What are the mechanisms underlying ES effects at the micro, meso, and macro scales? Models of ES effects can be generally trifurcated into characterizing the microscopic, mesoscopic, and macroscropic levels of description (Bestmann et al., 2015). There are several putative mechanistic models of ES, mainly surrounding tDCS effects on cortical function. At the microscopic level, models tend to focus on neuronal membrane polarization induced by electrical currents. While many models and empirical research emphasizes sliding-scale models (i.e., anodal excitatory, cathodal inhibitory), more recent research suggests a much more complex and nonlinear series of functional activity changes with tDCS (Batsikadze et al., 2013; Bonaiuto and Bestmann, 2015; Molaee-Ardekani et al., 2013; Rawji et al., 2018; Silvanto et al., 2008; Vergallito et al., 2023). On the mesoscopic scale, models tend to focus on the effects on activity of relatively distributed populations and networks of cortical and subcortical neurons. Research has showcased tDCS effects on resting-state functional network connectivity in the motor network, sensorimotor network, default mode network, frontal–parietal network, and self-referential network (Chan et al., 2021; Fischer et al., 2017; Mencarelli et al., 2020; Peña-Gómez et al., 2012), including both intrinsic activity within a given network and extrinsic activity between networks. Interestingly, there is a pronounced gap between microscopic and mesoscopic models and what should be expected behaviorally from ES administration. This gap unfortunately makes it difficult or impossible to predict the behavioral consequences of ES administration, whether related to performance improvement or degradation (Bestmann et al., 2015; Bonaiuto and Bestmann, 2015).

How do stimulation parameters modulate ES effects, and what is their optimal combination? There is a complex parameter space surrounding the administration of ES, including the targeted brain region, the number and size of electrodes, the polarity, intensity, timing (i.e., online versus offline), duration of stimulation, and whether the stimulation is personalized to individual structural or functional brain characteristics. Each of these parameters independently influences the behavioral and functional effects of ES, which likely have yet unknown interactive effects (Ehrhardt et al., 2021; Filmer et al., 2019; Friehs and Frings, 2019; Ho et al., 2016; Jacobson et al., 2012; Kessler et al., 2013; Opitz et al., 2015; Saturnino et al., 2019; Truong et al., 2013; Živanović et al., 2021). Given the inherent flexibility of ES stimulation parameters and their variability across studies, it is difficult to predict the optimal combinations of parameters to influence cognitive function (Au et al., 2017; Khorrampanah et al., 2020; Wagner et al., 2016). This is a challenging combinatorial modeling problem, especially given that very few studies manipulate two or more parameters across multiple (i.e., >2) levels. Recent research has attempted to use hierarchical Bayesian meta-regressions to predict the magnitude of tDCS effects as a function of 10 stimulation parameters (including electrode number, polarity, intensity, timing, duration), and identify optimal parameter combinations for maximizing effects on five performance domains (motor skill acquisition, visual search, working memory, vigilance, inhibitory control) (Santander et al., 2024). The model did not successfully converge on optimal solutions to confer reliable predictions to the five behavioral outcomes, largely due to extensive variability across research studies (e.g., experimental methods, tasks, and outcome measures). A relatively domain-general outcome suggested an advantage to anodal versus cathodal stimulation, but this pattern was not found in any specific outcome domain. The authors conclude that there is an urgent need for research to parametrically manipulate and probe independent and interactive effects across relatively complex parameter spaces.

How do individual differences influence ES effects? The effects of individual differences on ES-related outcomes have been examined in three primary ways. First, studies have examined how individual differences in brain structural and functional characteristics affect physiological and behavioral responses to ES. For example, individual differences in prefrontal cortical thickness influence tDCS effects on decision-making (Filmer et al., 2019). Additionally, tDCS-induced changes in resting-state functional connectivity are associated with differences in visual object-matching task performance (Pupíková et al., 2022). Secondly, studies have examined how individual differences in personality traits affect physiological and behavioral responses to tES (Brunyé et al., 2014, 2015; Krause and Cohen Kadosh, 2014). For example, tDCS affects reading speed of social sentences in readers with low scores on the behavioral approach and inhibition scales, but not those with high scores (Reyes et al., 2021); and trait anxiety modulates the effects of tDCS on creative task performance (Xiang et al., 2021). Third, studies have examined how individual differences in baseline knowledge or task proficiency affect physiological and behavioral responses to ES (Splittgerber et al., 2020). For example, tDCS increases the creativity of improvised instrument play for novices but harms expert performance (Rosen et al., 2016); and those with lower baseline reading proficiency show greater positive effects of tDCS on cross-language speech production (Bhattacharjee et al., 2020). Given the inherent heterogeneity of military personnel, the effects of relatively invariant individual characteristics is an important research topic. Individually tailored paradigms are likely needed, factoring in complex interacting variables (e.g., stimulating parameters, physiological, anatomical, and genetic differences). Personalized protocols do however raise challenges in a military setting, such as time constraints and practicalities. Recent research has further demonstrated large intra-individual variability in responses to repeat sessions of tDCS in that individuals did not respond consistently to tDCS when applied repeatedly over time (Willmot et al., 2024).

Can ES support and optimize performance, or does it truly enhance performance beyond baseline functioning? While the notion of cognitive enhancement is intriguing (Bostrom and Sandberg, 2009; Farah, 2015; Farah et al., 2014), it is challenging to truly demonstrate enhancement as opposed to performance sustainment or optimization, per se (Brunyé et al., 2020). Several philosophical positions conceptualize that enhancement must improve functioning of an individual beyond their normal range (Agar, 2013; Menuz et al., 2013). True enhancement would transcend the biological limits shaped by millennia of evolution—defining such biological limits will be critical, both within and across individuals. Demonstrating true enhancement requires quantitatively establishing baselines of an individual’s optimal biological performance potential under optimal conditions. For example, under an idealized set of hypothetical conditions, what could an individual achieve for reaction times, accuracy levels, or any other outcome of interest? Only when biotechnology-induced performance exceeds what has been established as innate optimal performance can it truly be deemed enhancement. In most cases, including when ES mitigates the performance deleterious effects of a contextual factor (e.g., sleep deprivation, stress, fatigue) (Hart-Pomerantz et al., 2024), performance is being sustained or optimized relative to a control condition that attempts to mimic some aspects of active experimental conditions (e.g., sham procedures with ES). These are important empirical comparisons, but they may not allow us to quantify the biological limits of performance or make inferences about enhancement. Moreover, military personnel might already be at their performance upper limit due to their training, limiting the effectiveness of ES. For example some tDCS studies in military samples did not observe any additional performance benefits (Blacker et al., 2020; Willmot et al., 2023). Visual search is required in many professions where an undetected threat, such as a weapon, can put the well-being of others at risk. Given the importance of detecting these threats, researchers have used various experimental techniques to improve performance in visual search tasks, albeit with varying degrees of success. Here, we explore two promising techniques to improve visual search using ecologically valid synthetic aperture radar stimuli: object recognition training and search strategy training. Search strategy training is intended to make observers search more systematically through a display, whereas object recognition training is intended to improve observers’ ability to recognize critical targets. Search strategy training was implemented by instructing participants to scan through the display in a pre-specified pattern. Object recognition training was implemented by having participants discriminate between targets and non-targets. We also manipulated whether observers received anodal or sham transcranial direct current stimulation (tDCS) during training, which has been shown to improve visual search performance and target learning. To measure the effectiveness of the training and stimulation conditions, we tested object recognition accuracy and overall visual search performance before and after three sessions of increasingly difficult training. Results indicated that object recognition training significantly improved object recognition accuracy relative to the search strategy group, whereas search strategy training was effective in improving visual search accuracy in those who adhered to the training. However, tDCS did not interact with training type, and although both training types yielded significant improvements, training-related improvements were not significantly different between the different approaches. This evidence suggests that strategy-based training could be as effective as the more prototypical object recognition training. Moreover, interactions between baseline performance and tDCS effectiveness have been demonstrated (Splittgerber et al., 2020). Whether ES can further improve performance in high performing individuals remains an open question.

Do trade-offs exist between targeted and untargeted performance domains? Using ES to enhance cognitive function raises questions about potential trade-offs between targeted and untargeted brain regions or tasks, aligning with the concept of net zero-sum dynamics within the nervous system. The net zero-sum framework posits that enhancements in one cognitive domain may be counterbalanced by detriments in another, necessitating careful consideration of cost–benefit interactions across various levels of neural processing (Brem et al., 2014; Luber, 2014). The concept underscores the need to assess potential costs alongside any enhancements. Multi-parameter considerations encompassing magnitude, duration, reversibility, and the level of impact across micro to macro scales are essential for delineating cost–benefit relationships. Within this framework, the notion of processing power emerges as pivotal, representing the brain’s capacity to allocate resources dynamically across different cognitive demands. Trade-offs, reflecting competition between sub-processes independent of top-down control, may elucidate interactions among neural elements. For instance, the speed-accuracy trade-off in decision-making tasks illustrates resource allocation dynamics, where competing demands for processing power influence task performance. While identifying the cost of ES-related enhancement remains challenging, the conceptualization of net zero-sum dynamics underscores the intricate interplay between cognitive enhancements and associated costs, necessitating nuanced framing and empirical investigation. Given the inherent complexity of military training and operations, we believe these investigations are highly warranted as military personnel are increasingly expected to perform multiple complex tasks simultaneously, recruiting diverse cognitive processes. If we are able to facilitate a few of those processes, what might happen to the other untargeted processes?

How will the use of ES devices integrate into existing military training and potentially be integrated into existing battle ensembles worn by military personnel? Schedules within military training establishments are increasingly facing time constraints and the introduction of an additional training requirement would add stress to an already overburdened system. It might be that the best solution is to identify critical periods of training where insertion of ES alongside existing training can yield performance improvement, mitigate performance decrements, or facilitate accelerated recovery. Another option could be to introduce effective interventions into the ongoing physical and wellbeing activities undertaken by military personnel during normal operation periods, thereby increasing the possibility of effective acceptance and implementation. Integrating devices into military personnel’s existing individual ensemble would involve a systematic process from research and development to acquisition and integration. Initially, the readiness and feasibility of ES technology would be assessed using the technology readiness level scale, considering factors such as safety, effectiveness, and compatibility with existing equipment (Small Business Association, 2024). Research and development efforts would focus on optimizing ES devices for military use, involving the development of prototype devices for testing and evaluation. This phase would also encompass human factors engineering to ensure the usability and ergonomic design of the devices, maximizing ease of use for warfighters. Interoperability with other equipment and systems, cybersecurity precautions, as well as considerations for sustainment and logistics, would be critical during the integration process. Insights from previous attempts to integrate EEG into warfighters’ helmets can inform this endeavor (Ko et al., 2019; Von Rosenberg et al., 2016; Zaman et al., 2022). While EEG integration faced challenges related to signal quality, comfort, and practicality, lessons learned from these efforts can guide the development and integration of other neurotechnology devices. Particularly, the importance of user-centered design, robust testing and evaluation, and ongoing lifecycle management to ensure the successful integration and utilization of neuroenhancement technologies in military operations is emphasized.

What are the ethical and regulatory considerations for ES application in military contexts? The use of ES in military contexts for enhancing cognitive functions and performance raises several ethical, legal, safety, and regulatory considerations. These include informed consent, autonomy, maintaining warfighter dignity and morality, potential risks to soldiers’ health and well-being, societal implications, and compliance with existing medical device regulations (Cinel et al., 2019; Davis and Smith, 2019; Puscas, 2020). Military personnel are a potentially vulnerable group who may face pressure to accept neuroenhancement due to authority relationships, hierarchical command structures, and the potential rights they forego when they enlist (Latheef and Henschke, 2020).

Regular and prolonged use of ES may carry yet unknown long-term effects for brain function, mental health, and neurological integrity. While acute effects are often reported as mild (e.g., tingling, itching), the consequences of repeated or long-term use remain largely unknown, raising questions about cumulative risks, dependency, and reversibility. If ES induces lasting neural plasticity or cognitive changes, what are the implications for warfighters once they leave military service? This issue extends to the ethical responsibility of the military and medical oversight bodies in ensuring that enhancement technologies do not compromise the long-term well-being of personnel, during and beyond their service. Another ethical issue is dual-use concerns, specifically the possibility that neuroenhancing technologies developed for military applications could be repurposed for punitive, coercive, or otherwise unauthorized civilian and non-civilian uses. This weaponization of neurotechnology, including potential application in interrogation, must be carefully considered and addressed through policy frameworks and regulatory oversight. An additional concern arises when the enhancement of one function coincides with diminished activity in other brain regions (Cinel et al., 2019). In future multi-domain operations that demand high levels of adaptability and multitasking, this could prove problematic.

For ES to be valuable and justifiable in military contexts, the positive effects must be superior, additive, proliferative, or associated with fewer adverse events than existing enhancement methods such as cognitive training (Davis and Smith, 2019). The use of stimulation to not only restore and maintain, but also aim to enhance human capabilities beyond natural limits raises ethical questions around fairness, merit, and the redefinition of military virtues such as courage, loyalty, and sacrifice (Puscas, 2020). Fairness and equity must also be considered, particularly regarding access to systems and potential performance disparities that can result. If ES confers advantages in certain domains, it could create disparities between enhanced versus non-enhanced personnel, and potentially lead to new stratification within military ranks where some are prioritized for promotions or special roles, whereas others are disadvantaged. Finally, it is possible that ES could create unrealistic expectations of warfighters, or cause overreliance or overestimation of its potential; indeed, if military personnel believe it provides a strong advantage for mitigating stress, fatigue, or other suboptimal states, they may take more risks and jeopardize the safety of themselves and others (Wilde, 1982).

Proactive engagement with stakeholders, including military personnel, ethicists, policymakers, and the broader scientific community, is essential to address these ethical and regulatory challenges, and establish guidelines that balance the potential benefits of ES with ethical and safety considerations in military training and operations. To this aim, a number of frameworks and regulatory guidelines on neuroenhancement are published by various government organizations and academia (Emanuel et al., 2019; Maslen et al., 2014). Furthermore, the Hybrid Minds project seeks to establish a foundational framework for ethically and legally evaluating neurostimulation systems embedded in intelligent neuroprostheses to ensure safety and ethics (Jotterand and Ienca, 2023). This framework is uniquely shaped by insights from user experiences and perspectives, along with input from scientists and engineers, helping to motivate international declarations regarding neurotechnologies and their relevance for human rights (Bublitz, 2024).