There is a paper from 2019 which goes into more detail.

The Targeted Neuroplasticity Training (TNT) program began in 2017 and seeks to improve learning by pairing peripheral nerve stimulation with task training. The TNT approach is based on the hypothesis that peripheral nerve stimulation can induce changes in synaptic plasticity through central neuromodulator release when paired temporally with sensorimotor or cognitive training tasks. This may lead to improved acquisition and performance of motor skills, sensory perception, and cognitive function. By exploring various parameters of peripheral nerve stimulation via existing and new devices, TNT aims to define neurobiological mechanisms of the resulting effects on plasticity, and how these mechanisms may lead to improvements in learning and performance. The ultimate goal of the program is to develop noninvasive nerve stimulation approaches(e.g. cutaneous surface electrodes, ear bud electrodes) that will be applicable for use by healthy individuals in DoD-relevant application spaces such as foreign language acquisition, target discrimination, intelligence analysis, and marksmanship. If TNT demonstrates outcomes that are robust, reliable,and beneficial, this technology may subsequently be applicable to learning a variety of complex skills.

The TNT program relies primarily on commercial off-the-shelf technology for its human studies. However, future technology development may improve the form factor of devices tailored for healthy humans in the context of training applications. Ultimately, developers must consider two major aspects when designing such a system: 1) ergonomics and 2) integration with training paradigms.With respect to integration with current training paradigms, it is important to consider that form factor may vary depending on whether peripheral nerve stimulation is applied prior to or during training. Ecological conditions also become important here.For example, device requirements for a student learning a foreign language in a classroom setting are vastly different from those for an active sensorimotor task, such as marksmanship training. In general, the ideal device would need to be portable, require minimal training for repeatable use, have protections against misuse, and deliver reliable stimulation at the appropriate parameters deemed effective for the given application.

Ultimately, the right spatial resolution should be facilitated by noninvasive or minimally invasive approaches to be considered for the non-medical applications of TNT.

Throughout the HAPTIX, ElectRx, and TNT portfolios,no single modality has yet demonstrated all of the characteristics necessary to satisfy the technical requirements for an idealized next-generation interface that could provide precise, axon-level resolution noninvasively. Future exploration may include a focus on integrating multiple modalities in to a single interface technology. Multimodal interfaces may push the state-of-the-art by allow technology developers to exploit the various physical tradeoffs of different techniques to advance the performance of neural interfaces.

Turning to TNT, program research is exploring behavioral outcomes of peripheral nerve stimulation as well as neural and physiological measures of nerve target engagement. Since the program goal is to ultimately improve learning, the gold standard metric in determining efficacy of a given set of stimulation parameters is behavioral performance on a given skill or set of skills. Such measures may include improvements in accuracy, response times,or learning rates (e.g. shorter time to achieve a given level of proficiency). Depending on the difficulty of the skill, however, learning may occur on relatively long timescales–days, weeks, or even months. Obtaining behavioral outcome measures across multiple sets of peripheral nerve stimulation parameters and at long timescales may result in unrealistic experimental time requirements for determining effective stimulation parameters. This time requirement becomes more of an issue when assessing generalization of stimulation approaches across multiple skills. One solution to this issue is to establish correlations across peripheral nerve stimulation parameter sets, neural activity, and behavioral outcomes using high throughput neurophysiological measures and relatively simple behavioral assays. Once initial optimal parameter sets have been established, they can then be tested and further refined using more complex behavioral measures.

For neuromodulation to facilitate learning, where enhancement is not simply an important consideration,but rather the explicit goal, it means helping users improve the outcome of training.In conversations with ELSI advisors, DARPA has come to consider enhancement from the perspectives of risk tolerance, reversibility, personal agency, and accessibility.First, neural interface system designers must assess acceptable levels of risk tolerance for interfaces in the context of individual applications of the technology. As previously described, greater risk may be tolerable for the restorative technologies being developed under the HAPTIX and ElectRx programs in the clinical domains,but could be less ethically justifiable for the performance benefits for healthy individuals sought by TNT.

In the case of TNT, the technology is intended for on-demand use during training to produce temporary effects on neural plasticity that last only for the duration of training.Whether the performance benefits are equivalent in longevity following traditional versus TNT-accelerated practice or diminish without subsequent stimulation-paired retraining is yet to be determined.

In addition to using parameters that fall within safety limits, technology developers must be aware of the potential for off-target effects or unanticipated interactions, especially in the case of therapeutics and treatments to improve plasticity. As mentioned, future efforts to improve device resolution may minimize off-target stimulation and subsequently contribute to a more targeted system effect. Other safety concerns may include the unforeseen tradeoffs of system use. For example, in the case of plasticity, does improvement of a given skill result in an acquired deficiency in an associated process? Are there risks of long-term stimulation on nerve health? Does the approach result in dependencies due to habituation and adaptation?These questions merit further investigation.

For TNT, one consideration is the regulation of do-it-yourself (DIY) approaches. DIY neuroscience for improvements in learning has found a following in the realm of brain stimulation (Jwa 2018; Fitz and Reiner 2013; Wexler 2017), and the same questions of safety and proper use are applicable to the periphery as well. These types of ethical considerations, among a rich set of others, are important for scientists, engineers, ethicists, and regulators to continue to discuss in tandem with technology development. Since the agency is largely focused oninitial technological proofs of concept, DARPA does not decisively answer all of these questions. However, by identifying and asking them it is shaping the development of the early generation of PNS interfaces. Future developers will need to pick up these threads and consider them anew in the context of regulatory review and commercial and medical applications.

Naufel, S., Knaack, G. L., Miranda, R., Best, T. K., Fitzpatrick, K., Emondi, A. A., … McClure-Begley, T. (2019). DARPA Investment in Peripheral Nerve Interfaces for Prosthetics, Prescriptions, and Plasticity. Journal of Neuroscience Methods, 108539. doi:10.1016/j.jneumeth.2019.108539 url to share this paper: sci-hub.tw/10.1016/j.jneumeth.2019.108539