A study published today offers some of the best evidence yet that humans, like many other creatures, can sense Earth’s magnetic field. But it doesn’t settle other questions that have swirled around this contentious idea for decades: If we do have a subconscious magnetic sense, does it affect our behavior? And does it arise from an iron mineral found in our brains, as the authors believe?
“I think this paper will make quite a splash,” says Peter Hore, a physical chemist at the University of Oxford in the United Kingdom. But, he adds, “Independent replication is crucial.”
A variety of species—bacteria, snails, frogs, lobsters—seem to detect Earth’s magnetic field, and some animals, such as migratory birds, rely on it for navigation. But testing for the sense in humans has been tricky. Experiments in the 1970s that asked blindfolded participants to point in a cardinal direction after being spun around or led far from home yielded inconsistent results.
Biophysicist Joe Kirschvink at the California Institute of Technology (Caltech) in Pasadena is a veteran of the search. Using electroencephalography (EEG), his team recorded brain activity from electrodes on the scalp to search for some response to changes in a highly controlled magnetic field equal in strength to Earth’s.
In the experiment, each of the 34 participants sat quietly in a dark aluminum box that shielded them from electromagnetic noise such as radio waves. By changing the flow of electric current through coils lining the box, the researchers created a magnetic field that sloped steeply downward, like Earth’s own field at the midlatitudes of the Northern Hemisphere. Then they rotated the field, as would happen if a person turned their head.
In an EEG study with a different design, published in 2002, other researchers failed to find any brain response to a changing field. Kirschvink says data analysis techniques used at the time were not powerful enough to detect an effect. The new study, published in eNeuro, found that the rotating field sometimes elicited a marked drop in waves of the α frequency, which are typical of a brain that is awake but at rest. Many EEG studies use α to track responses to visual information, says Mary MacLean, a neuroscientist at the University of California (UC), Santa Barbara, who was not involved in the work. A change in α, she says, “is generally a good indicator of the degree to which people are engaging in sensory processing.”
The effect showed up in less than a third of participants, which could indicate that genetic factors or past experiences influence a person’s sensitivity to a magnetic field, says cognitive neuroscientist Shinsuke Shimojo, another member of the Caltech team. Mysteriously, the change registered only when the field was rotated counterclockwise.
“What they show is very exciting and seems robust,” says Stuart Gilder, a geophysicist at Ludwig Maximilian University in Munich, Germany. But the results call for follow-up tests, such as measuring how different field strengths and rotation speeds affect brain activity, he adds.
“I’m not surprised there’s an effect,” says Margaret Ahmad, a biologist at Sorbonne University in Paris, who notes that magnetic fields are known to affect human and other mammalian cells in a dish. “There’s something in a cell that is different in the presence of a magnetic field,” she says. “We see this effect in human embryonic kidney cells; you’re not going to convince me that an effect in brain cells is of any greater or lesser significance.”
The Caltech team is still far from explaining how magnetoreception is possible, scientists say. “I’m convinced that something in the brain is responding to a magnetic field in a particular way,” Maclean says. “I just have no idea … what mechanism that really represents.”
The mechanism of magnetoreception is only settled for certain bacteria, which harbor magnetite crystals that align with Earth’s magnetic field. Bird beaks and fish snouts also contain magnetite, as does the human brain. Gilder and his colleagues recently found that it is most concentrated in lower, evolutionarily ancient regions—the brain stem and cerebellum. But no one has identified the proposed sensory cells that contain magnetite.
Other groups suggest a protein in the retina called cryptochrome, which senses incoming light, also responds to magnetic fields. But Kirschvink’s team contends its new results tip the scales in favor of magnetite. When they reversed their magnetic field to point upward, its rotations no longer elicited a change in brain activity. Magnetite, like a compass needle, responds to a field’s direction, whereas cryptochrome would respond identically to fields with opposite polarity.
“If the results are real, I think that rules out cryptochrome as the source of these effects in humans,” Hore says, though it might play a role in other animals.
But is a change in brain waves alone evidence of a “sense”? Some aren’t convinced. “If I were to … stick my head in a microwave and switch it on, I would see effects on my brain waves,” says Thorsten Ritz, a biophysicist at UC Irvine. “That doesn’t mean we have a microwave sense.”
More convincing would be evidence that the brain actually processes magnetic information in a way that influences behavior, Ritz says. He is intrigued by a study from a South Korean research team, published last month in PLOS ONE, which found that, in the absence of visual or auditory cues, men who had fasted for about 20 hours could sometimes orient themselves in a direction they previously associated with food.
Kirschvink’s team has experiments in progress that aim to unearth subtle consequences of a magnetic sense—for example, manipulating the magnetic field to bias a person’s best guess at a cardinal direction. “That would really supersolidly establish that humans have a full-fledged magnetosensory system,” says Caltech neuroscience graduate student Connie Wang, who is first author on the new paper. The team also wants to test whether careful training could bring magnetic sensations into consciousness.
If humans really use a magnetite-based sensor, there are other concerns to explore, Kirschvink says, such as whether the magnets in aviation headsets could impair pilots’ sense of direction, and whether the strong magnetic field generated by MRI machines could somehow alter our magnetite.
Three years ago, Kirschvink gave a preview of these results at a meeting of the United Kingdom’s Royal Institute of Navigation, which meets every 3 years in Egham. On 12 April, at the society’s next meeting, he’ll take the stage to defend his ideas to an audience of skeptics, with data in hand. “We’re going to have a fun session,” he says.