Background: Traditionally, the brain and spinal cord have been studied as separate systems due to the challenges of simultaneously imaging their activity. However, optically pumped magnetometer (OPM)-based imaging is uniquely versatile, allowing flexible sensor placement on different parts of the body. Using OPMs, we have developed a novel system for concurrent imaging of brain and spinal cord activity (Mardell et al., 2022).
Aim: This preliminary work aimed to investigate endogenous interactions between the brain, spinal cord, and muscle involved in sensorimotor control during simple movement.
Methods: Healthy participants (n=3) performed a tonic contraction task with their right and left hands. We recorded brain and spinal cord activity using OPMs positioned on the head and neck, while also recording electromyography (EMG) data from the thumb abductor muscle. During the task, they were asked to follow a target contraction level (~10 % maximal voluntary contraction) with their rectified, smoothed EMG trace as precisely as possible. The data recorded from sensors over the neck were reconstructed in source space using a Bayesian minimum norm inversion (Litvak et al., 2011). Multiple linear regression and canonical variate analysis (CVA) were applied (Friston et al., 1996) to identify within- and cross frequency coupling between brain-EMG, brain-spinal cord, and spinal cord-EMG activity in the frequency range of 5-30 Hz.
Results: Reconstructed current flow showed patches of activity concentrated over the lower cervical region of the spinal cord source space in all participants during both right and left contractions. For brain-muscle functional connectivity, we found evidence for within-frequency associations in the beta band (15-30 Hz; family-wise error (FWE) corrected p < .05) as described previously (Conway et al., 1995) as well as cross-frequency interactions (CVA chi-squared p <. 05) in all participants. Interactions between the spinal cord and EMG were exclusively cross-frequency (CVA chi-squared p < . 05), whereas brain-spinal cord interactions showed both strong within- (FWE corrected p < .05) and cross-frequency (CVA chi-squared p <. 05), oscillatory functional connectivity. These results were consistent across participants.
Implications: Our results provide evidence for within- and cross-frequency oscillatory interactions between the brain, spinal cord, and muscle during voluntary movement. This research demonstrates the utility of OPMs in studying endogenous spinal cord activity. Our OPM-based system, allowing concurrent imaging of the brain and spinal cord, opens new possibilities for advancing our understanding of how communication is coordinated in the central nervous system, both in health and disease.