Comparison of pre-amplifier topologies for use in brain-computer interface with capacitively-coupled EEG electrodes

PurposeInterest in brain computer interfaces (BCI) has recently increased due to the need for quality of life technologies for disabled people. While brain signal processing and its applications have been widely studied for many decades, BCI seldom requires attention to practical and efficient brain signal sensing methods. Noninvasive electroencephalogram (EEG) measurements using wet adhesive Ag/AgCl electrodes are universally employed for BCI, but have limitations in practical acceptability with regard to portability, comfort and aesthetic design. In order to translate the results of laboratory experiments into practical use, EEGs should be recorded easily without requiring scalp preparation and regardless of the presence of hair.MethodsIn this paper, general requirements for capacitive measurement of EEG are presented and four different frontends for capacitive EEG electrodes are evaluated: (a) basic voltage follower scheme with high value resistor bias network (Rb), (b) voltage follower scheme with active guarding (second op-amp), (c) reverse current of signal diodes to providing bias current, (d) electrode scheme without any external bias network. We explore the use of capacitively-coupled electrodes for BCI technologies through the use of current popular BCI paradigms such as steady state visual evoked potential, P300 and sensory motor rhythm.ResultsOur experimental results indicate that capacitive electrode technology allows the acquisition of spontaneous EEG signals through hair with average correlation coefficient of 0.7949, 0.7946, 0.6333, 0.6549 for each capacitive electrode at O2 and 0.8433, 0.7822, 0.6253, 0.5427 for each capacitive electrode at C4. Although signal quality is lower and the movement artifacts are larger than those of conventional electrodes, SSVEP was successfully recorded through hair without spectral difference between SSVEP peaks and stimulus peaks except low stimulus frequency (5.45 Hz). P300 responses was measured with significant coefficient of determination (>0.005) except electrode (d). Sensory motor rhythm was suppressed during right hand imagery movement with log ratio value less than zero for all electrodes.ConclusionsFurther studies are required to apply capacitive measurement technology to uses in diagnostic EEG, but the method can currently be used for simple BCI applications.