Coming full circle: In vivo Veritas, or expanding the neuroscience frontier

The 25-century long history of brain science can be divided into four distinct 'Eras': Era 1 (∼2000 years) started around 500 BC with reductionist studies of human and animal brains using invasive in vivo and ex vivo methods; Era 2 (∼200 years) started in the 17^th century and introduced the first invasive methods to study the function of living tissues; Era 3 started around 1838 and is the ongoing era of cellular neurophysiology; finally, Era 4 (in statu nascendi) is the era of non-invasive, holistic yet mechanistic, studies of the brain. Animal experimentation is becoming increasingly more holistic as multimodal imaging and recording techniques are combined in a single experiment on the brain of awake behaving animal. This newly emerged approach can be called in vigilo (from Latin 'in awake' or 'in vigilant state'), by analogy to the earlier introduced terms in vivo or in vitro. We introduce the Special Issue "In Vigilo Veritas: New Frontiers of Optical Imaging and Electrical Recording in the Brain of Awake Behaving Mice", which features original research articles and reviews that represent some of the finest examples of a truly multimodal studies, where behavioural readouts and tasks are combined in the same longitudinal experiment and on the same mouse with the two-photon imaging, optogenetics and/or electrophysiological recordings. This exciting multi-methodological approach creates a fertile ground for breakthrough discoveries in neurophysiology and neuropsychology of an awake behaving mammalian brain.