A SSVEP Stimuli Encoding Method Using Trinary Frequency-Shift Keying Encoded SSVEP (TFSK-SSVEP).

Area of Science:

• Neuroscience
• Biomedical Engineering
• Signal Processing

Background:

• Steady-State Visually Evoked Potential (SSVEP) offers a high information transfer rate for brain-computer interfaces (BCIs).
• A key limitation of SSVEP is the scarcity of usable frequencies for distinct stimuli.
• Existing methods struggle with limited frequency and phase coding options, especially for portable BCI devices.

Purpose of the Study:

• To develop and evaluate a novel stimuli encoding method for SSVEP-based BCIs.
• To overcome the limitation of scarce frequencies in traditional SSVEP systems.
• To enable a greater number of unique stimuli while maintaining high information transfer rates.

Main Methods:

• A Frequency Shift-Keying (FSK) method was developed to encode SSVEP stimuli using three distinct frequencies for 'Bit 0', 'Bit 1', and 'Bit 2'.
• EEG signals were acquired from specific channels (Oz, O1, O2, Pz, P3, P4) at 250 SPS using ADS1299.
• Signal processing involved detrending, FFT-based FIR band-pass filtering, quadrature demodulation, derivative calculation, and window-based bit stream conversion.

Main Results:

• The FSK encoding method theoretically allows for at least 2^n stimuli (where n is the bit command length) without compromising the Information Transfer Rate (ITR).
• The processing pipeline successfully extracted valid peaks and converted them into bit streams.
• The proposed method is suitable for monitor-based stimuli and portable BCI devices with limited computational capabilities.

Conclusions:

• The developed FSK-based stimuli encoding method effectively expands the stimulus set for SSVEP BCIs.
• This technique addresses the frequency scarcity issue inherent in traditional SSVEP systems.
• The method is practical for implementation on standard monitors and resource-constrained BCI devices.