[ARTICLE] Enhancing cognitive control training with transcranial direct current stimulation: a systematic parameter study – Full Text

Abstract

Background

Cognitive control (CC) is an important prerequisite for goal-directed behaviour and efficient information processing. Impaired CC is associated with reduced prefrontal cortex activity and various mental disorders, but may be effectively tackled by transcranial direct current stimulation (tDCS)-enhanced training. However, study data are inconsistent as efficacy depends on stimulation parameters whose implementations vary widely between studies.

Objective

We systematically tested various tDCS parameter effects (anodal/cathodal polarity, 1/2 mA stimulation intensity, left/right prefrontal cortex hemisphere) on a six-session CC training combined with tDCS.

Methods

Nine groups of healthy humans (male/female) received either anodal/cathodal tDCS of 1/2 mA over the left/right PFC or sham stimulation, simultaneously with a CC training (modified adaptive Paced Auditory Serial Addition Task [PASAT]). Subjects trained thrice per week (19 min each) for two weeks. We assessed performance progress in the PASAT before, during, and after training. Using a hierarchical approach, we incrementally narrowed down on optimal stimulation parameters supporting CC. Long-term CC effects as well as transfer effects in a flanker task were assessed after the training period as well as three months later.

Results

Compared to sham stimulation, anodal but not cathodal tDCS improved performance gains. This was only valid for 1 mA stimulation intensity and particularly detected when applied to the left PFC.

Conclusions

Our results confirm beneficial, non-linear effects of anodal tDCS on cognitive training in a large sample of healthy subjects. The data consolidate the basis for further development of functionally targeted tDCS, supporting cognitive control training in mental disorders and guiding further development of clinical interventions.

 

Introduction

Continuously changing environments require dynamic adaptation by means of filtering and evaluating internal and external stimuli to orchestrate goal-directed behaviour. This is especially important for situations in which distractions might influence efficient responses. Important information is maintained, while non-relevant stimuli must be suppressed or ignored. Dysfunctions of cognitive control (CC) processes are at the core of many psychopathological conditions [1,2], comprise the intentional selection of thoughts, emotions, and behaviours based on current task demands [3] involving functions of attention, memory, and emotional control [4], and are associated with altered patterns of brain activation [5,6]. The prefrontal cortex (PFC), particularly the dorsolateral prefrontal cortex (dlPFC), is known to be highly involved in CC processes [7] by means of processes related to working memory [8], encoding of task relevant rules and responses [9], and emotion regulation [10].

Transcranial direct current stimulation (tDCS) has been put forward as a means to influence these processes by modulating the likelihood of neuronal firing in response to a stimulus [11]. At the macroscopic level, within the common and safe range of stimulation parameters (1–2 mA, up to 30 min of stimulation [12]), it is supposed that anodal tDCS predominantly enhances, while cathodal tDCS mainly reduces the excitability and spontaneous activity of the targeted and connected areas [13]. This polarity-dependent modulation of brain activity by tDCS has a remarkable potential to influence corresponding cognition and behaviour [[14][15][16]]. However, tDCS does not induce cortical activity per se. It develops its effects particularly in interaction with spontaneous neuronal activity [17,18]. This activity-dependent influence on brain networks allows for a ‘functional targeting’ of stimulation when tDCS is directly coupled with the respective cognitive or behavioural process [19], where the target regions are activated (i.e. by a task) and further specifically modulated by the stimulation [20]. Correspondingly, tDCS effects have been found especially in neuronal correlates of task features that were active during stimulation [21]. Therefore, the combination of tDCS with task training is suggested to have a synergistic ‘neuroenhancing’ effect that is currently subject of extensive research [[22][23][24][25]]. However, available data are still inconsistent as efficacy depends on stimulation parameters that vary widely between studies. For a meaningful clinical application, a sustainable enhancement of adaptive plasticity would be most desirable [26]. Based on this notion, a specific activation of the CC network and concomitant tDCS holds promise to provide new treatment strategies for cognitive and behavioural disorders [[27][28][29]]. In a plethora of studies, stimulation has already shown to enhance CC by changing emotion regulation processes [30], improving frustration tolerance [31], modulating emotional vulnerability [32], dissolving attentional biases [33], augmenting working memory training [16], and increasing multitasking capacity [34]. However, reliability of results and the plausibility of approaches leaves room for improvement, not at least because studies often yield varying results even for similar tasks [[35][36][37][38]]. Therefore, reliable knowledge about the efficacy of parameter settings is mandatory for further advancements [39].

To this aim, we systematically tested different standard stimulation parameters (anodal/cathodal tDCS with 1/2 mA to the left/right dlPFC) in 162 healthy subjects, combining repeated CC training (6 sessions within 2 weeks) with tDCS, and additionally analysed pre- and post-training assessments. We applied a modified adaptive paced auditory serial addition task (PASAT) to challenge and train CC [40]. This task requires continuous updating of working memory with parallel distracting performance feedback; it is known to activate CC [31], critically involves resources within the PFC [41], and adapts task difficulty to individual performance [42]. We hypothesized that adding anodal but not cathodal tDCS to PASAT-induced neuronal activity of the dlPFC [43,44] can enhance cognitive training effects [45,46], improve performance of the PASAT or similar, even more challenging tasks [16,31,[46][47][48][49][50]], and that higher stimulation intensity does not increase efficacy [51]. Furthermore, we wanted to test if the laterality of stimulation matters. Therefore, PASAT performance under eight different tDCS conditions (combined N = 119) was compared to a sham intervention group (N = 43). Analyses were conducted hierarchically, allowing us to narrow down the responsible factors for the most efficient combination of CC training and tDCS.[…]

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