Valid identification of blink artefacts: are they larger than 50 μV

Summary Can artefacts, in particular of blink potentials, be validly identified in EEG data by defining EEG amplitudes as artefacts whenever their absolute values exceed 50 ~V? Does the performance of this 50/.W criterion change when the data have been high-pass filtered (simulating a low time constant)? These questions were studied in data of an auditory oddball task recorded from young and elderly healthy adults and from Alzheimer patients. The performance of the 50 ~V criterion heavily 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-pass filtering. A qualitative ffect of the 50 #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 filtered data, this effect occurred not only at Pz but also at Cz. These results lead to the conclusion that the 50 IzV criterion does not in general perform well, and in particular may bias results when records are made from Pz or from Cz only using a short time constant. Key words: Blink artefacts; EEG By blinking, an electrical field is generated that overlaps EEG potentials at the scalp, thereby constitut-ing one of the most relevant artefacts when EEG potentials are measured. This artefact may be elimi-nated by either excluding trials with blink artefacts from further analysis or by subtracting the blink arte-facts from the EEG potentials (Verleger et al. 1982; Berg 1986; cf., Donchin et al. 1977). But both methods, rejection and correction, require that the blink artefact has been identified with certainty. Usually, this is sim-ply accomplished by recording the vertical EOG and identifying blink potentials in this record, using either an amplitude criterion or a covariance criterion (by comparing a template of a blink potential with th