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Abstract
The aim of this work was to test if a novel transcranial direct current stimulation (tDCS) montage boosts the accuracy of lower limb motor imagery (MI) detection by using a real-time brain-machine interface (BMI) based on electroencephalographic (EEG) signals. The tDCS montage designed was composed of two anodes and one cathode: one anode over the right cerebrocerebellum, the other over the motor cortex in Cz, and the cathode over FC2 (using the International 10–10 system). The BMI was designed to detect two MI states: relax and gait MI; and was based on finding the power at the frequency which attained the maximum power difference between the two mental states at each selected EEG electrode. Two different single-blind experiments were conducted, E1 and a pilot test E2. E1 was based on visual cues and feedback and E2 was based on auditory cues and a lower limb exoskeleton as feedback. Twelve subjects participated in E1, while four did so in E2. For both experiments, subjects were separated into two equally-sized groups: sham and active tDCS. The active tDCS group achieved 12.6 and 8.2% higher detection accuracy than the sham group in E1 and E2, respectively, reaching 65 and 81.6% mean detection accuracy in each experiment. The limited results suggest that the exoskeleton (E2) enhanced the detection of the MI tasks with respect to the visual feedback (E1), increasing the accuracy obtained in 16.7 and 21.2% for the active tDCS and sham groups, respectively. Thus, the small pilot study E2 indicates that using an exoskeleton in real-time has the potential of improving the rehabilitation process of cerebrovascular accident (CVA) patients, but larger studies are needed in order to further confirm this claim.
Highlights
Transcranial direct current stimulation is a non-invasive brain stimulation technique based on weak direct electrical current transferred between electrodes over the scalp in order to modulate the neural membrane resting potential (Nelson et al, 2014; Rodríguez-Ugarte et al, 2016b; Lefaucheur et al, 2017)
The results of E1 and the preliminary results of the pilot test in E2, seem to support the hypothesis that this novel transcranial direct current stimulation (tDCS) montage improves the real-time classification of lower limb motor imagery (MI) tasks
An anode was placed over the cerebellum, since this improves the brain’s learning abilities according to several studies (Mandolesi et al, 2003; Ferrucci et al, 2013; Shah et al, 2013; Hardwick and Celnik, 2014)
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique based on weak direct electrical current transferred between electrodes (from anode to cathode) over the scalp in order to modulate the neural membrane resting potential (Nelson et al, 2014; Rodríguez-Ugarte et al, 2016b; Lefaucheur et al, 2017). The vast majority of the studies focus their tDCS experiments on improving the performance of the upper limbs, the speech, or the balance; where the areas stimulated are either the motor cortex, the frontal area or the cerebellum (Monti et al, 2013; Hortal et al, 2015; Foerster et al, 2017) In these studies, the range of current density used is typically between 0.04 and 0.06 mA/cm with a duration of 15 or 20 min (Marquez et al, 2013) and electrode sizes of about 35 cm. Stimulation with such big electrode surface areas gives only a vague idea of the areas of the brain that are important in producing the results
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