Towards an optimization of stimulus parameters for brain-computer interfaces based on steady state visual evoked potentials

Abstract

Efforts to construct an effective brain-computer interface (BCI) system based on Steady State Visual Evoked Potentials (SSVEP) commonly focus on sophisticated mathematical methods for data analysis. The role of different stimulus features in evoking strong SSVEP is less often considered and the knowledge on the optimal stimulus properties is still fragmentary. The goal of this study was to provide insight into the influence of stimulus characteristics on the magnitude of SSVEP response. Five stimuli parameters were tested: size, distance, colour, shape, and presence of a fixation point in the middle of each flickering field. The stimuli were presented on four squares on LCD screen, with each square highlighted by LEDs flickering with different frequencies. Brighter colours and larger dimensions of flickering fields resulted in a significantly stronger SSVEP response. The distance between stimulation fields and the presence or absence of the fixation point had no significant effect on the response. Contrary to a popular belief, these results suggest that absence of the fixation point does not reduce the magnitude of SSVEP response. However, some parameters of the stimuli such as colour and the size of the flickering field play an important role in evoking SSVEP response, which indicates that stimuli rendering is an important factor in building effective SSVEP based BCI systems.

Figure 1. Time course of the experimental paradigm.

Figure 2. Relative power increase for SSVEP response to different colours of stimuli.

Mean computed for all the subjects. Error bars indicate standard error of the mean (SEM). Horizontal lines above the bars indicate statistically significant differences on the level of p<0.001 (***), p<0.01 (**), and p<0.05 (*).

Figure 3. SSVEP responses to stimuli of different colours; organization of the plot as in Figure 2 .

Figure 4. SSVEP responses to stimuli of different sizes; organization of the plot as in Figure 2 .

Figure 5. SSVEP responses to stimuli of different sizes; organization of the plot as in Figure 2 .

Figure 6. SSVEP responses to stimuli with and without fixation point; organization of the plot as in Figure 2 .

Figure 7. SSVEP responses to stimuli with and without fixation point; organization of the plot as in Figure 2 .

Figure 8. SSVEP responses to circle and square stimuli; organization of the plot as in Figure 2 .

Figure 9. SSVEP responses to circle and square stimuli; organization of the plot as in Figure 2 .