Abstract
Sustained cognitive demands may result in cognitive fatigue (CF), eventually leading to decreased behavioral performance and compromised brain resources. In the present study, we tested the hypothesis that transcranial direct current stimulation (tDCS) would counteract the behavioral and neurophysiological effects of CF. Twenty young healthy participants were tested in a within-subject counterbalanced order across two different days. Anodal tDCS (real vs. sham) was applied over the left prefrontal cortex. In the real tDCS condition, a current of 1.5 mA was delivered for 25 min. Cortical oxygenation changes were measured using functional Near Infrared Spectroscopy (fNIRS) on the frontal cortices. CF was triggered using the TloadDback task, a sustained working memory paradigm that allows tailoring task demands according to each individual’s maximal cognitive capacity. Sustained cognitive load-related effects were assessed using pre- versus post-task subjective fatigue and sleepiness scales, evolution of performance accuracy within the task, indirect markers of dopaminergic activity (eye blinks), and cortical oxygenation changes (fNIRS) both during the task and pre- and post-task resting state periods. Results consistently disclosed significant CF-related effects on performance. Transcranial DCS was not effective to counteract the behavioral effects of CF. In the control (sham tDCS) condition, cerebral oxygen exchange (COE) levels significantly increased in the right hemisphere during the resting state immediately after the induction of CF, suggesting a depletion of brain resources. In contrast, tDCS combined with CF induction significantly shifted interhemispheric oxygenation balance during the post-training resting state. Additionally, increased self-reported sleepiness was associated with brain activity in the stimulated hemisphere after recovery from CF during the tDCS condition only, which might reflect a negative middle-term effect of tDCS application.
Highlights
Cognitive fatigue (CF) can be defined as temporary compromised mental resources developing over time on sustained cognitive demands/effort
Inter-Hemispheric Correlations and Relationships Between Sleepiness and cerebral oxygen exchange (COE). In light of these results, we investigated the relationships between increased self-reported sleepiness in the Real Transcranial DCS (tDCS) condition during Rst3 and the variations in oxygenation observed during the same period
Transcranial DCS over the left prefrontal cortex failed to counteract CF-related modifications, possibly because participants were stimulated in a condition in which maximal cognitive resources are recruited to cope with the ongoing task
Summary
Cognitive fatigue (CF) can be defined as temporary compromised mental resources developing over time on sustained cognitive demands/effort. CF is responsible for decreased behavioral performance and increased propensity to errors (Boksem et al, 2005; Lorist et al, 2005) following sustained cognitive demands. This makes it a topic of interest both in experimental and ecological settings. Optimizing work schedules was shown to reduce CF (Rose and Curry, 2009), whereas amphetamines (Ilieva et al, 2015; Spencer et al, 2015) and caffeine (Lorist and Tops, 2003; Van Duinen et al, 2005; McIntire et al, 2014; Urry and Landolt, 2015) stimulants can improve cognitive functioning on the short term and delay the development of CF. Variability is high among studies and participants (Kim et al, 2014), and positive effects have been questioned (Horvath et al, 2015b) or reported to be beneficial in specific conditions, e.g., for low performers only (Tseng et al, 2012)
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