Precise manipulation of neurons across cortical layers requires optical approaches that maintain single-cell specificity deep within scattering brain tissue. Two-photon optogenetics provides targeted photostimulation in vivo and has expanded experimental access to defined neuronal populations beyond the limits of single-photon excitation. However, its application remains largely confined to the upper cortical layers (∼250–300 µm) in mice due to scattering. Here we introduce three-photon temporally focused computer-generated holography (3P TF-CGH) for precise optogenetic activation beyond this depth limit. We characterize 3P excitation of multiple excitatory and inhibitory opsins in organotypic slices, demonstrating cubic power dependence, efficient photocurrents and preserved temporal fidelity. We demonstrated that temporally focused holography maintains tight axial confinement in scattering tissue and minimizes superficial out-of-focus excitation under conditions required for deep targeting. In vivo, we combined 3P holographic stimulation at 1700 nm with simultaneous 3P calcium imaging at 1300 nm to achieve reliable neuronal activation across cortical layers down to 800 µm. Photostimulation remained stable across repeated trials without detectable physiological perturbation. By extending cell-resolved optogenetic control well beyond the established depth limits of 2P approaches, 3P holographic optogenetics enables non-invasive, all-optical interrogation of deep cortical circuits.
Electrical signals in neurons underlie perception, movement, memory and behaviour, yet many unfold too rapidly to be captured by conventional optical imaging. Calcium imaging has transformed neuroscience but provides an indirect and relatively slow readout of electrical activity. By directly measuring membrane potential changes, voltage imaging enables millisecond-scale recording of action potentials, subthreshold dynamics and signal propagation across neural circuits. Recent advances in voltage-sensitive dyes and genetically encoded voltage indicators have made voltage imaging increasingly practical, motivating the development of fluorescence microscopy methods optimized for high-speed acquisition. However, voltage imaging remains constrained by trade-offs among imaging speed, spatial resolution, signal-to-noise ratios and photodamage. In this Review we discuss high-speed optical microscopy strategies that address these challenges and highlight the need for co-design among voltage indicators, imaging systems and computational analysis. The Review discusses recent advances in neural voltage imaging, with an emphasis on relevant microscopy techniques and outstanding challenges in the field.
