Neuromodulation Through the Skull: Can tdc stimulators improve performance?
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Brain stimulation has been associated with improvements in cognitive and sports performance, but exactly how effective it is remains up for debate.
This week Halo neuroscience raised $13m in series B funding for their sports performance headphones1. These deliver transcranial direct-current stimulation (tDCS) to the motor cortex area of the brain’s grooved cortical surface. However, also this week, a leading neuroscience research group from New York University published compelling data arguing that tDCS does not work, at all2.
Non-invasive neuromodulation improves brain cell communication
By placing an electrode on the scalp above the area of the brain they are trying to stimulate, these devices aim to pass direct current through the skull and stimulate the brain tissue beneath. In general the devices deliver a small current less than 2 milliAmps (mA) in amplitude. This amplitude limit avoids causing any side-effects, such as activating pain fibres in the scalp.
The devices are thought to stimulate the brain by priming the neurons to be more active. It is thought that more-active neurons induce more neuroplasticity, the strengthening and weakening of connections between neurons. When performing an activity or task whilst using a tDCS device the priming enables your brain to better reinforce and fine-tune the brain cell activity underlying the movement. This improves the efficiency of neural circuitry involved in performing the task. Meaning the next time you perform the task you will have improved the efficiency of the neural communication underlying it.
The scalp is an efficient filter of electrical current
However, Prof. György Buzsáki claims there is a limit to the applicability of tDCS devices. The latest findings from his lab compared the efficiency of transcranial current stimulation through both rat skull and human cadaver skull. The group found that up to 75% of the applied current is prevented from reaching the brain by the skin of the scalp. Effectively meaning that the current amplitudes necessary to evoke meaningful brain cell activity are much higher than the 2mAs utilised by most devices. A similar level attenuation was observed in live patients in 2017, when a group tested transcranial electrical stimulators on patients undergoing surgery for epilepsy. They found that when stimulating at 2mA a cortical field potential of 0.4V/m was achieved, and this is the lower limit for stimulation necessary in animal models3.
Performance enhancing headphones
Despite this, clinical trials using tDCS devices have shown they can increase neurotransmitter levels in the brain, improve exercise performance by reducing fatigue, and even enhance memory and improve stuttering.
Several papers cited by Halo neuroscience suggest that sporting performance can be improved by wearing their headsets, and they are currently being used by elite athletes as part of their training regimes. The company says that using their headsets which resemble a pair of headphones that contain electrodes in the band that sits on the scalp, can improve performance. They say improvements can be seen at the level of fine motor control, such as that needed to perform a golf swing, up to improving the endurance needed to compete in an Iron Man competition.
However, a mini-review4 paper also cited by Halo neuroscience discusses the applicability of headset-type devices explaining that, in some controlled studies, improvements in performance were not seen.
The new doping? -Electrical brain modulation
The evidence base in support of transcranial direct current brain stimulation for a host of applications is large and is still growing.
However, sceptics warn of the risks to public health in response to reports of people making DIY brain stimulators in attempts to improve performance or treat themselves for psychiatric conditions5.
Prof. Buzsáki’s group’s evidence that the electrical field generated by direct current stimulation is largely filtered out by the skin of the scalp raises questions for the applicability of tDCS devices. And suggests that it is not suitable for applications requiring stimulation of deeper brain areas.
However, Halo claim their headset device works well and has generated noticeable improvements for wearers. Perhaps this is because the motor cortex, the brain area they aim to stimulate, sits directly under the headset’s band on the surface of the brain? Would the headsets work if they were required to stimulate deeper structures, for example?
If the devices do improve athletic performance, does this make them a performance enhancer? And if so, should elite athletes be allowed to use them?
Also, despite the supportive evidence for the use of tDCS devices, scientists are not exactly sure of the mechanism by which they modulate brain cells. Is it ethically sound for them to be used when a mechanism of action is still unknown?
Further work is needed to truly understand the neuromodulatory effects of direct current stimulation and their application in the treatment of brain conditions and sports performance.
2. Vöröslakos, M., Takeuchi, Y., Brinyiczki, K., Zombori, T., Oliva, A., Fernández-Ruiz, A., ... & Berényi, A. (2018). Direct effects of transcranial electric stimulation on brain circuits in rats and humans. Nature Communications, 9(1), 483.
3. Huang, Y., Liu, A. A., Lafon, B., Friedman, D., Dayan, M., Wang, X., ... & Parra, L. C. (2017). Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation. eLife. Published online February 7, 2017.
4. Edwards, D. J., Cortes, M., Wortman-Jutt, S., Putrino, D., Bikson, M., Thickbroom, G., & Pascual-Leone, A. (2017). Transcranial direct current stimulation and sports performance. Frontiers in human neuroscience, 11, 243.
5. Miller, G. (2014). Inside the Strange New World of DIY Brain Stimulation. WIRED, May, 5.