Transcranial Direct Current Stimulation Does Not Counteract Cognitive Fatigue, but Induces Sleepiness and an Inter-Hemispheric Shift in Brain Oxygenation

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.

Systematic review of combined functional near-infrared spectroscopy and transcranial direct-current stimulation studies

“…Immediately following online 1.5-mA anodal stimulation of the left PFC, an initial increase in ipsilateral frontal cortical oxygenation (cerebral oxygen exchange: pre ¼ −3.17 × 10 14 versus post ¼ −4 × 10 14 ; no units) was followed by a decrease 20 min later (post 2 ¼ −2.45 × 10 14 ; no units). 67 In the longer term, 1 month after five sessions of anodal right PFC tDCS online training in older adults, a decrease in task-evoked HbO 2 change in the bilateral PFC region was again observed. 59 Decrease in PFC activation correlated with improved task performance regardless of 1 mA, 2 mA, or sham stimulation.…”

“…In the immediate period following online stimulation, cortical hemodynamics demonstrated increased cortical activation, 47,67 suggested to be due to the increase in blood flow required for motor memory consolidation, although this does appear to decline over time. 59,67 Offline anodal stimulation demonstrated an increase in cortical activation in three cognitive studies 54,58,60 and two motor studies, 47,66 although the latter was not significant compared to sham. Evidence suggests different neurophysiological mechanisms may be responsible for online and offline effects, [27][28][29]31 which may explain the different activation patterns demonstrated in this study.…”

“…66 Another approach was to measure responses in the same general brain region, but not in the exact same surface location. 55,67,68 The remaining studies used a variety of methods to integrate tDCS electrodes and fNIRS optodes over an identical surface location simultaneously. The majority utilized commercial devices that combine tDCS with fNIRS within a premade headcap and precludes any further technical equipment alterations by the investigator.…”

“…tDCS-modulated brain activation evoked by cognitive tasks was investigated in seven studies, all of which utilized sham stimulation protocols to assess effectiveness of tDCS. An online (task with stimulation) protocol was utilized in four studies 48,59,67,72 with the general trend supporting a reduction in cortical activation. This was observed in the bilateral PFC during 1-mA anodal HD right PFC stimulation compared to sham stimulation during a spatial working memory (WM) task.…”

“…For example, despite all including a sham group, exposure to sham could be either prior to active stimulation, 48 or always poststimulation, 54 or sometimes without a washout period between the two modes. 67 One study was performed in the elderly population and utilized repeated sessions. 59 Furthermore, there was variation in the tasks implemented between and within studies along with a noticeable difference in the time lag for poststimulation fNIRS measurement periods and a lack of reporting for all cortical areas measured.…”