A Better Way of Delivering Noninvasive Brain Stimulation?

Megan Brooks

February 09, 2018

Scientists may have developed a better way of delivering noninvasive brain stimulation, early research shows. 

Investigators at NYU School of Medicine in New York City believe a novel method of delivering transcranial electric stimulation (TES) may offer greater efficacy than the current method, which is not strong enough to affect brain circuits.

"In its current substantiation of TES stimulation, it is unlikely that it has a direct effect on the neurons in the brain. Thus, alternative explanations should be searched for explaining the claimed behavioral and clinical effects," lead investigator, György Buzsáki, MD, PhD, neuroscience professor, told Medscape Medical News.

TES is being investigated as a potential treatment of various neurologic and psychiatric conditions.

Prior research by NYU Langone scientists questioned widely accepted mechanisms for noninvasive TES, suggesting current protocols may not deliver adequate stimulation to affect neuronal activity in the human brain.   

Dr Buzsáki and colleagues now show that a novel stimulation method may be more effective at affecting neuronal circuits, and they have established a minimum current threshold required to affect brain oscillations in people.

The study was published online February 2 in Nature Communications.

Current Performance Enhancer

In experiments in rats and human cadaver brains, the researchers found that roughly 75% of scalp-applied currents are attenuated by soft tissue and skull.  Using intracellular and extracellular recordings in rats, they determined that at least 1 mV/mm voltage gradient (magnitude of electric fields) is needed to affect neurons.

"We applied currents with traditional electrodes in human cadavers and show that the induced electric fields are too weak with the commonly used 1- to 2-mA stimulation. We also show that at least 4 to 7 mA is needed to induce the desired 1 mV/mm," Dr Buzsáki told Medscape Medical News.

"In TES practice, currents larger than 2 mA are avoided because higher intensities are associated with skin sensation, phosphenes, and other side effects," the investigators note in their article.

They designed an "intersectional short-pulse" stimulation method that allows simultaneous TES and recording of scalp electroencephalography and concentrates sufficiently high current intensities into the brain, while keeping the charge density and sensation on the scalp surface relatively low.

Using this method, "we could directly assess and calibrate the needed intensity to affect brain patterns (occipital alpha waves). We have not seen any direct effect up to 4 mA but demonstrate that above 4.5 mA we can see direct and immediate changes in the amplitude of alpha oscillation," Dr Buzsáki said.

"If the goal is to directly stimulate neurons in the brain, which is the goal of the majority of people working in this field, including us, we have to work hard to figure out novel stimulation methods to deliver enough charges into the brain yet decrease the effect on the scalp. Our novel stimulation protocol and device are in the right direction but need to be improved and perfected," he added.

Commenting on the study for Medscape Medical News, Anli Liu, MD, assistant professor of neurology at NYU School of Medicine, said, "new devices which deliver a higher intensity of current, without causing excessive sensory discomfort, need to be developed. Intersectional short-pulse is one such method."

"Investigators using the present generation of TES devices and protocols should include more rigorous controls to explore the contributions of sensory and placebo effects on behavioral outcomes," Dr Liu added. "Animal researchers could explore alternative mechanisms of TES, including peripheral nerve and glial contributions. Finally, many in the TES community propose that the effects of low-intensity electrical stimulation may be cumulative. We need to test these hypotheses in higher primates and humans," said Dr Liu. 

The study had no commercial funding. The authors have disclosed no relevant financial relationships.

Nat Commun. Published online February 2, 2018. Full text

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