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A technique of half-field alternate visual stimulation, combined with differential EEG signal measurement, is applied to acquire steady-state brain-evoked signals. Taking the difference of two signals, measured with properly placed electrodes, enhances the visual-evoked potential (VEP) and suppresses the noise components. An array of different-flash-frequency light-emitting-diode (LED) pairs forms a multiple-choice table. By fixating at different LED pairs, the user communicates (by the VEP of corresponding frequency) his/her decision about the selection of the table entry.
Materka et al. (Thu,) studied this question.