Neuroscientists can now capture millisecond electrical changes in neurons in the cortex of an alert mouse, allowing tracing of neural signals, including subthreshold events, in the brain. The new technique combines all-optical scanning with two-photon fluorescence imaging to produce a 2D rasterized picture every 1,000-3,000 milliseconds. That and another technique that allows 3D imaging of large areas of the mouse cortex to a depth of 650 microns will aid study of neural circuits.