Quantification of SSVEP responses using multi-chromatic LED stimuli: Analysis on colour, orientation and frequency

Most LED visual stimulators used in steady state visual evoked potential (SSVEP) brain-computer interface (BCI) use single LED sources to elicit SSVEP responses. In this study, we tested the hypothesis that different orientations would have different responses in different participants and aimed to develop a portable LED based stimulus design which consists of a small number of RGB LEDs arranged in a line which can be oriented horizontally or vertically. The colour and frequency of the flicker were controlled by a portable microcontroller platform. The study investigated the performance of the SSVEP from five participants when the LED stimulus was displayed vertically and horizontally for a period of 30 seconds. The frequency range used was from 7 Hz to 11 Hz with three primary colours: red, green and blue in both orientations. Furthermore, we also compared the effect of vertical and horizontal orientations using four different frequencies and three colours to test visual fatigue reduction. The results of the analysis using band-pass filtering and Fast Fourier Transform showed that the green horizontal LED stimulus orientation gave the highest response and viewing comfort in all the participants rather than the vertical orientation.