Vestibular Extremely Low-Frequency Magnetic and Electric Stimulation Effects on Human Subjective Visual Vertical Perception

Abstract

Electric fields from both extremely low-frequency magnetic fields (ELF-MF) and alternating current (AC) stimulations impact human neurophysiology. As the retinal photoreceptors, vestibular hair cells are graded potential cells and are sensitive to electric fields. Electrophosphene and magnetophosphene literature suggests different impacts of AC and ELF-MF on the vestibular hair cells. Furthermore, while AC modulates the vestibular system more globally, lateral ELF-MF stimulations could be more utricular specific. Therefore, to further address the impact of ELF-MF-induced electric fields on the human vestibular system and the potential differences with AC stimulations, we investigated the effects of both stimulation modalities on the perception of verticality using a subjective visual vertical (SVV) paradigm. For similar levels of SVV precision, the ELF-MF condition required more time to adjust SVV, and SVV variability was higher with ELF-MF than with AC vestibular-specific stimulations. Yet, the differences between AC and ELF-MF stimulations were small. Overall, this study highlights small differences between AC and ELF-MF vestibular stimulations, underlines a potential utricular contribution, and has implications for international exposure guidelines and standards. © 2022 Bioelectromagnetics Society.

Keywords: alternating current stimulation; extremely low-frequency magnetic fields; human vestibular system; magnetic induction; subjective visual vertical.

Figure 1

Experimental stimulations. All three panels show both the MF custom coil system centered over the mastoid process and the binaural electrode montage (green circles) delivering the DC and AC currents. The left panel shows a view from behind, the middle panel shows a view from the left side, and the right panel a view from above. AC = alternating current; DC = direct current; MF = magnetic fields.

Figure 2

Schematic representation of the protocol for one participant. Each session is made of 20 blocks. Each 30‐s block represents one experimental condition: CTRL, DC, AC, or MF. Each AC and MF experimental condition was given at four different frequencies: 20, 60, 120, and 160 Hz. During each 30‐s block, two consecutive SVV trials were executed. Block order is semi‐randomized, as a minimum of six randomized blocks are set between two MF blocks at 20 Hz. A 1‐min rest period is given between each experimental block. AC = alternating current; CTRL = control trials; DC = direct current; MF = magnetic fields.

Figure 3

Boxplot representation of dSVVmean between CTRL (left) and DC (right). dSVV CTRL is equal to CTRL minus CTRL (i.e. =0). DC is significantly lower than 0 (t29 = −2.0104, P = 0.027, R ² = 12%), which shows a mean misperception towards the anodal stimulation. AC = alternating current; CTRL = control trials; DC = direct current.

Figure 4

Boxplot representation of dSVVmean (left panel), dSVVstd (middle panel), and dVel (right panel) distributions comparing AC and MF stimulations. Individual measurements are presented as swarm plot over each boxplot. dSVVstd (F _1,29 =  7.86, P = 0.009, ${\mathbf{\eta }}_{\mathbf{G}}^{\mathbf{2}}$ = 2%) and dVel (F _1,29 = 9.04, P = 0.005, ${\mathbf{\eta }}_{\mathbf{G}}^{\mathbf{2}}$ = 2%) yielded significant differences between AC and MF. AC = alternating current; MF = magnetic fields.

Figure 5

Magnetic flux density distribution around the exposure device for a 20‐Hz stimulation. On the left panel, the black lines show the outer boundaries casing, and the grey lines show the outer boundaries of the solenoid. The vestibular system (represented as the two yellow structures into the skull) lies approximately 3 cm from the casing of the coil. The right panel shows the dB/dt values along the Mediolateral axis at the vestibular level. The dashed line represents the position of the coil casing (black) and the vestibular system (red) along the mediolateral axis.