A marker circuit to enable recording of auditory evoked potential using a wireless EEG system (EPOC) and a portable computer

This paper describes a circuit that enables auditory evoked potential recordings using a wireless EEG device (Emotiv EPOC) and a portable computer as the stimulus machine. The need arises since neither the portable computer nor EPOC has serial/parallel ports to send/receive marker signals that indicate the stimulus onset. The circuit produces a pulse on the EEG channels when it detects an audio stimulus. First it amplifies the audio tones produced by the stimulus machine and detects their envelope. If the envelope exceeds the threshold, it will activate the emitter side of an optocoupler. This in turn switches on the photo-transistor of the optocoupler and produces a pulse that is electrically isolated from the stimulus machine. The pulse's amplitude is reduced to at most 300 microvolts using a voltage divider and is fed to a pair of the EEG channels. The delay between the output pulse and the onset of the 1 kHz audio tone is under 1 ms when the tone is instantly switched on and hence insignificant. Each of the two EEG channels was also biased to DRL via a 4.7 kohm resistor so that it is free from EEG and movement artifact.