Closed-Loop Phase-Dependent Vibration Stimulation Improves Motor Imagery-Based Brain-Computer Interface Performance

Abstract

The motor imagery (MI) paradigm has been wildly used in brain-computer interface (BCI), but the difficulties in performing imagery tasks limit its application. Mechanical vibration stimulus has been increasingly used to enhance the MI performance, but its improvement consistence is still under debate. To develop more effective vibration stimulus methods for consistently enhancing MI, this study proposes an EEG phase-dependent closed-loop mechanical vibration stimulation method. The subject’s index finger of the non-dominant hand was given 4 different vibration stimulation conditions (i.e., continuous open-loop vibration stimulus, two different phase-dependent closed-loop vibration stimuli and no stimulus) when performing two tasks of imagining movement and rest of the index finger from his/her dominant hand. We compared MI performance and brain oscillatory patterns under different conditions to verify the effectiveness of this method. The subjects performed 80 trials of each type in a random order, and the average phase-lock value of closed-loop stimulus conditions was 0.71. It was found that the closed-loop vibration stimulus applied in the falling phase helped the subjects to produce stronger event-related desynchronization (ERD) and sustain longer. Moreover, the classification accuracy was improved by about 9% compared with MI without any vibration stimulation (p = 0.012, paired t-test). This method helps to modulate the mu rhythm and make subjects more concentrated on the imagery and without negative enhancement during rest tasks, ultimately improves MI-based BCI performance. Participants reported that the tactile fatigue under closed-loop stimulation conditions was significantly less than continuous stimulation. This novel method is an improvement to the traditional vibration stimulation enhancement research and helps to make stimulation more precise and efficient.

The Effects of Sensory Threshold Somatosensory Electrical Stimulation on Users With Different MI-BCI Performance

“…Visual and somatosensory stimulation are two commonly used guidance strategies (Ren et al, 2020). Compared with visual stimulation, somatosensory stimulation has the advantage of not occupying the vision and providing more natural guidance for MI tasks (Zhang et al, 2021). St-SES, which can provide somatosensory afference to the targeted limb, has been demonstrated to enhance the activation of the sensorimotor cortex during MI tasks and the performance of MI-BCI (Corbet et al, 2018).…”

Can Vibrotactile Stimulation and tDCS Help Inefficient BCI Users?

“…Researchers have observed that stimulation triggered by the EEG alpha falling phase outperformed continuous stimulation with respect to the motor evoked potential (MEP) [35] amplitude and motor imagery BCI performance [20]. In this respect, our study applied vibrotactile stimulation according to the EEG alpha (8 ~ 13Hz) phase at the electrode channels on the left (C3) and right motor cortex (C4) as in [20]. Speci cally, during the vibrotactile stimulation session, the subjects were instructed to place their left and right index ngertips on each vibration motor.…”

“…For each trial, the epoch was extracted from the contralateral EEG channel as often as 500ms every 50ms to calculate the alpha phase. The epoch extracted was band-pass ltered in the [8-13]Hz frequency range using a 10 th -order elliptical in nite impulse response (IIR) lter, and the phase was calculated using the Fast Fourier Transform (FFT)-based phase tracking algorithm, as proposed and adopted previously [20,36]; among the FFT amplitudes, the dominant alpha frequency component between 8 and 13Hz and the corresponding phase were used to obtain a simple sine function to predict the upcoming phase. When the phase predicted was falling, the vibration was delivered through the left or right vibration motor for 100ms according to the motor imagery class, and the inter-stimulation interval was set to 100ms.…”

“…In the vibrotactile stimulation, it is known that somatosensory stimulation modulates corticospinal excitability and the stimulation can increase motor evoked potentials (MEPs) [18]. Previous studies have investigated vibrotactile stimulation's effects during motor imagery and found that it improved motor imagery BCI performance by enhancing the brain activity on the electrode channels around the motor cortex [19,20]. Further, they have observed that vibrotactile stimulation during a motor imagery task enhanced eventrelated desynchronization (ERD) and improved motor imagery BCI performance.…”

“…The di culty in conducting a comparative study originates from the stimulation paradigm. In general, vibrotactile stimulation is delivered while subjects are performing a motor imagery task because the stimulation enhances the cortical activation that motor imagery induces [20] and thus concurrent stimulation effects can be investigated. However, with tDCS, the stimulation effects are investigated following the stimulation session because recording EEG during the stimulation session is not applicable, and appropriate stimulation is necessary to modulate brain activity.…”