Threshold and Mechanisms of Magnetophosphene Perception

Background

The effect of Magnetic Fields (MF) on the human body has long been a hot topic in scientific research. Extremely Low-Frequency Magnetic Fields (ELF-MF) are widespread in daily life, primarily originating from power lines (⁵⁰⁄₆₀ Hz) and household appliances. These magnetic fields can induce electric fields and currents within the human body, which may in turn modulate brain functions. A specific phenomenon—magnetophosphene, a flickering visual perception induced by magnetic fields—is one of the bases for international electromagnetic field exposure guidelines.

The magnetophosphene phenomenon was first observed by French physician Jacques-Arsène d’Arsonval in 1896, and it was later validated in some small, non-replicative studies. In recent decades, there has been relatively limited research on magnetophosphenes, especially experimental data at household frequencies (i.e., 50 Hz and 60 Hz) is still lacking, resulting in uncertainty about the perception threshold of magnetophosphene and ongoing debate about the exact site of action (whether in the retina or the visual cortex).

Authors and Sources

This article was written by Alexandre Legros, Janita Nissi, Ilkka Laakso, Joan Duprez, Robert Kavet, and Julien Modolo, among others. The research team is from Lawson Health Research Institute, Western University, Aalto University, Kavet Consulting LLC, and Univ Rennes and INSERM. The article was published in the journal “Brain Stimulation” on May 11, 2024.

Research Process and Methods

The study used a stimulation method called Transcranial Alternating Magnetic Stimulation (TAMS), which delivers sinusoidal electric fields similar to Transcranial Alternating Current Stimulation (TACS) in vivo. With this method, the study quantified magnetophosphene perception in 81 volunteers under different magnetic field intensities and frequencies (20 Hz, 50 Hz, 60 Hz, and 100 Hz).

Specific Steps and Methods of the Study

1. Participants and Exclusion Criteria:

   • A total of 81 healthy volunteers were recruited.
   • Exclusions were made for participants with eye or retinal issues, claustrophobia, head injuries, neurological and cardiovascular diseases, and those with metal devices or implants above the neck.

2. Magnetic Field Exposure System:

   • MRI gradient amplifiers powered the coil system, with one set for localized exposure (retina and occiput) and another for whole-head exposure.
   • The system could generate magnetic flux densities up to 50 mT within the frequency range of 20 to 100 Hz.

3. MRI Gradient Amplifiers and Coil System:

   • MTS 0106475 MRI gradient amplifiers (Horsham PA, now owned by Performance Control Inc.) were used.
   • The coil system consisted of hollow copper wires cooled by water, and manufactured using the “wet winding” technique to ensure the coils were compact and without air gaps.

4. Localized Exposure to the Eye and Occiput:

   • A single 176-turn coil was used; for retinal exposure (RET), the coil’s center was tangent to the outer side of the eyeball, and for occiput exposure (OCC), the coil’s center was at the back of the head.

5. Whole-Head Exposure:

   • Composed of a pair of 99-turn coils arranged in a Helmholtz-like configuration with a separation of 20.6 cm, mounted on a motorized platform to ensure no contact with participants.

6. Experimental Procedure:

   • Volunteers were exposed to magnetic fields ranging from 0 to 50 mT (in 5 mT increments) for 5 seconds per exposure, with 5-second intervals between exposures, over five trials. Magnetic field presence was recorded by pressing a button.

7. Exposure Response Analysis:

   • A mixed logistic regression model was used to analyze the perception response at each frequency, fitting the binary (yes/no) data to the magnetic field intensity variation curve.

8. Dose Measurement Analysis:

   • Finite element methods were used to calculate the induced electric field and current density in 14 human head models.

Research Results

As shown in Figure 1, the logistic regression curves for different frequency and exposure mode combinations were illustrated. Table 1 shows the regression coefficient values and their statistical significance. The results indicated statistically significant perception thresholds at all frequencies for whole-head and retinal exposures. Only the occiput exposure mode showed significance at 60 Hz and 100 Hz. Once magnetophosphene perception was reported, all participants (81) described the phosphenes as colorless “white” dots, suggesting that the source of magnetophosphene perception is rods in the retina.

Conclusion and Significance

Research Conclusions

The study found that TAMS reliably induces magnetophosphene perception more effectively than TACS, without causing scalp sensations. The probability of perception based on frequency can be quantified through binary logistic regression. The results support an interaction between induced current density and retinal rod cells.

Scientific and Practical Value

This study has direct implications for international safety guidelines, helping establish perception thresholds under ELF-MF exposure, and providing new potential for the differential diagnosis of retinal diseases and for neurostimulation therapies.

Research Highlights

1. The novel TAMS method does not cause scalp sensation, outperforming traditional TACS.
2. The study provides the first human data on magnetophosphene perception thresholds at frequencies from 20 Hz to 100 Hz.
3. Dose measurement analysis strongly suggests that the perception site of phosphene lies in retinal rod cells.

Other Valuable Information

The MF exposure system used in this study has unique technological innovations, capable of generating higher magnetic flux density levels than previous experimental systems, and has significant practical advantages.