An SSVEP-based BCI using high duty-cycle visual flicker

Steady-state visual-evoked potential (SSVEP)-based brain-computer interfaces (BCIs) have generated significant interest due to their high information transfer rate (ITR). Due to the amplitude-frequency characteristic of the SSVEP, the flickering frequency of an SSVEP-based BCI is typically lower than 20Hz to achieve a high SNR. However, a visual flicker with a flashing frequency below the critical flicker-fusion frequency often makes subjects feel flicker jerky and causes visual discomfort. This study presents a novel technique using high duty-cycle visual flicker to decrease users visual discomfort. The proposed design uses LEDs flashing at 13.16Hz, driven by flickering sequences consisting of repetitive stimulus cycles with a duration T (T 76 ms). Each stimulus cycle included an ON state with a duration T $ _ON and an OFF state with a duration T$ _OFF ( T T $ _ON T $ _OFF), and the duty cycle, defined as T$ _ON/ T, varied from 10.5 to 89.5. This study also includes a questionnaire survey and analyzes the SSVEPs induced by different duty-cycle flickers. An 89.5 duty-cycle flicker, reported as a comfortable flicker, was adopted in a phase-tagged SSVEP system. Six subjects were asked to sequentially input a sequence of cursor commands with the 25.08-bits/min ITR.