The effect of brain anodic and cathodic tDCS on PRP at different stimulus‑onsets asynchrony in non-fatigue and mental fatigue conditions

Hafezi, Somayeh; Doustan, Mohammadreza; Saemi, Esmaeel · 2023 · Crossref

DOI: 10.21203/rs.3.rs-3771354/v1

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Summary

This study investigated the effects of anodal and cathodal transcranial direct current stimulation (tDCS) on the psychological refractory period (PRP) and response time to the second stimulus (RT2) under both non-fatigue and mental fatigue conditions. The research was motivated by the need to understand how neuromodulation influences dual-task performance, particularly in contexts where mental fatigue impairs executive functions and decision-making, such as in competitive sports. The study aimed to determine if tDCS applied to the posterior lateral prefrontal cortex (pLPFC) could mitigate PRP effects and whether these effects varied across different stimulus-onset asynchronies (SOAs) and fatigue states. The study employed a quasi-experimental design with 39 male university students randomly assigned to anodal, cathodal, or sham tDCS groups. Participants underwent four consecutive daily sessions of 20-minute tDCS stimulation targeting the left pLPFC. Mental fatigue was induced using a 30-minute Stroop task. Performance was assessed via a dual-task paradigm involving visual letter and color stimuli presented at ten different SOAs (50 to 1500 ms). Assessments were conducted at pre-test, post-test (one day after stimulation), and follow-up (four days after post-test) phases under both fatigue and non-fatigue conditions. Statistical analysis included mixed ANOVA to evaluate changes in RT2 and PRP across groups, time points, fatigue states, and SOAs. The results indicated that tDCS had no significant effect on RT2 or PRP when comparing stimulation groups. However, significant main effects were observed for testing phase and fatigue. RT2 decreased significantly from pre-test to post-test and follow-up across all groups, suggesting practice effects rather than stimulation benefits. PRP also decreased significantly from post-test to follow-up. Crucially, PRP was significantly lower under mental fatigue conditions compared to non-fatigue conditions. Additionally, PRP and RT2 were significantly longer at shorter SOAs (50–150 ms) compared to longer intervals. In fatigue conditions, active stimulation groups performed better than the sham group at higher SOAs, though this did not reach significance for overall PRP reduction. The findings supported the response integration phenomenon, contradicting the strict bottleneck model, as response times to the first stimulus increased with longer intervals between stimuli. The study concludes that while tDCS applied to the pLPFC did not significantly alter PRP or RT2 compared to sham stimulation, mental fatigue significantly reduced PRP, suggesting altered parallel processing capabilities. The lack of significant tDCS effects implies that this specific neuromodulation protocol may not be effective for enhancing dual-task performance or mitigating fatigue-related deficits in response selection. The results highlight the complexity of central processing in dual tasks and suggest that fatigue modifies, rather than simply impairs, the mechanisms of response integration.

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• dual task multitasking