Before revision:  Electroencephalogram (EEG) is the most frequently used input signal in BCIs . However, EEG signals are weak, easily contaminated by interferences and noise, non-stationary for the same subject, and varying among different subjects .
After revision:  Electroencephalograms (EEGs) used in BCIs are weak, easily contaminated by interferences and noise, non-stationary for the same subject, and varying among different subjects .