4. Discussion
impaired motor functions, brain injuries, stroke, cord injury, tetraplegia
STROKE, TETRAPLEGIA, AMYOTROPHIC LATERAL SCLEROSIS, CORTEX
In this study, we examined the functional contribution of the left dorsolateral prefrontal cortex (DLPFC) to the motor imagery (MI) using excitatory repeated transcranial magnetic stimulation (rTMS).Stimulation of the left DLPFC influences neural activity in the PrecuneusR leading to s statistically significant increase in the theta-band power on the intervals preceding the MI performing. Based on the connectivity analysis, we highlight the vital role of the functional interaction between these areas. The Phase Locking Value (PLV) shows that functional connectivity between the left DLPFC and the PrecuneusR exists in the resting state and grows on the interval preceding MI (PLV We found a negative correlation between the MI response latency and the precuneus theta-band power. High theta-band power corresponded to the short time between the cue and the formation of the event-related desynchronization. Despite the strong negative correlation between the theta-band power and latency, we did not report a decreased latency in the rTMS group. The mean response latency and its variance declined, but their change was statistically insignificant (The correlation between MI response latency and theta-band power in PrecuneusR may indicate that preactivation of Precuneus facilitates processing of the visual cue. This hypothesis is supported by the high posterior theta-band power on the post-cue interval, which probably reflects cue processing (The obtained results may shed light on the neural interactions behind MI. The literature reports that the left DLPFC is a hub of the fronto-parietal central executive network (CEN) involved in executive functions [These networks may interact as follows. The rTMS targets the left DLPFC and activates task-positive CEN. Strengthening connectivity between the DLPFC and PrecuneusR during the pre-cue interval deactivates task-negative DMN. Other studies support this hypothesis by reporting a negative correlation of the theta-band power with the DMN activation [Recently, researchers reported the ability of rTMS to modulate functional connectivity [Here, we applied excitatory rTMS and expect to facilitate functional connectivity. The rTMS may affect brain activity through different mechanisms. First, rTMS at frequencies of 5 Hz or higher enhances cortical excitability [It is widely accepted that training in MI-based brain-computer interface (BCI) is an effective approach in neurorehabilitation therapy for people with impaired motor functions, such as patients with tetraplegia, spinal cord injury, and brain injuries like stroke or amyotrophic lateral sclerosis [Several limitations exist in this study. One was the lack of behavioral measures, making it impossible to relate the imaging findings to behavioral consequences. We overcome this limitation to some extent by developing an algorithm for estimating motor imagery brain response time by ERD formation latency. Another limitation is the absence of individual MRIs of the subjects. We positioned the TMS coil over the left DLPFC using a neuro-navigation system based on the MRI average head model brain. So, we cannot ascertain that we stimulated the exact location of the left DLPFC for all subjects. Other limitations include the small sample size and the across-subject design, which may contribute to large inter-subject variations.