A wireless marker system to enable evoked potential recordings using a wireless EEG system (EPOC) and a portable computer

As wireless EEG devices have become affordable at low cost, have a small form factor and quick setup time, they can be deployed at universities and schools for teaching purposes. However they have not been applied for evoked potential recording since they lack an option to receive stimulus markers. Meanwhile evoked potential recording is required for functional assessment of the sensory systems such as auditory and visual. This paper describes a wireless system that embeds information about the stimulus in the EEG channels. The transmitter unit is connected to the stimulus device to detect the stimulus and transmit the stimulus information to the receiver unit. The receiver unit attached to two of the EEG electrodes decodes the information and generates a pulse across the electrodes. The pulse width conveys the information about the stimulus. Hence the stimuli are synchronised with the EEG data allowing users to evaluate the evoked potentials in the offline processing. The wireless marker system was verified with audio stimuli consisting of 1000Hz and 1200Hz tones and reliably generated pulses with 100ms and 200ms width respectively. The delay between the onset of the tone and the onset of the pulse was 19.3 +/- 0.1ms. Since the variability of the delay was under 1ms and so negligible, the evoked potentials could be evaluated reliably. The evoked potential could be shifted back by 19.3ms to compensate for the delay. The system was also verified with a black-and-white checkerboard pattern stimuli and reliably generated pulses with 100ms width when the pattern reversed. The delay between the onset of the reversal and the onset of the pulse was 6.4ms. Similarly the variability of the delay was negligible.