A novel multiple frequency stimulation method for steady state VEP based brain computer interfaces.

The objective is to increase the number of selections in brain computer interfaces (BCI) by recording and analyzing the steady state visual evoked potential response to dual stimulation. A BCI translates the VEP signals into user commands. The frequency band from which stimulation frequency can be selected is limited for SSVEP. This paper discusses a method to increase the number of commands by using a suitable combination of frequencies for stimulation. A biopotential amplifier based on the driven right leg circuit (DRL) is used to record 60 s epochs of the SSVEP (O(z)-A(1)) on 15 subjects using simultaneous overlapped stimulation (6, 7, 12, 13 and 14 Hzs and corresponding half frequencies). The power spectrum of each recording is obtained by frequency domain averaging of 400 ms SSVEPs and the spectral peaks were normalized. The spectral peaks of the combination frequencies of stimulation are predominant compared to individual stimulating frequencies. This method increases the number of selections by using a limited number of stimulating frequencies in BCI. For example, six selections are possible by generating only three frequencies.

Design and implementation of High Performance Visual Stimulator for Brain Computer Interfaces.

An algorithm for implementing visual stimulators on generic computers has been developed for Brain Computer Interfaces (BCIs). It uses the hardware counter present in these systems to derive accurate timing. Simultaneous display of 20 patterns (e.g. 3×3 checkerboards) modulated at different frequencies is possible. The pattern used for stimulating the Steady State Visual Evoked Potential (SSVEP) can be changed with ease. The stimulators are evaluated using software counters. High accuracy (less than 0.73% error) and precision (0.1% coefficient of variation) is recorded for 20 patterns set with frequencies between 6 Hz and 15 Hz.