Extraction of SSVEP Signals of a Capacitive EEG Helmet for Human Machine Interface
Abstract— The use of capacitive electrodes for measuring EEG eliminates the preparation procedure known from classical noninvasive EEG measurements. The insulated interface to the brain signals in combination with steady-state visual evoked potentials (SSVEP) enables a zero prep human machine interface triggered by brain signals. This paper presents a 28channel EEG helmet system based on our capacitive electrodes measuring and analyzing SSVEPs even through scalp hair. Correlation analysis is employed to extract the stimulation frequency of the EEG signal. The system is characterized corresponding to the available detection time with different subjects. As demonstration of the use of capacitive electrodes for SSVEP measurements, preliminary online Brain-Computer Interface (BCI) results of the system are presented. Detection times lie about a factor of 3 higher than in galvanic EEG SSVEP measurements, but are low enough to establish a proper communication channel for Human Machine Interface (HMI).
helmet based design. This provides a direct access to EEG signals even through scalp hair. Visual stimulation of the brain causes visual evoked potentials (VEPs), which can be measured in the region of the visual cortex . If the stimulation repetition frequency increases above 6 Hz, the VEPs come to a steady state, called steady-state visual evoked potentials (SSVEP). These SSVEP can easily be recorded with the EEG and are suitable for a communication interface in a BCI application. Several SSVEP-BCI systems have been demonstrated by now , , . Stimulation is often done by LED or ﬂickering areas on a PC screen as separated areas or checkerboards. The signal processing is generally done by power spectral density analysis or correlation analysis . II. SENSORS & SYSTEM A. Sensor The capacitive coupling between the body and our sensor is based on a metallic electrode plate insulated from the skin by a thin plastic layer. This electrode plate is connected to an impedance converter in the signal processing unit (SPU) on top of the electrode plate. This unit also contains a combined ampliﬁer and ﬁlter circuit (Fig. 1). The electrodeSPU package is shielded from the environment by a metallic case to prevent the inﬂuence of external electric noise. In addition the electrode itself is shielded by an internal active guard circuit surrounded by shielding layers, effectively shielding static charges.
I. INTRODUCTION Noninvasive measurement of the electroencephalogram (EEG) by galvanic contact via Ag/AgCl electrodes is the standard measurement procedure in neuro-medicine. This provides an access to sub-µV brain signals for diagnostics and neuroscience. One application of this noninvasive EEG is the use in the ﬁeld of Brain-Computer-Interfacing. The BCI provides a new human communication interface to computers and machines by interpreting brain signals to deduce control commands for the computer. This is interesting for handicapped people to control, e.g., a prosthesis or a wheelchair and for normal people as an alternative Human Machine Interface or for gaming purposes , , . Capacitive electrodes are an alternative way to interface the noninvasive EEG, avoiding the electrochemical contact between the skin and the metal of the Ag/AgCl electrode via electrolyte gel by using an insulator between electrode and skin. The biosignal is measured by capacitive coupling between body and electrode. These capacitive electrodes have been already evaluated for EEG application , . Also dry electrodes have been tested for BCI applications . With capacitive electrodes the preparation procedure for measuring the EEG is reduced dramatically, especially for a high number of channels. Such systems increase the convenience for the user because no application of electrolyte gel is needed. Here we integrate capacitive electrodes in a This work was...
Please join StudyMode to read the full document