A brain-computer interface (BCI) is a system that is able to acquire brain signals and translate them into an input for a control system. Typical applications of BCIs span several domains, including the rehabilitation of patients with severe motor disabilities, prosthetic control, and augmentation of virtual reality environments. Most widespread BCIs are based on data recorded with an electroencephalograph (EEG), a device where a set of electrodes is placed on a subject's scalp to record the electric potential. Scalp EEG is a non-invasive and relatively cheap solution that provides measurements of brain activity with high temporal resolution. On the other hand, EEG is known to suffer from poor spatial resolution.