In the substudies, brain activity of healthy human volunteers will be monitored by functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG), electroencephalography (EEG), or simultaneous MEG-EEG. Anatomical MRI will be collected in some subjects to allow better localization of brain dynamics. Because transcranial direct-current stimulation (tDCS) is ideally suited to modulate the slow component of scale-free brain activity, we will also investigate the effect of tDCS on brain activity and/or behavioral performance in cognitive tasks. tDCS will be administered sequentially, but not simultaneously, with brain-activity monitoring by fMRI/MEG/EEG. Most experiments will be conducted in conjunction with cognitive tasks. In some experiments, we will study resting-state brain function, during which spontaneous brain activity is collected without a specific externally administered task.