A new microcomputer-based online ocular artefact removal (OAR) system

In many clinical and psychological situations it is often helpful to remove ocular artefacts (OAs) from the electroencephalogram (EEG) signals so that the true EEG can be analysed. These artefacts are associated with movements in the ocular system (eyelids, eyeball etc). Although satisfactory OA removal is now possible offline, online OA removal has hitherto been unsatisfactory. The previously reported online methods required the co-operation of subjects which cannot always be guaranteed, involved time-consuming manual calibration, and at best can only deal with one type of OA as they assume a constant correction factor. In the paper a new online system for removing OAs from the EEG signals is described which overcomes these disadvantages and offers additional advantages, such as flexibility. The system is based on the Motorola 68000 microprocessor and uses the numerically stable UD factorisation algorithm which allows continuous adaptive OA removal. A description of the online algorithm, and of the hardware and software of the OAR system, is presented. Preliminary results obtained with both normal and patient subjects showed that the system gives satisfactory OA removal.