Background
Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that has been known to induce long-lasting alterations of cortical excitability. Due to its ability to promote cerebral plasticity, tDCS has been regarded as a promising therapeutic option for post-stroke rehabilitation. However, basic neurobiological mechanisms of tDCS are not fully understood. In this study, we examined the effects of anodal tDCS on electroencephalography (EEG) using an auditory cortical stroke rat model. Through the study, we tried to figure out the underlying electrophysiological changes after tDCS and its potential role in stroke recovery.
Method
A total of 12 male Sprague-Dawley rats were enrolled and photothrombotic stroke was induced using the Rose Bengal dye targeting the right primary auditory (A1) cortex. Immediately after the stroke induction, each rat underwent the microelectrode implantation surgery for the EEG acquisition from the bilateral frontal and A1 cortex. In addition, an epicranial electrode was mounted onto the skull over the right A1 cortex for the tDCS stimulation. Then, they were randomized to either anodal tDCS group (n = 6) or the control group (n = 6). After 3 days of recovery, the rats in the anodal tDCS group received 10 days of stimulation (5 consecutive days of tDCS followed by a tDCS-free interval of 2 days and 5 additional days of stimulation) for 15 minutes at 500 μA, while the rats in the control group were sham-stimulated for control. After each stimulation session, a frequency modulated sound (frequency linearly rising from 8 to 12kHz with 750ms duration) was applied and auditory evoked potentials (AEPs) were acquired from each EEG channel. Finally, amplitude, latency of AEPs recorded from the right A1 cortex were evaluated and brain connectivity using partial directed coherence (PDC) was calculated before and after the stimulation.
Results
Peak amplitudes were increased and latencies were shorted in both groups when comparing the results before and after the stimulation. However, no significant difference was observed between the anodal tDCS and the control group. In the connectivity analysis, the PDC between right and left A1 cortex and between right A1 and right frontal cortex significantly increased after the anodal tDCS (p-value < 0.05), while there was no significant change in the control group.
Conclusion
Application of tDCS seems to promote both inter- and intrahemispheric connectivity and we believe that the increased connectivity will potentially facilitate stroke recovery. To the best of our knowledge, this is the first animal study to use the auditory cortical stroke model and evaluate the effects of transcranial direct current stimulation. Further studies with larger sample sizes will be conducted to increase the value and robustness of our findings.
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