Mental tasks induce common modulations of oscillations in cortex and spinal cord

Spike trains from spinal motor neurons contain low-frequency components that modulate muscle force, and higher-frequency components (above 10 Hz) that do not. The functional role of these higher-frequency components in motor control is still debated. We investigated whether mental tasks that modulate the power of cortical oscillations produce corresponding modulations in spinal motor neuron activity above 10 Hz without affecting force output. Such coupling would indicate that some higher-frequency components are not merely arising as a byproduct of force generation nor indirectly contributing to motor control, but simply reflect cortical oscillations propagating to spinal motor neurons. If voluntary power modulations of these higher-frequency oscillations do not affect force output, they could potentially serve as control signals for neural interface applications such as movement augmentation or motor neuroprostheses.

We recruited 15 human participants and recorded high-density electromyography signals (HD-EMG) from the tibialis anterior muscle, as well as electroencephalography (EEG) signals. The cumulative spike train (CST) was computed from the activity of spinal motor neurons decoded from HD-EMG signals. The participants performed sustained dorsiflexion concurrent with foot motor imagery, hand motor imagery, mental arithmetic, or no specific mental task. We analysed the bandpower correlation between EEG and CST signals as well as evaluated the task discriminability of CST bandpower signals with a linear classifier.

At the intra-muscular coherence peak, we found statistically significant power correlations between CST and EEG in two separate analyses: first, when correlating across individual trials regardless of the mental task, and second, when correlating across the four mental tasks (Kendall's τ coefficient [Formula: see text], [Formula: see text] respectively; mean ± std. dev.). To evaluate the potential of the CST as a control signal, we classified the mental tasks based on CST bandpower and obtained classification accuracies slightly but significantly above chance level ([Formula: see text]; chance level = 25%).

These results show that mental tasks can simultaneously modulate the power of cortical and spinal oscillations. This supports the notion that cortical oscillations not contributing to ongoing force control can propagate to the spinal level. We further demonstrate that mental tasks can be classified from CST bandpower, but classification performance is limited by the low signal-to-noise ratio.