Neuroplastic alterations and variability of transcranial alternating current stimulation (tACS) on the human motor cortex among caffeine-adapted subjects

by Ornela Merkohitaj
Doctoral thesis
Date of Examination:2023-07-26
Date of issue:2023-06-29
Advisor:Prof. Dr. Paulus Walter
Referee:Prof. Dr. Walter Paulus
Referee:PD Dr. Jana Zschüntzsch
Persistent Address: http://dx.doi.org/10.53846/goediss-9967

 

 

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Abstract

English

Transcranial alternating current simulation (tACS) at a "ripple" frequency of 140 Hz may interfere with ongoing brain oscillations and thus causes changes and modulates cortical excitability. Response variability further hinders transcranial electric stimulation (tES) techniques to be used in clinical practice. Therefore, this study aimed to better understand the factors that might cause a response variability in plasticity induction studies. We concretely examined the effects of time of day, caffeine and alertness on plasticity aftereffects of tACS (140Hz, 1mA) on the motor cortex (M1) in moderate caffeine consumers (200-400 mg/day). This study (n = 30, Male : Female = 1 : 1 ) was a randomized, double-blind, cross-over study and consisted of 8 sessions. An experimental session lasted 2,5 hours. First, the motor threshold was determined, which was followed by the oral admission of caffeine (200mg) or placebo tablet and a 45-minute waiting period. After that, tACS or Sham tACS was applied for 10 minutes over the M1 in a randomized order: Actual tACS stimulation, morning; Sham stimulation, morning; Actual tACS stimulation, afternoon; Sham stimulation, morning. At the same time, the level of alertness was monitored by pupillometry for 11 minutes as the spontaneous oscillations in pupillary size reflect the level of central nervous activation. The aftereffects of stimulation were recorded every 5 minutes till 30 minutes post stimulation. At the end of experiment, the participant filled in a questionnaire regarding short term adverse effects or stimulation-related sensations. The data analysis showed a consistent reduction in cortical excitability (except tACS placebo morning) probably due to the light deprivated situation. tACS induced excitatory effects compared with Sham, particularly in the placebo morning sessions. Furthermore, we observed that caffeine increased alertness and that there was no significant difference between Actual or Sham tACS in caffeine sessions. Finally, our analysis found non-significant correlations between vigilance and motor cortex excitability in the Sham or Actual stimulation sessions except a significant positive correlation in the 1.0 mA tACS placebo sessions.
Keywords: tACS; tACS
 
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