New evidence for a human magnetic sense that lets your brain detect the Earth's magnetic field
Posted by admin on 18th March 2019

Do human beings have a magnetic sense? Biologists know other animals do. They think it helps creatures including bees, turtles and birds navigate through the world.

Scientists have tried to investigate whether humans belong on the list of magnetically sensitive organisms. For decades, there’s been a back-and-forth between positive reports and failures to demonstrate the trait in people, with seemingly endless controversy.

The mixed results in people may be due to the fact that virtually all past studies relied on behavioral decisions from the participants. If human beings do possess a magnetic sense, daily experience suggests that it would be very weak or deeply subconscious. Such faint impressions could easily be misinterpreted – or just plain missed – when trying to make decisions.

So our research group – including a geophysical biologist, a cognitive neuroscientist and a neuroengineer – took another approach. What we found arguably provides the first concrete neuroscientific evidence that humans do have a geomagnetic sense.

How does a biological geomagnetic sense work?

Life on Earth is exposed to the planet’s ever-present geomagnetic field that varies in intensity and direction across the planetary surface.

The Earth is surrounded by a magnetic field, generated by the movement of the planet’s liquid core. It’s why a magnetic compass points north. At Earth’s surface, this magnetic field is fairly weak, about 100 times weaker than that of a refrigerator magnet.

Over the past 50 years or so, scientists have shown that hundreds of organisms in nearly all branches of the bacterial, protist and animal kingdoms have the ability to detect and respond to this geomagnetic field. In some animals – such as honey bees – the geomagnetic behavioral responses are as strong as the responses to light, odor or touch. Biologists have identified strong responses in vertebrates ranging from fish, amphibians, reptiles, numerous birds and a diverse variety of mammals including whales, rodents, bats, cows and dogs – the last of which can be trained to find a hidden bar magnet. In all of these cases, the animals are using the geomagnetic field as components of their homing and navigation abilities, along with other cues like sight, smell and hearing.

Skeptics dismissed early reports of these responses, largely because there didn’t seem to be a biophysical mechanism that could translate the Earth’s weak geomagnetic field into strong neural signals. This view was dramatically changed by the discovery that living cells have the ability to build nanocrystals of the ferromagnetic mineral magnetite – basically, tiny iron magnets. Biogenic crystals of magnetite were first seen in the teeth of one group of mollusks, later in bacteria, and then in a variety of other organisms ranging from protists and animals such as insects, fish and mammals, including within tissues of the human brain.

Chains of magnetosomes from a sockeye salmon.
Mann, Sparks, Walker & Kirschvink, 1988, CC BY-ND

Nevertheless, scientists haven’t considered humans to be magnetically sensitive organisms.

Manipulating the magnetic field

Schematic drawing of the human magnetoreception test chamber at Caltech.
Modified from ‘Center of attraction’ by C. Bickel (Hand, 2016).

In our new study, we asked 34 participants simply to sit in our testing chamber while we directly recorded electrical activity in their brains with electroencephalography (EEG). Our modified Faraday cage included a set of 3-axis coils that let us create controlled magnetic fields of high uniformity via electric current we ran through its wires. Since we live in mid-latitudes of the Northern Hemisphere, the environmental magnetic field in our lab dips downwards to the north at about 60 degrees from horizontal.

In normal life, when someone rotates their head – say, nodding up and down or turning the head from left to right – the direction of the geomagnetic field (which remains constant in space) will shift relative to their skull. This is no surprise to the subject’s brain, as it directed the muscles to move the head in the appropriate fashion in the first place.

Study participants sat in the experimental chamber facing north, while the downwards-pointing field rotated clockwise (blue arrow) from northwest to northeast or counterclockwise (red arrow) from northeast to northwest.
Magnetic Field Laboratory, Caltech, CC BY-ND

In our experimental chamber, we can move the magnetic field silently relative to the brain, but without the brain having initiated any signal to move the head. This is comparable to situations when your head or trunk is passively rotated by somebody else, or when you’re a passenger in a vehicle which rotates. In those cases, though, your body will still register vestibular signals about its position in space, along with the magnetic field changes – in contrast, our experimental stimulation was only a magnetic field shift. When we shifted the magnetic field in the chamber, our participants did not experience any obvious feelings.

The EEG data, on the other hand, revealed that certain magnetic field rotations could trigger strong and reproducible brain responses. One EEG pattern known from existing research, called alpha-ERD (event-related desynchronization), typically shows up when a person suddenly detects and processes a sensory stimulus. The brains were “concerned” with the unexpected change in the magnetic field direction, and this triggered the alpha-wave reduction. That we saw such alpha-ERD patterns in response to simple magnetic rotations is powerful evidence for human magnetoreception.