Remote neural monitoring - stimulation using interference

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

For electroencephalography please refer to the following Wikipedia article: https://en.wikipedia.org/wiki/Electroencephalography

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, whereas the dominant variation frequency of the RF was identical with the dominant EEG frequency, as determined by the spectral density analysis (Fourier transform).

Notes based on the article by Li X.P. et al (2014)

When an electromagnetic wave propagates in a nonlinear dielectric body such as the brain which is multi-layered and of low dielectric loss (low dissipation) the amplitude and phase of the propagating electromagnetic wave are changed according to the permittivity of the dielectric.

Neuronal activation is linked to ion concentration change within the extracellular fluid resulting in the change of permittivity of the fluid. The permittivity of a material describes how the electric flux between two point charges is affected by the material (i.e. decreased compared to vacuum). It is also a measure of the ability of the material to store an electric field in its polarization. The extracellular fluid is considered as a dielectric of dynamic nature as movements of ions create polarization densities. In order to study the electromagnetic wave propagation in the brain, the dielectric is equated to the extracellular fluid.

Remote cardiorespiratory monitoring comprises the use of different RADAR and LIDAR techniques such as Doppler RADAR or LIDAR  for determination of the movement of the thoracic wall which is linked to respiration and heartbeat. The principle of Doppler RADAR consists of the transmission of an electromagnetic wave towards a subject and the reception of the backscattered wave which is characterized by a change of phase (phase shift), or in other terms, it is phase-modulated.

An empirical way to understand the change of phase is to consider a rotating object which collides slightly with an obstacle in the course of its rotation. As a result, the object will lose energy, its velocity will be slightly decreased and it will be “left behind” by a few degrees, meaning that its angular position with respect to a predetermined axis will change. The inferred angle represents the new phase or the shifted phase.

Similarly, an electromagnetic wave pushes/pulls ions such as Na+, K+, Mg2+, Ca2+ and Cl− in the brain — in a notion which is relevant to the collision mentioned previously — and as a result it loses energy, it is “left behind” and its phase changes.

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 spectral density analysis (Fourier).

Specifically, by using a 30 GHz (milimeter wave) they measured phase change in the range of 0.2 to 0.6 degrees and respective amplitude change (in dB). The dominant brain frequency was determined to be 2.2 Hz which was expected due to rat anesthesia which is linked to the slow-wave sleep mode of the brain corresponding to the delta range (0.5-4 Hz).

The accompanying theoretical study of the authors (Li X.P. et al (2014)) derived the relationship between the electromagnetic wave phase change φ, and the value of the permittivity of the dielectric, ε (Equation 1- Li X.P. et al (2014)). In their simulation, they modelled neuronal activation as an increase of permittivity by 100% (in a spherical site of a 6mm radius) and they determined theoretically a change of phase of 0.53 degrees, which is in line with the experimental results (model validation). 

Figure 1: Excerpt of Figure 5 from Li X.P. et al (2014) demonstrating:

(A) variation of phase change of the EM wave (propagating through the rat brain) and its Fourier

(B) variation of amplitude change of the EM wave and its Fourier

(C) EEG of rat and its Fourier

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.