Coordination of Both Hands and Spinal Cord Neuroregulation: How Transcutaneous Spinal Cord Stimulation Changes the Neural Substrate of Bimanual Actions

Background: Humans use their arms in complex ways, often requiring coordination of both hands. Neurological disorders have limited this distinctive feature of the human motor system. Understanding how neuroregulation techniques alter the neural mechanisms behind the coordination of both hands is an important step in designing effective rehabilitative interventions. Non-invasive activation of the spinal cord through transcutaneous spinal cord stimulation (TSCS) has promoted recovery of motor function after spinal cord injury. Although many studies have attempted to capture the underlying neural mechanism of these effects using various electrophysiological tools, the impact of TSCS on cortical rhythms recorded by electroencephalography (EEG), especially during bimanual actions, is not yet clear.

Researchers investigated the impact of cervical TSCS on sensory-motor cortical oscillations in 12 neurologically intact participants. They examined changes in motor kinematics during the application of TSCS, as well as the levels of cortical activation and connectivity between the two hemispheres during the execution of unimanual and bimanual arm reaching movements (representative of daily life activities). Behavioral assessment results showed improvements in movement time and errors for bimanual cooperative target movements during TSCS application, but no differential effect of TSCS was found in performances of unimanual and bimanual separate target movements.

Study Source: This paper was co-authored by Behdad Parhizi, Trevor S. Barss, Alphonso Martin Dineros, Gokul Sivadasan, Darren Mann, and Vivian K. Mushahwar. The authors hail from multiple research institutions and the study was published in the 2024 issue of the Journal of Neuroengineering and Rehabilitation.

Study Methods and Results

Methods

This study designed three arm reaching standard movements: 1. Unimanual Visually Guided Reaching (VGR) 2. Bimanual Separate Target VGR 3. Bimanual Cooperative Target VGR

Following the application of cervical TSCS, a Kinarm exoskeleton robot was used to perform the above movements through elbow and shoulder joints in a horizontal plane. Subjects performed these tasks with and without TSCS, recording cortical activity using EEG, with special attention to α-band (8-12Hz) and β-band (13-30Hz) cortical activities related to the sensory-motor process.

Results

The study showed that in the α-band, cervical TSCS significantly increased the spectral power in the primary motor cortex during unimanual movements and bimanual separate target movements and increased the spectral power in the primary sensory cortex during α-band unimanual movements. In the β-band, TSCS significantly increased the spectral power in the primary motor and sensory cortex during the execution of cooperative bimanual movements and unimanual movements. Additionally, significant increases in interhemispheric connectivity within the α-band of the primary motor cortex were observed only while performing unimanual tasks during TSCS.

Conclusion and Significance

These findings indicate that cervical TSCS, as a neuroregulation technique applied to the spinal cord, provides initial information on the supraspinal effects of TSCS during the execution of unimanual and bimanual arm movements, and confirms the inhibitory effects of TSCS at the cortical level reported in prior research. These findings may lead to the use of TSCS for designing better rehabilitative interventions for future upper-limb functional recovery.

The limitations of the study include its conduct in neurologically intact subjects, and further research is needed to infer the impact of TSCS on individuals with neurological impairments. Moreover, this study did not evaluate the short-term or long-term effects of post-stimulation motor performance without stimulation, nor did it test indicators such as intra-cortical inhibition (IHI) or short-interval intracortical inhibition (SICI) in participants, which could provide valuable knowledge about the interactions and networks of cortical inhibition and excitation.