**2. Overview of EEG-based BCI**

Brain signals can be retrieved via invasive or non-invasive method. Invasive methods such as electrocorticography (ECoG) or intracortical point signal acquisition have the advantages of high temporal and spatial resolution with low artifact vulnerability. But with time, the signal quality and sensitivity would diminish. Non-invasive methods include electroencephalogram (EEG), magnetoencephalography (MEG), functional MRI (fMRI) and near-infrared spectroscopy (NIS) in general have high temporal resolution but with variable signal quality and spatial resolution [1].
