Imaging input and output dynamics of neocortical networks in vivo: Exciting times ahead

Many remarkable properties of neocortical networks are not detectable in the activities of single neurons. How the properties of single neurons and their synaptic connections combine to form networks that are capable of striking sensory processing and higher brain functions remains a major enigma. To understand how neuronal networks function, the activities of numerous single neurons must be studied simultaneously. The last few decades have seen the emergence of powerful functional imaging techniques that cover broad spatial and temporal scales (Fig. 1A), from single molecules to the intact human brain, which can be imaged noninvasively by techniques such as EEG, magnetoencephalography, and functional MRI. However, comprehensive understanding of neuronal computations requires spatial resolution at the level of single cells, and the speed of communication in these networks demands temporal resolution within milliseconds. As an additional imperative for understanding neuronal information processing, the inputs (synaptic potentials) must be distinguished from the outputs [action potentials (APs)]. Without such dissection, we cannot fully understand perception, higher brain functions, and behavior. Up to now, a comprehensive description of network input and output activity at the level of a single cortical neurons has not been possible. However, in a recent issue of PNAS, Kerr et al. (1) demonstrated that, by employing two-photon calcium imaging of bulk-labeled tissue, local input and output activities in the neocortex can be dissected in vivo. This approach should facilitate the exploration of basic mechanisms underlying neocortical development, function, and plasticity.