The Effect of Two Methods of Transcranial Direct Current Stimulation (tDCS) and Stimulus-Response Asynchrony on Backward-Compatibility Effect in Mental Fatigue Conditions

The present study aimed to investigate the effect of anodal and cathodal brain tDCS and stimulus-response asynchrony on the backward-compatibility effect (BCE) in conditions of acute mental fatigue.

The participants were 39 boys (20 to 24 years old). The instruments included the informed consent form, the Edinburgh handedness questionnaire, the dual reaction time instrument, the Stroop software, the visual analog scale to evaluate fatigue severity (VAS-F), and the tDCS device. The participants in the pre-test were tested in two conditions of non-fatigue and mental fatigue with a dual reaction time instrument. The tests consisted of two three-choice visual stimulus-response (letters and colors) with ten different stimulus onset asynchronies (SOAs). Then the subjects were randomly assigned to three Anodal, Cathodal, and Sham stimulation groups. The intervention consisted of four consecutive 20-minute sessions of tDCS stimulation on the DLPFC area. One day and four days after the last stimulation session, the tests were repeated. For inferential data analysis, Mixed Model ANOVA and One-way ANOVA tests were used at a significance level of 0.05.

The results showed that there is a difference between different SOAs in response time to the first stimulus (RT1) which is a representation of backward adaptation, and at lower SOAs, the RT1 is shorter. Also, there is a difference in RT1 at low SOAs between fatigued and non-fatigued conditions. However, the effect of cathodal and anodal tDCS on RT1 was very small.

In general, the results showed that at least some central aspects related to the response can be processed in parallel. Fatigue also affects this parallel processing.