A 40-year-old male with a chronic cervical SCI participated in this study. Prior to enrollment, his injury, which occurred 12 years before recruitment, was graded as ASI A, with a C4 neurological level of injury, according to the International Standards for Neurological Classification of SCI (ISNCSCI).
This study was approved by the Human Subjects Ethics Sub-committee of the Hong Kong Polytechnic University (HSEARS20190121002; 9 Feb 2019) and the participant provided written informed consent.
This study implemented a two-phase crossover design. After a two-week baseline period, the first phase involved five weeks of tSCS training three times per week, and a second phase introduced BCI motor priming before tSCS training for a further five weeks (Figure 1A). There was a two-week washout period between phases, and a follow-up assessment was conducted 4 weeks after the end of the second phase.
Hand and arm training consisted of repetitive uni- and bimanual exercises in conjunction with tSCS. A typical session focused on a number of grasp types, including palmar grasping, pinching, pinching with rotation, and finger isolation. Tasks included flipping playing cards, moving ping pong balls between containers, scooping rice with a spoon, and stacking blocks, among others. Tasks were adjusted relative to functional improvements to maintain a degree of difficulty. For example, ping pong balls were replaced with marbles and then by small beads as the study progressed. Hand training was performed continuously over the 60-min session with two brief pauses when the participant was given a break from tSCS.
A constant current stimulator (DS8R; Digitimer, Oxford, United Kingdom) delivered stimulation in bursts of ten 100 mus long biphasic rectangular pulses at a frequency of 30 Hz, reflecting recent clinical work. Two round cathode electrodes (3.2 cm; Axelgaard Manufacturing Co, Fallbrook, CA, USA) were positioned at and below the level of injury between (i) C4 and C5, and (ii) C5 and C6 spinous processes. Cathodes were fastened to the skin with hypoallergenic tape to ensure a snug contact throughout the session. Inter-connected anode electrodes (8.9 x 5.0 cm; Axelgaard Manufacturing Co, Fallbrook, CA, USA) were placed symmetrically on the shoulders, above the acromion. In order to increase the likelihood of activating spinal structures, which lie below multiple layers of skin, fat, muscle, and vertebrae, stimulation intensity was set to highest tolerable degree (mean +- standard deviation; C4-C5: 49.0 +- 4.6 mA, C5-C6: 40.8 +- 5.1 mA). Current intensity was determined at the beginning of each session by gradually increasing the current from zero mA in 2.5 mA increments. This continued until the participant verbally communicated that the stimulation was causing a painful sensation, as indicated by reference to the fifth increment (moderate-severe discomfort) of the Visual Analogue Scale for pain intensity. The participant reported habituation after prolonged stimulation, therefore stimulation intensity was re-evaluated after 10 min.
Stimulation was applied for a total of 60 min during each session. To avoid heating and skin irritation from prolonged high-intensity stimulation, there was a 2-min break every 20 min where the stimulator was switched off. Hemodynamic parameters (blood pressure and heart rate) were monitored during breaks to track any incidence of autonomic dysreflexia.
To motivate the participant to engage in motor priming, as well as record sensorimotor rhythms, we devised a game-like brain-computer interface priming paradigm based on the "BCI2000" platform. The participant positioned his wheelchair opposite a computer screen and was fitted with an EEG cap, as shown in Figure 1B. Conductive gel was injected into each electrode and signal quality was verified by visual inspection. Modulation of beta band power (14-25 Hz) from the participant's electroencephalogram (EEG) was used to guide a virtual basketball toward one of two targets. The participant underwent 300 repetitions of the priming task, divided into 10 runs, each separated by 10-60 s breaks to avoid fatigue. Each repetition, or "trial", began with a photo-realistic basketball at the center-left of the computer screen, and a target either at the top-right or bottom-right (see Figure 1C). The ball moved horizontally at a fixed rate from left to right. The participant attempted mirror symmetric bimanual finger flexion and extension to push the ball to the upper target and relaxed for the ball to fall downwards. The participant was encouraged to imagine the sensation of clutching a real basketball as they performed the movement, in line with kinesthetic motor imagery protocols. Each trial lasted for 4 s and there was a 1.5-2.5 s inter-trial interval. Once priming was completed, the electrode gel was removed from the participant's hair, and the participant immediately proceeded to tSCS training.
In order to control the onscreen ball, EEG was recorded with a biosignal amplifier (g.USBamp; gtec, Schiedlberg, Austria) at a sampling rate of 256 Hz from ten active electrodes positioned at FC3, FC4, C1, C2, C3, C4, C5, C6, CP3, and CP4, according to the international 10-10 system. Electrode AFz was used as ground and the reference electrode was placed on the right earlobe. Through the BCI2000 platform, incoming EEG were spatially filtered with a small Laplacian filter to enhance the spatial resolution at electrodes C3 and C4, approximating the area above the sensorimotor cortices. The spatially filtered data was transformed into the frequency domain using an autoregressive spectral estimation. The mid-beta frequency band (18-26 Hz) was found to be the most reactive band during movement and was used to influence the vertical trajectory of the ball. The sum of spectral power from electrode C3 and C4 was found every 50 ms from a 400 ms long window and vertical cursor control was determined by solving a linear equation. A detailed explanation of this procedure was described by Wolpaw and McFarland. A 5-min calibration session at the beginning of each session trained the program to classify between attempted movement and rest. The setup was identical to that of the above priming strategy. However, the ball only moved in the horizontal direction, with no vertical displacement.
The current setup required 30 min for BCI priming, including 10 min for setup and 5 min for equipment removal. The tSCS component required around 10 min to apply electrodes and establish stimulation parameters. Including breaks, a session of BCI priming with tSCS never exceeded 100 min.
The Graded Redefined Assessment of Strength, Sensibility and Prehension (GRASSP) and grip strength were the primary measure of functional outcome. Grip strength was measured with the Vive Precision grip strength tester (Vive Health, Naples, FL, USA). GRASSP tested the strength of upper-limb muscles (Anterior deltoid, elbow flexors, elbow extensors, wrist extensors, extensor digitorum (DIII), opponens pollicis, flexor pollicis longus, finger flexors (DIII), finger abductors, first dorsal interossei), sensation on the dorsal and palmar sides of the hands, and fine and gross motor skills, quantified by scoring functional tasks (these included grasping and pouring water from a bottle, unscrewing the lids from jam jars, moving pegs between holes, inserting and rotating a key in a lock, inserting coins into a slot, screwing a nut onto a bolt).
Secondary outcome measures included the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), and the Spinal Cord Independence Measure (SCIM).
Further, the Modified Ashworth Scale (MAS) was used to quantify spasticity in the following upper-limb movements: shoulder abduction, elbow extension, elbow supination, wrist extension, and finger extension. The assessment was performed with the participant in the supine position and a trained physiotherapist graded each movement depending on the level of rigidity during flexion and extension. The minimum and maximum score for each unilateral movement was 0 (no spasticity) and 4 (velocity-dependent resistance to movement). A score of 1.5 was given when 1+ was selected [a detailed description of the MAS assessment was given by Charlambous et al. ]. The sum of scores from the left and right side were found for each movement.
All outcome measures were performed at the beginning and end of each intervention phase. Primary outcome measures were also performed in the middle of each five-week phase, and again at a 4-week follow-up session. Primary outcome measures were measured twice at baseline: once two weeks prior to the beginning of the first intervention phase, and once immediately before the first training session (refer to Figure 1A). Functional outcome measures were performed on different days from hand training sessions, and stimulation was not applied during any assessments.
An offline analysis of the participant's EEG during BCI motor priming was performed to determine if sensorimotor cortical activity was modulated during and/or across sessions. EEG was first band-pass filtered from 1 to 40 Hz with a 3rd order Butterworth filter. Next, we calculated the power spectrum density during each trial, that is, from one to three seconds relative to the appearance of the ball (t=0 s). The pre-trial period (-1.5 to -0.5 s) was also found relative to the appearance of the ball. The mean power across the beta band (18-26 Hz) was subtracted from and divided by the mean of the resting state beta power to give the percentage ERD/ERS relative to pre-trial power.