Filtering and aliasing of muscle activity in EEG frequency analysis

Muscle artifact contains activity at frequencies more than half the usual acquisition rates for spectral EEG that may contribute power at conventional EEG frequencies by aliasing. In spectral analysis work, excessive EMG activity is usually rejected by visual inspection, but significant amounts may still remain and special steep low-pass filters are usually used to inhibit aliasing. A 24 dB / octave variable filter was used in various combinations with conventional EEG amplifier filters to study aliasing of EMG activity. Aliased EMG activity entered as noise across all frequencies. With the usual amount of EMG activity that is present in EEG undergoing frequency analysis, a 6dB / octave 35 Hz low-pass filter is equivalent to teh 24 dB / octave 35 Hz low-pass filter in preventing aliasing using a sampling rate at least 128 Hz.