Eliciting steady state visual evoked potentials near the visual perception threshold

Steady state visual evoked potentials (SSVEP) are widely used in EEG research as they offer a relatively high signal to noise ratio allowing the investigation of visual processing at the cortical level. Presentation of a repetitive visual stimulus (flicker, RVS) at a frequency in the range from approximately 1 to 100 Hz, elicits an oscillatory response at the same frequency of the stimulus and/or harmonics which can be observed in the electroencephalogram (EEG), particulary at occipital sites. The frequency and the modulation depth of the RVS determine the strength of the corresponding SSVEP response. While a stronger response allows for a higher signal-to-noise ratio, it does also negatively influence comfort and safety. By relying on visual perception research which characterize the flicker perception thresholds for different frequencies and modulation depths, we focus in this paper on analyzing the SSVEP response near the perception threshold and we show that it is possible to design quasi-imperceptible RVS which elicits a sufficiently strong SSVEP response.