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. Long-term imaging of the cortex in macaque monkeys becomes more reliable 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.
Cell-type specificity can be established post-mortem in macaque monkeys with antibody-staining in 1mm thick iDisco-cleared slices of cortical tissue and imaged with light-sheet microscopy.
The neighboring image shows blocks of macaque monkeys V1/V2, stained for the three main classes of interneurons (in collaboration with Nicolas Renier at ICM).