1) Too small a signal caused by the electrodes being too close together and possibly shorted to some extent by leaking electrode jelly. Considerable care needs to be taken to correctly site electrodes. The CFAM3 recording system requires the user to define electrode sites for recordings and these should be set with a measuring tape if necessary. Care should be taken to ensure that surplus electrode jelly is wiped off from the periphery of the recording electrodes.
2) Eye movement and low frequency waves from sweating and psycho-galvanic responses may originate from the forehead region. These are normally avoided or minimised by a parietal or occipital electrode placement and particularly by transverse (left-right) placements. The CFAM amplitude-frequency analysis band-pass filter attenuates lower frequencies considerably but these potentials may still be noticeable on the unprocessed EEG trace in particular.
3) Facial movement may cause electrical interference mainly from the temporalis muscles and from scalp movement affecting the electrodes. This is normally only seen in conscious patients, in some sleep stages and during seizures. The CFAM trace indicates when muscle activity is being detected at the recording electrodes.
4) Excessive scalp muscle potentials may on occasion produce EEG waveforms that can be mistaken for precursors of seizure activity. Experience is necessary to recognise this unusual condition. Scalp muscle activity near electrodes usually produces some energy in the EEG frequency band. Typically this is recognised by excursions on the muscle trace matching excursions on the percentage beta trace. Note that muscle relaxants given for surgery may not completely abolish scalp muscle, particularly in response to 'painful' stimuli.
5) ECG (EKG) pick-up. This is normally most likely to be observed in deeply comatose patients with low amplitude EEG traces. It may substantially exceed the EEG amplitude. Maternal EKG is also a common contaminant of intra-partum fetal EEG monitoring (see "Monitoring Cerebral Function").
