The other half sits in an entirely different evidence position, and it is the half a performance director should approach with the scepticism this series applies to any category selling certainty ahead of proof. Brain-training apps, EEG-based focus trainers and transcranial stimulation devices have proliferated, typically promising substantial gains in reaction time, decision-making and competitive performance. The published literature supports a far more measured conclusion: the effects are real in places, but small, conditional, and concentrated in exactly the applications the marketing tends not to lead with. 

Transcranial direct current stimulation is the most studied case and the most instructive. The most rigorous synthesis, a meta-analysis of 43 controlled trials, found that anodal stimulation of the motor cortex produced a small favourable effect on physical performance, a Hedges’ g of 0.29, with large heterogeneity and significant moderation by exercise type, training status and whether commercial or research-grade equipment was used (Winker et al., 2024). A small effect is not no effect, and for endurance tasks, where the perception of effort and central fatigue are limiting factors, the signal is most defensible. But two things should give a buyer pause. The heterogeneity is large enough that the prediction interval crosses zero, meaning some applications show no benefit or harm. And the effect is concentrated in physical, not cognitive, outcomes. A separate systematic review looking specifically at national- and international-level athletes found a single session improved strength or endurance in most studies but sport-specific tasks in fewer than half, with the high-level athlete evidence too heterogeneous to pool (Yu et al., 2024). 

That second point is the one the marketing inverts, and it is the single most useful thing a buyer can know about this category. Neurotech is sold overwhelmingly on cognitive promises, sharper focus, faster decisions, better reaction time. Yet across the meta-analytic literature, the detectable effects of stimulation cluster in physical domains such as strength and endurance, while the aggregate effect on cognitive performance tends to come out smaller and less consistent. So the strongest evidence is for a physical effect, and the headline cognitive claims that actually sell the devices are the least supported part of the literature. A category that leads with its weakest evidence is telling you something about itself. 

The commercial layer compounds the problem. The direct-to-consumer devices an athlete is most likely to encounter are not the research-grade equipment most studies used, and the gap between the two introduces variability the literature has barely addressed. For an elite team, the only defensible posture is experimental: a tool worth piloting for a specific, measured use case, most plausibly an endurance application, with a clear endpoint and an honest comparison, and never a generally validated performance enhancer deployed on the strength of a vendor’s reaction-time claim. The same applies to commercial EEG focus and neurofeedback platforms, where a systematic review of randomised trials found genuine potential but a field held back by non-standardised methods and inconsistent choices about which EEG signals to target (Cheng et al., 2024). 

There is a further wrinkle that the category rarely raises about itself. Because some of these effects are real, however small, brain stimulation has already acquired a nickname in the ethics literature, neurodoping, and with it a question sport has not resolved: whether techniques such as tDCS should sit on the prohibited list at all, a question the World Anti-Doping Agency has had under active discussion (Petersen, 2021). If a device reliably improved performance, would it be permitted? The current effects are too small and too inconsistent for that to be a live regulatory issue, which is convenient for everyone, but it exposes the honesty problem at the heart of the marketing. A vendor cannot simultaneously claim that a device produces a meaningful competitive edge and that it raises no integrity question at all. Either the effect is large enough to matter, in which case sport will eventually have to rule on it, or it is small enough to be ignored by regulators, in which case it is probably too small to justify the purchase. The marketing tends to want it both ways.