Mental Tasks Induce Common Modulations of Oscillations in Cortex and Spinal Cord
We investigated whether the same modulations in spinal motor neurons parallel power modulations of cortical oscillations induced by mental tasks. We recruited 15 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 found significant power correlations between CST and EEG across trials irrespective of the mental task and across mental tasks at the intra-muscular coherence peak ({tau}_trial = 0.08 {+/-} 0.10, {tau}_task = 0.33 {+/-} 0.19, respectively; mean {+/-} std. dev.). CST power in beta and low-gamma bands could provide a novel control signal for neural interface applications, as power changes in these bands are not translated into actual force changes. To evaluate the potential of CST bands as a control signal, we classified the mental tasks from CST bandpower with a linear classifier and obtained classification accuracies slightly but significantly above chance level (30% {+/-} 5%; chance level = 25%). These results show for the first time that mental tasks can modulate the power of cortical and spinal oscillations concurrently. This supports the notion that movement-unrelated oscillations can leak down from the cortex to the spinal level. We further show that mental tasks can be classified from CST, although further research is necessary to boost the classification performance to an adequate level for neural interface applications.