Viability of tDCS in Military Environments for Performance Enhancement: A Systematic Review

INTRODUCTION

Transcranial electrical stimulation (tES) as a method of cognitive enhancement in both diseased and healthy individuals has gained popularity. Its potential for enhancing cognition in healthy individuals has gained the interest of the military. However, before it being implemented into military training or operational settings, further work is needed to determine its efficacy and safety. Although a considerable amount of literature exists, few studies have specifically evaluated its use in enhancing cognition relative to operational, military tasks. Therefore, in a first step to evaluate its efficacy, we completed a systematic literature review of studies using transcranial direct current stimulation (tDCS), a type of tES, to enhance cognitive processes in healthy individuals.

METHODS

A systematic literature review was conducted to identify literature published between 2008 and 2018 that used a method of tES for cognitive enhancement. As part of a larger literature review effort, 282 articles were initially retrieved. These were then screened to identify articles meeting predetermined criteria, to include those using various methods of tES, resulting in 44 articles. Next, the articles were screened for those using tDCS or high-definition tDCS, resulting in 34 articles for review and information extraction.

RESULTS

Of the 34 articles reviewed, 28 reported some degree of enhancement (eg, improved accuracy on tasks and reduced reaction times). Areas of cognitive enhancements included executive functioning, creativity/cognitive flexibility, attention/perception, decision-making, memory, and working memory. However, the precise outcomes of enhancement varied given the range in tasks that were used to assess the constructs. Additionally, the stimulation parameters in terms of intensity applied, duration of stimulation, and brain region targeted for stimulation varied.

CONCLUSIONS

The conclusions to be drawn from this systematic literature review include the identification of a brain region for targeting with stimulation to enhance a broad range of cognitive constructs applicable to military tasks, as well as stimulation parameters for duration and intensity. The dorsolateral prefrontal cortex was most frequently targeted in the studies that found enhanced performance across several cognitive constructs. Stimulation intensities of 2 mA and durations of 20 minutes or longer appeared frequently as well. Although several parameters were identified, further work is required before this type of technology can be recommended for operational use.

Role of Sensorimotor Cortex in Gestural-Verbal Integration

Action comprehension that is related to language or gestural integration has been shown to engage the motor system in the brain, thus providing preliminary evidence for the gestural-verbal embodiment concept. Based on the involvement of the sensorimotor cortex (M1) in language processing, we aimed to further explore its role in the cognitive embodiment necessary for gestural-verbal integration. As such, we applied anodal (excitatory) and sham transcranial direct current stimulation (tDCS) over the left M1 (with reference electrode over the contralateral supraorbital region) during a gestural-verbal integration task where subjects had to make a decision about the semantic congruency of the gesture (prime) and the word (target). We used a cross-over within-subject design in young subjects. Attentional load and simple reaction time (RT) tasks served as control conditions, applied during stimulation (order of three tasks was counterbalanced). Our results showed that anodal (atDCS) compared to sham tDCS (stDCS) reduced RTs in the gestural-verbal integration task, specifically for incongruent pairs of gestures and verbal expressions, with no effect on control task performance. Our findings provide evidence for the involvement of the sensorimotor system in gestural-verbal integration performance. Further, our results suggest that functional modulation induced by sensorimotor tDCS may be specific to gestural-verbal integration. Future studies should now evaluate the modulatory effect of tDCS on semantic congruency by using tDCS over additional brain regions and include assessments of neural connectivity.