Exploring the Modulation of Motor Cortical Excitability by Duration of Transcranial Direct Current Stimulation

Abstract

The motor cortex(MC) was important in human learning and memory, and the level of cortical excitability is a way of assessing its state. To non-invasively modulate cortical excitability, transcranial direct current stimulation (tDCS) is a widely employed technique. tDCS can be utilized to either enhance or inhibit motor MC excitability. However, in most studies, the optimal duration effect of tDCS on cortical excitability has been neglected. Additionally, there are no systematic analysis of tDCS and cortical excitability indexes. To bridge these gaps, we designed an experiment to systematically analyze the duration of tDCS and MC excitability it triggered. We recruited 5 healthy, right-handed subjects. They don’t have history of psychiatric disorders. Throughout the 30-minute tDCS stimulation at 1 mA, we continuously applied TMS to characterize the motor evoked potential. Our experiment revealed the following: applying 1 mA tDCS to the participants for 30 minutes effectively increases cortical excitability, and this effect can last for at least 30 minutes after stimulation. We then analysed cortical excitability by measuring the motor-evoked potentials (MEPs) induced by TMS during the tDCS stimulation process. We found that the relationship between cortical excitability and tDCS duration does not appear to be linear. Instead, two peaks were observed at 2-5 minutes and 15-25 minutes, respectively. A decrease in cortical excitability was observed at 26-28 minutes. Finally, we analysed the response of different participants to tDCS and found that individuals vary in their response to tDCS. Additionally, the relationship between cortical excitability and tDCS duration is inconsistent across different individuals.