatus and method for detecting functional cellular activity within a volume of a tissue. The method includes inserting a three-dimensional array of optical emitters and optical detectors into a volume of a tissue, where the tissue volume includes one or more cells labeled with an optical reporter of cellular activity; illuminating the one or more cells with photons from the optical emitters of the three-dimensional array to generate optical signals from the optical reporter that labels the one or more cells; and detecting the optical signals using the optical detectors of the three-dimensional array, where the illumination includes one-photon excitation of the optical reporter.