Performance evaluation of electroencephalograph with negative capacitance converter

Spontaneous electroencephalogram (EEG) and evoked potentials measured from the scalp are considered to be attenuated by impedance of mediated biological tissues such as skull, biomembrane and cortex. Voltage loss at these tissues may deteriorate the measured signal. In this study, we explored a possibility that an electroencephalograph bearing enhanced input-impedance could detect more sensitively the short latency somatosensory-evoked potential (SEPs) and high frequency oscillations (HFOs) in SEPs. We introduced a negative capacitance converter (NCC) into the electroencephalograph at front end. We expected the NCC to reduce floating capacitance in shielded wires between electrode and the electroencephalograph, and consequently to enhance the input-impedance of the device especially in higher frequency region. SEPs and HFOs were measured in eight subjects with the proposed device and compared with those measured with a commercial electroencephalograph. 7 of 8 showed larger amplitude of SEPs when EEG measured with the proposed device, but only 4 out of 7 showed larger amplitude of HFOs.