Multichannel EEG-EMG coupling analysis using a variable scale symbolic transfer entropy approach

The coupling strength (CS) of the electroencephalogram (EEG) and electromyogram (EMG) reflects the connection between the cerebral cortex and active muscles in motion control. Conventional symbolic transfer entropy analysis (STE) methods are limited to effectively reveal the dynamic characteristics. In order to quantitatively analyze multi-channel EEG and EMG coupling, a variable scale symbolic transfer entropy (VSP-STE ) analysis approach was proposed in this paper, and a CS expressive method was presented to quantitatively measure the corticomuscular CS of EEG and EMG signals during grip tasks. The influence of the scale parameter on EEG-EMG transfer entropy was first analyzed and compared at different grip force levels. The optimized scale parameter was then applied to the STE calculation in experimental data analysis. Results demonstrated the dominance of the C3 and C4 EEG channels in motion control, as well as the contralateral control mechanism of the brain. In addition, the premotor cortex ( FC5 and FC6) also played an important role in corticomuscular coupling. Results showed that the EMG→ EEG transfer entropy increased as the grip force increased (5, 10, 20 kg), and the right hand (dominant hand) showed higher EEG→EMG transfer entropy than the left hand in all the 3 subjects. The two-way (EEG→EMG, EMG→-EEG) coupling strengths were also increased as the grip force increased. The statistical analysis showed that at 5, 10, 20 kg grip force levels, EMG→ EEG coupling strength of left hand was 0.0330 ± 0.0058,0.0373 ± 0.0040, 0.0451 ± 0.0055 respectively, and that of right hand was 0. 0352 ± 0. 0029, 0. 0432 ± 0. 0035, 0. 0603 ± 0. 0018 respectively. There was a significant difference (p <0. 05) of coupling strength, except for that between 5 kg and 10 kg of grip force in left hand. For EEG-+EMG, coupling strength of left hand is 0. 0253 ± 0. 0047, 0. 0379 ± 0. 0026, 0.0481 ±0.0068, and that of right hand is 0.0333 ± 0.0041, 0.0510 ± 0.0057, and 0.0649 ± 0.0085 respectively. The difference of coupling strength was significant for all. In conclusion, our results show that corticomuscular coupling is a two-way process and the coupling strength varies with different channels and gripping strength. VSP-STE can be used to describe the nonlinear synchronizing characteristics and information interaction of the cerebral cortex and neuromuscular tissue.