The deleterious effect of ocular artefacts on the quantitative EEG, and a remedy.

The effect of ocular artefacts on spectral EEG parameters is assessed statistically. These artefacts are caused by movements of the eyeball and/or of the lid. Further, methods for correcting ocular artefacts are presented and evaluated. This methodological study is based on data from an investigation comparing the EEG of schizophrenic patients (n = 17) with healthy controls (n = 15). Ocular artefacts are monitored by the bipolar vertical and the bipolar horizontal electro-oculogram (EOG). It is shown that the influence of ocular artefacts on the measured electrical activity in the frontal region is larger than the cerebral potentials which the EEG is ideally intended to record. The more frequent occurrence of blinks and eye movements in schizophrenic patients may lead to an artificial enhancement of slow frequency EEG power for schizophrenics and eventually "false significances". In contrast to this, we found more significant group differences when correcting for EOG artefacts than without it. This can be attributed to a very much inflated sample variability of the uncorrected EEG, due to the individually varying EOG power. We conclude that it may not be sufficient to select visually epochs for analysis that are considered artefact-free. Rather, one should monitor EOG artefacts and apply an appropriate correction.