A revolution in optical imaging techniques has made it possible to visualize the activity pattern of large populations of individual neurons, as well characterize their cell-type specificity. This is due to the development of multi-photon imaging in combination with highly sensitive activity-dependent fluorescent markers.
2-photon imaging is now sufficiently powerful to image down to the deep layers of the cortex in small animals such as mice, but is still somewhat limited for larger animals. Reaching down the deep layers of the cortex in macaque monkeys is potentially possible by going to 3-photon imaging.
The neighboring image shows a neuronal population in mouse auditory cortex, expressing a functional indicator (GCaMP6s, in green) and structural indicator for inhibitory neurons (tdTomato, in red), imaged at a depth of ~600um with our 3-photon microscope in the DIP.