For Electroencephalography please refer to this Wikipedia Section: 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 and the dominant variation frequency of the RF was identical with the dominant EEG frequency, as determined by the spectral density analysis (Fourier).
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: Figure 5 (excerpt) 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