Evaluation of suitable frequency differences in SSVEP-based BCIs

Abstract

Frame-based frequency approximation methods are a pop- ular approach to realize visual stimuli that can be used to elicit steady-state visual evoked potentials (SSVEPs) at various frequencies on computer screens and allows the development of multi-target Brain- Computer Interface (BCI)-Systems. In this paper we investigate appro- priate selection of visual stimuli for multi-target BCIs using a frequency approximation method. Twelve sets of frequencies from different bands and with different resolutions have been compared among each other during an on-line BCI-task with six healthy subjects. Our results con- firm that equidistant frequency sets are not optimal, as the results from the sets with lower frequency ranges (<12 Hz) surpass those of the mid- range sets, even if a higher resolution is used. Interestingly, the study shows that SSVEPs elicited by stimuli from lower bands with a very high frequency resolution of 0.05 Hz could still be classified with ade- quate accuracy (around 90%). The results confirm that careful stimuli choice has high impact on SSVEP based BCI performance.