Effect of posterized naturalistic stimuli on SSVEP-based BCI
Most visual stimuli used in steady-state visual evoked potential (SSVEP)-based brain-computer interface (BCI) are simple and elementary. Examples of such stimuli are checkerboard patterns and sinusoidal gratings. These stimuli exhibit distinct contrasts and edges that conform well to the simple cortical cells behavior first observed by Hubel and Weisel. Hence, they are effective in eliciting VEP. On the other hand, the use of naturalistic stimuli is known to advance our understanding of early visual system. However, naturalistic stimuli are spectrally and spatially complex. They may not elicit the optimal VEP and the results obtained may not easily correlate to the stimulus parameters. Hence, we proposed to posterize natural images to generate naturalistic stimuli suitable for SSVEP-BCI. The posterization process considers both the edge and contrast information of the input image. This study elucidates the effect of posterized naturalistic stimuli on SSVEP amplitudes and phases by exploring the relationship between the number of tones of posterized visual stimuli and their effect on the power spectra and phase synchrony of attended stimuli. Results show that posterized visual stimuli at four tone display a significant effect on the dominant frequency response. Our findings suggest the effectiveness of posterized naturalistic stimuli and should advance the use of naturalistic stimuli in SSVEP-BCI.