Remote neural monitoring and stimulation

Remote neural monitoring (RNM) or remote EEG using microwaves/radiowaves demonstrated experimentally in the rat

Li X.P. et al (2014) demonstrated experimentally in the rat that neuronal activation can be sensed/monitored using an RF/microwave frequency as its phase change, which varies with permittivity in the examined brain site. The variation frequency of the RF electromagnetic wave was correlated with the EEG and the dominant variation frequency of the RF was identical with the dominant EEG frequency, as determined by the spectral density analysis (Fourier transform).

Use of temporal interference for brain stimulation / activation - Neurons follow the frequency of the interference pattern

Reading notes from (Grossman N. et al 2017) published in the Cell journal 

In the context of electrostimulation, if two electric fields of high frequencies differing by a small amount are applied to the brain and the resulting interference pattern, i.e. their envelope modulation frequency, corresponding to their difference is within a certain frequency range, the neurons will be able to follow it. The notion is similar to demodulation by the neurons. The neural membrane acts similarly to a low-pass filter and as a result neural electrical activity cannot follow a very high frequency oscillating electric field superior to 1000 Hz (Hutcheon and Yarom, 2000).

Grossman N. et al report in their study published in the Cell journal that they used interferential stimulation with two sinusoids, 2.01 KHz and 2 KHz, resulting in a a ΔEq envelope frequency of 10 Hz which recruited neurons to fire at 10 Hz, exactly like direct 10 Hz stimulation which would be expected to affect neural activity significantly (Miranda et al., 2013).

The amplitude of the envelope modulation at a specific point is determined by the vectorial sum of the two applied field vectors at that point and as a result it can have a maximum at a distant location in the brain, away from the electrodes, even deep in the brain. By altering the positions of the electrodes, the location of the envelope modulation peak could be steered within the tissue. Similarly, by changing the current ratio of the electrodes, the peak could be moved towards the electrode with the less current.

It was proven that a peak envelope modulation of 10 Hz was accomplished at a deep site, with lower envelope modulation amplitudes in more superficial structures. The researchers could activate the hippocampus without also recruiting the overlying cortex. Also by steering the envelope peak, it was possible to activate different motor cortex functional features as demonstrated by the induced motor patterns in mice (movement of forepaws, whiskers, and ears).

In conclusion, temporal interferential stimulation allows to steer the stimulation target without changing the position of the electrodes, only by altering the current intensity delivered to each electrode. In this way, deep brain structures can be stimulated without effect on neighboring locations.