The previous section outlined some of the promising avenues that might be explored in using NIBS for cognitive enhancement in older people. However, it was also clear that there are several unknowns and sources of variance in applying these techniques to older people. Here I will highlight some of the key issues that will need to be addressed in this topic.

Overarching much of this discussion is the practical question of whether NIBS can have any effect in natural settings. All studies cited so far have been lab-based studies, where controlled doses of stimulation have been delivered in a controlled environment, with a carefully controlled outcome measure. How well do lab-based findings translate to people’s natural environment? At present, there have been few scientific attempts that have used brain stimulation in the person’s home. TMS would be impractical for this use, but two studies have used tDCS with reasonable success (Andrade 2013; Hagenacker et al. 2014). In a less controlled environment, the so-called DIY-tDCS movement has seen people construct and use tDCS devices in their own home. The purposes, protocols and results of at-home users vary widely (Jwa 2015), and harnessing the enthusiasm of this community would be a valuable source of information in designing safe and effective protocols for naturalistic stimulation procedures (Charvet et al. 2015; Davis 2016).

One issue that is particularly relevant for tDCS is that we do not have a principled means of setting dosage, or for producing a given change in performance in a given person (Datta et al. 2012). At present, our best method for setting dosage is to use computational models to predict how the electrical energy will spread through the head from the point of delivery on the scalp. These models have so far been successful in understanding how to target perilesional areas of the stroke-affected brain (Datta et al. 2011), or in determining how best to alleviate chronic pain (Mendonca et al. 2011); however, there are many gaps in our knowledge (Bestmann and Ward 2017). One particular problem is that many existing models use a ‘standard’ head model, drawn from an MRI scan of a healthy young adult. However, if NIBS protocols are designed around people with this type of head anatomy, they may lead to dangerous concentrations of energy in people with non-standard anatomy such as the very young (Davis 2014) or people with eating disorders (Widdows and Davis 2014). A recent review of the safety of tDCS protocols indicated a wide variation across model heads in the current density on the surface of the brain, given a particular current density at the scalp; the implication is that a head model should be tailored to the individual, or at the very least to the population of interest (Bikson et al. 2016; Fregni et al. 2015). The reason for this uncertainty is that these groups have smaller or thinner skulls than healthy adults. This is illustrated in Fig. 1, where the head anatomy of three younger people is contrasted with that of three older people. The differences mean that the electrically insulating bone layer may offer the brain less protection than might be expected (Ivancich et al. 1992). The loss of bone density and mineralisation in older age is likely to present similar problems when NIBS is used in older people (Blunt et al. 1994). TMS also lacks a comprehensive set of principles for dose-setting, although protocols may at least be designed around individualized motor thresholds (Stokes et al. 2013).

A further source of variability, and possible risk, in brain stimulation is the interaction of stimulation with other treatments that the person may be receiving. As we age, we naturally accumulate conditions that require pharmacological management, such as treatments for blood pressure, high cholesterol or other conditions of later life. Relatively little is known about how the presence of pharmacological agents may affect NIBS, partly because people who are taking psychoactive medications are usually excluded from NIBS experiments on grounds of caution. tDCS and TMS induce changes in neurotransmitter levels in the brain (Stagg et al. 2009; Stagg and Nitsche 2011), so drugs that also manipulate these same mechanisms may interact with NIBS either by cancelling out the effect or by having an additive effect. Since tDCS is known to rely on synaptic mechanisms to produce its longer-lived effects (e.g. Nitsche et al. 2009), it would seem to follow that manipulating neurotransmitter levels would modulate tDCS effects, and indeed there appears to be some evidence of enhanced effect of tDCS with pharmacological support in older people (Prehn et al. 2016). These issues of drug interactions, and related problems of co-occurring conditions as the life course progresses, require careful research to establish clear safety and dosage guidelines. In parallel with these issues, there is the additional complication that individuals may respond to NIBS in different, even opposing, ways. NIBS to the motor cortex is known to be variable across individuals (Wassermann 2002; Wiethoff et al. 2014), and the effects of tDCS of opposing polarities may induce identical effects (Batsikadze et al. 2013). Troublingly, NIBS may induce quantitatively or even qualitatively different effects in the older person (Fujiyama et al. 2014; Heise et al. 2014). Overall, it is clear that considerable research is needed to reduce variability in the response to NIBS, and to understand how these techniques can be used to generate consistent, reliable effects in individual persons.

One problem that older people share with children and with people with eating disorders is the greater possibility of diminished capacity to consent to the procedures of enhancement. All parties in the procedure, including the participant and the person delivering the NIBS, must be certain that the participant has not declined in cognitive function to the point where they may not understand the implications and risks of the procedure. As with many medical treatments delivered to the older person, there are likely to be concerns about whether a person of declining cognitive health can make effective decisions about the intervention (Moye and Marson 2007; Stanley et al. 1984). It will require careful regulation to ensure that people are not coerced into receiving NIBS, nor is it unfairly withheld.

A wider ethical concern is whether we want to live in a society that permits, or even relies upon, widespread cognitive enhancement. Allowing unregulated enhancement presents a number of potential harms (Davis 2017). First, there is the issue that cognitive enhancement may only be available to those who can afford it, leading to, or widening, a divide in health between the richer and the poorer in society (Marmot et al. 2012). Second, by focusing on cognitive performance, we risk losing respect for natural human variation, which is one of the factors that define our humanity (Jordan 1921), and instead narrow the range of acceptable domains of achievement. A third risk is that we lose respect for these very achievements that result from performance under enhancement, just as we are disappointed when athletes are found to have enhanced their performance through use of illicit drugs (Santoni de Sio et al. 2016). Ethical concerns around enhancement raise important questions about the nature and the value of the activities we engage in, and understanding these concerns will be crucial in establishing clear boundaries for the use of enhancement technology in daily life.

Overall, it is evident that there is a wide range of behavioural targets for enhancement, and there are also a number of possible means for generating those enhancements. A key challenge for the near future will be to determine which cognitive faculties are most amenable to enhancement with NIBS, and to establish reliable protocols to deliver those changes. This will be possible only with a coordinated approach of careful experimentation built on solid theoretical foundations and on a good understanding of the neural and anatomical changes of healthy ageing.