An international team of neuroscientists and geoscientists from Caltech, the University of Tokyo, Princeton University and Tokyo Institute of Technology has discovered that the human brain can detect Earth-strength magnetic fields.
The Earth is surrounded by a magnetic field, generated by the movement of the planet’s liquid core. At the planetary surface, this magnetic field is fairly weak, about 100 times weaker than that of a refrigerator magnet.
Many migratory (such as birds, turtles, eels and lobsters) and non-migratory (fruit flies, cockroaches, honeybees) animals are equipped with a special sense called magnetoreception that allows them to detect this field to perceive direction, altitude or location.
Although magnetoreception has been well-studied in these creatures, scientists have not yet been able to determine whether humans share this ability.
Dr. Joseph Kirschvink, a geoscientist from Caltech and Tokyo Institute of Technology, Caltech neuroscientist Dr. Shin Shimojo and their colleagues set out to address this long-standing question using electroencephalography (EEG) to record adult participants’ brain activity during magnetic field manipulations.
“Magnetoreception, the perception of the geomagnetic field, is a sensory modality well-established across all major groups of vertebrates and some invertebrates,” they said.
“Many past attempts have been made to test for the presence of human magnetoreception using behavioral assays, but the results were inconclusive.”
“To avoid the cognitive and behavioral artifacts, we decided to use EEG techniques to see directly whether or not the human brain has passive responses to magnetic field changes.”
EEG, which records brain electrical activity and reflects information processing in many interconnected neurons, is an ideal tool to study subconscious processes in which physical stimuli are picked up by the brain but do not enter conscious awareness. This happens with all sorts of sensory stimuli, which can influence our cognition and behavior without us ever knowing that we saw, heard, or felt anything new.
“We thought that geomagnetic stimuli might be processed this way, so we looked at human brain waves for any signs that we have a magnetic sense,” the scientists said.
They asked 34 adult volunteers (24 male, 12 female) simply to sit in a testing chamber while they directly recorded electrical activity in their brains with EEG.
“In our experiment, the participant relaxes in an extremely comfortable chair inside a radiofrequency shielded test chamber,” they said.
“Inside the chamber is a set of large square coils, setup along three different directions (up/down, E/W, N/S) through which electric current flows to generate an Earth-strength magnetic field within. The field can point in any direction, whether up, down, N, S, or anywhere in between, and can simulate the local magnetic field in the Northern or Southern Hemisphere.”
“We can vary the magnetic field direction or intensity over short time intervals of 100 milliseconds while recording EEG to see if brain activity changes in response to a changing magnetic field.”
“While our human participants sat doing nothing with their eyes closed for about 7 minutes, we changed the direction of the magnetic field at irregular intervals, up to 100 times throughout the experiment. Each time, the field rotated clockwise or counterclockwise, analogous to turning your head left or right but without actually moving. The whole time, the field pointed steeply downwards (60-75 degrees from horizontal) like the Earth’s field in mid-latitudes of the Northern Hemisphere, where most of participants were born and raised.”
“By comparing brain waves following field rotations with brain waves in control trials when the field didn’t change, we can spot an EEG difference indicating that the brain is processing geomagnetic stimuli.”
After a downwards magnetic field rotated counterclockwise, some volunteers responded with a large drop in amplitude of their EEG alpha waves, up to a 60% decrease from pre-stimulus levels.
“Alpha waves are EEG oscillations that go up and down at a frequency around 10 Hz,” the researchers said.
“They dominate the EEG signal when we are awake with our eyes closed, and arise from the spontaneous, synchronized activity of millions of neurons. Their function is not well-understood, but they may reflect a relaxed mind with nothing in particular to focus on and no particular task to do.”
“When a stimulus suddenly appears and is processed, neurons fall out of synchrony with each other, the alpha rhythm is disrupted, and alpha waves get smaller as a result.”
This phenomenon is called alpha event-related desynchronization (alpha-ERD).
“In our experiment, alpha-ERD shows that the human brain can detect Earth-strength magnetic fields, demonstrating that we have a sensory system that processes the geomagnetic field all around us,” the authors said.
“Potentially, we and/or our nomadic hunter-gatherer ancestors could use a magnetic sense to navigate and survive.”
“Future studies of magnetoreception in diverse human populations may provide new clues into the evolution and individual variation of this ancient sensory system.”
The findings were published in the journal eNeuro.
Connie X. Wang et al. Transduction of the Geomagnetic Field as Evidenced from Alpha-band Activity in the Human Brain. eNeuro, published online March 18, 2019; doi: 10.1523/ENEURO.0483-18.2019