Abstract
BackgroundBrain-computer interfaces (BCIs) based on Steady State Visual Evoked Potential (SSVEP) have attracted more and more attentions for their short time response and high information transfer rate (ITR). The use of a high stimulation frequency (from 30 Hz to 40 Hz) is more comfortable for users and can avoid the amplitude-frequency problem, but the number of available phases for stimulation source is limited. To circumvent this deficiency, a novel protocol named Multi-Phase Cycle Coding (MPCC) for SSVEP-based BCIs was proposed in the present study.MethodsIn MPCC, each target is coded by a block word that includes a series of cyclic codewords, and each block word is corresponding to a certain flickering visual stimulus, which is a combination of multiple phases from an available phase set and flickers at single frequency. The methods of generating block code and extracting phase were presented and experiments were performed to investigate the feasibility of MPCC.ResultsThe optimal stimulation frequency was subject-specific, and the optimal duration was longer than 0.5 s. The BCI system with MPCC could achieve average discrimination accuracy 93.51 ± 5.62% and information transfer rate 33.77 ± 8.67%.ConclusionsThe MPCC has the error correction ability, can effectively increase the encoded targets and improve the performance of the system. Therefore, the MPCC is promising for practical BCIs.
Highlights
Brain-computer interfaces (BCIs) based on Steady State Visual Evoked Potential (SSVEP) have attracted more and more attentions for their short time response and high information transfer rate (ITR)
SSVEP refers to the response of cerebral cortex to a repetitive visual stimulus (RVS) oscillating at a constant frequency, and can be characterized by peaks at the fundamental frequency and its harmonics in the power spectral density (PSD) of recorded EEG signals [1]
During offline analysis, all experimental data were preprocessed firstly. They were down-sampled to 256 Hz, Cz was used as the re-referenced electrode and the electrodes O1, O2, Oz, PO3, PO4, P3, P4 and Pz were used for spatial filtering, since the occipital and parietal regions had the strongest response of SSVEP [1]
Summary
Brain-computer interfaces (BCIs) based on Steady State Visual Evoked Potential (SSVEP) have attracted more and more attentions for their short time response and high information transfer rate (ITR). SSVEP refers to the response of cerebral cortex to a repetitive visual stimulus (RVS) oscillating at a constant frequency, and can be characterized by peaks at the fundamental frequency and its harmonics in the power spectral density (PSD) of recorded EEG signals [1] It is an effective neural source compared to those in other BCI systems (e.g. motor imagery [2,3], P300 [4,5], and slow cortical potentials [6]), since it can achieve higher information transfer rate (ITR) with shorter time response [7,8]. Only few frequencies can elicit sufficiently strong SSVEPs for BCI purposes in the high frequency range (30 Hz ~ 40 Hz) [24]
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