VALID IDENTIFICATION OF BLINK ARTIFACTS - ARE THEY LARGER THAN 50 MU-V IN EEG RECORDS

Can artefacts, in particular of blink potentials, be validly identifie d in EEG data by defining EEG amplitudes as artefacts whenever their a bsolute values exceed 50 mu V? Does the performance of this 50 mu V cr iterion change when the data have been high-pass filtered (simulating a low time constant)? These questions were studied in data of an audit ory oddball task recorded from young and elderly healthy adults and fr om Alzheimer patients. The performance of the 50 mu V criterion heavil y depended on the distance from the eyes: most blinks were detected at Fz, very few at Pz and Oz. This rate further decreased after high-pas s filtering. A qualitative effect of the 50 mu V criterion occurred in the Alzheimer patients' Pz data: unidentified blink artefacts caused a late positive wave that mimicked a delayed P3. In the high-pass filt ered data, this effect occurred not only at Pz but also at Cz. These r esults lead to the conclusion that the 50 mu V criterion does not in g eneral perform well, and in particular may bias results when records a re made from Pz or from Cz only using a short time constant.