Abstract
Steady-state visual evoked potential (SSVEP) is widely used in vision research and applications. SSVEP is a periodic brain response induced by periodic visual stimulation at a certain frequency (F0). The response is mostly prominent at the driving frequency (F0), but also at higher harmonics (2*F0, 3*F0 etc) due to non-linearity in the stimuli and/or in the visual system. There are different designs of flickering stimuli, such as square-wave and luminance sinewave. Surprisingly, rarely any study has directly compared between different stimulation designs. Here, we tested 3 types of waveforms: square-wave, sinewave in luminance and sinewave in lightness (L* in CIE L*a*b* color space). Observers (N=7) viewed the stimuli (filled circle, radius=1.8°), flickering at F0=3Hz between black (0.3 cd/m2) and white (105 cd/m2) in a certain waveform, against black background on a CRT monitor. Each trial lasted 9 seconds, and 15 trials were tested for each waveform. EEG signals were averaged in each condition and converted to frequency domain by fast Fourier transformation. The result showed that, among three waveforms, sinewave in lightness produced the largest response at F0=3Hz (SNR: 5.6, 5.0, and 8.4 for square-wave, luminance sinewave, and lightness sinewave, respectively). In addition, we calculated the proportion of F0 response divided by the total response summed over all harmonics up to 45Hz. The sinewave in lightness also produced the largest proportion of F0 (0.13, 0.16, and 0.32). These results indicate that, compared with square-wave and sinewave in luminance, sinewave in lightness enhances F0 response and reduces non-linearity in SSVEP. This finding is in line with previous fMRI studies showing that the primary visual cortex codes lightness rather than luminance (e.g., Tsubomi, Ikeda & Naoyuki, 2012, JEP:HPP). We suggest that periodical stimuli modulated sinusoidally in lightness is optimal for SSVEP.