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Tham khảo tài liệu 'advances in robot navigation part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Brain-actuated Control of Robot Navigation 169 The figure above illustrates how challenging information extraction is even after digital filtering and ear-referencing EEG signals. Without these pre-processing steps however the signals are even less usable. We thus proceed to discuss the minimum pre-processing stages. 1.4.1 Frequency band filtering EEG signal energy is optimal in the 0Hz-80Hz range although historically most studies have ignored frequencies above about 45Hz as most processes of interest to the medical community take place below 45Hz . In this range there are three main sources of noise which must be removed or minimized i motion artifacts caused by electrode and cable movements including slow electrode drift which are mostly below 0.5Hz ii mains interference 50Hz in the UK 60Hz in the USA and iii muscle signals i.e. electromyography EMG e.g. from jaws facial muscle twitches swallowing etc. some of which actually overlaps with EEG as EMG produces relevant information between near 0Hz and about 500Hz up to 1kHz if implanted EMG is recorded . EMG cannot be fully removed due to the EEG EMG overlap but it can be minimized by avoiding muscle contractions in areas near the brain and by applying a lowpass digital filter to the EEG signal if EMG is simultaneously recorded it can be used with methods such as independent component analysis to reduce the EMG interference on the EEG signal . Most motion artifacts can be removed with a highpass filter at 0.5Hz some researchers will apply a cutoff as high as 5Hz if they wish to ignore the EEG delta band . Mains interference can be eliminated by referencing see next subsection . But if no referencing will be applied a notch or stopband filter must be used to remove mains interference. Overall both analogue and digital filters are needed as a first step in EEG processing. Typical filters suitable for BCIs are illustrated in Fig. 4 which are shown only as basic guidelines as researchers might use different filter .
