Complex-valued spatial filters for SSVEP-based BCIs with phase coding

Abstract

Brain–computer interface (BCI) systems based on

steady-state visual evoked potentials (SSVEPs) have gained consid- erable popularity because of the robustness and high information

transfer rate these can provide. Typical SSVEP setups make use

of visual targets flashing at different frequencies, where a user’s

choice is determined from the SSVEPs elicited by the user gazing

at a specific target. The range of stimulus frequencies available for

such setups is limited by a variety of factors, including the strength

of the evoked potentials as well as user comfort and safety with light

stimuli flashing at those frequencies. One way to tackle this lim- itation is by introducing targets flickering at the same frequency

but with different phases. In this paper, we propose the use of

the analytic common spatial patterns (ACSPs) method to discrim- inate between phase coded SSVEP targets, and we demonstrate

that the complex-valued spatial filters used for discrimination can

exceed the performance of existing techniques. Furthermore, the

ACSP method also yields a set of spatial patterns, separable into

amplitude and phase components, that provide insight into the

underlying brain activity.