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

Xing Zhao,Xia Wang,Xiaorong Hou,Dechun Zhao

doi:10.3389/fnhum.2017.00278

Abstract

SSVEP is a kind of BCI technology with advantage of high information transfer rate. However, due to its nature, frequencies could be used as stimuli are scarce. To solve such problem, a stimuli encoding method which encodes SSVEP signal using Frequency Shift–Keying (FSK) method is developed. In this method, each stimulus is controlled by a FSK signal which contains three different frequencies that represent “Bit 0,” “Bit 1” and “Bit 2” respectively. Different to common BFSK in digital communication, “Bit 0” and “Bit 1” composited the unique identifier of stimuli in binary bit stream form, while “Bit 2” indicates the ending of a stimuli encoding. EEG signal is acquired on channel Oz, O1, O2, Pz, P3, and P4, using ADS1299 at the sample rate of 250 SPS. Before original EEG signal is quadrature demodulated, it is detrended and then band-pass filtered using FFT-based FIR filtering to remove interference. Valid peak of the processed signal is acquired by calculating its derivative and converted into bit stream using window method. Theoretically, this coding method could implement at least 2n−1 (n is the length of bit command) stimulus while keeping the ITR the same. This method is suitable to implement stimuli on a monitor and where the frequency and phase could be used to code stimuli is limited as well as implementing portable BCI devices which is not capable of performing complex calculations.

Highlights

“Brain-Computer Interface” (BCI) is a very attractive technology which provides hands-free access to devices for the situation where the access to input device proves to be rather difficult, e.g., space exploration or provide patients who suffers Amyotrophic Lateral Scleroses” (ALS) or other body disabilities a mean to express themselves (Trejo et al, 2006; Parini et al, 2009)
Unlike to the traditional modulation of Binary Frequency-Shifted Keying” (BFSK) which is commonly seen in digital communication
TFSK-SSVEP as a mean to overcome the defects of stimuli in traditional SSVEP

Summary

Introduction

“Brain-Computer Interface” (BCI) is a very attractive technology which provides hands-free access to devices for the situation where the access to input device proves to be rather difficult, e.g., space exploration or provide patients who suffers ALS or other body disabilities a mean to express themselves (Trejo et al, 2006; Parini et al, 2009). There are many different approaches trying to restore the broken communication between brain and environment from many aspects. These methods include “Functional Magnetic Resonance Imaging” (fMRI) which is blood oxygenation level dependent and the invasive Electrocorticography (ECoG) as well as Electroencephalography (EEG). Comparing to other non-invasive methods, EEG provides temporal resolution up to milliseconds while it is cheaper, non-radioactive and provides high portability which makes EEG become a favorable choice when building a BCI system (Sitaram et al, 2007; Cincotti et al, 2008; Athanasiou and Bamidis, 2010; Becedas, 2012; Pistohl et al, 2012; Baranauskas, 2014)

Methods

Discussion

Conclusion