Do human beings have a magnetic sense? Biologists know that other animals do it. They think it helps creatures like bees, turtles and birds to sail around the world.
Scientists have been trying to determine if humans are on the list of magnetic-sensitive organisms. For decades, there has been a back and forth between positive relationships and failures to demonstrate the trait in humans, with seemingly endless controversy.
The mixed results in people may be due to the fact that virtually all previous studies were based on participants' behavioral decisions. If human beings possess a magnetic sense, the daily experience suggests that it would be very weak or deeply subconscious. Such weak impressions could easily be misinterpreted – or simply missed – when you try to make decisions.
For example, our research group – including a geophysical biologist, a cognitive neuroscientist and a neuroengineer – took another approach. What we have found probably provides the first concrete neuroscientific evidence that humans have a geomagnetic sense.
How does a geomagnetic biological sense work?
The Earth is surrounded by a magnetic field generated by the movement of the liquid core of the planet. This is why a magnetic compass indicates the north. On the surface of the Earth, this magnetic field is relatively weak, about 100 times lower than that of a refrigerator magnet.
Over the past 50 years, scientists have shown that hundreds of organisms in almost every branch of the bacterial, protist, and animal kingdom have the ability to detect and respond to this geomagnetic field. In some animals – such as honey bees – geomagnetic behavioral responses are as strong as responses to light, smell or touch. Biologists have identified strong responses in vertebrates: fish, amphibians, reptiles, many birds and various mammals, including whales, rodents, bats, cows and dogs. These can be driven in search of a hidden magnet. In all these cases, animals use the geomagnetic field as components of their navigation and navigation capabilities, as well as other clues such as sight, smell and hearing.
Skeptics rejected the early reports of these responses, largely because there did not appear to be a biophysical mechanism capable of translating the Earth's weak geomagnetic field into strong neuronal signals. This vision has been radically altered by the discovery that living cells are capable of building nanocrystals of ferromagnetic mineral magnetite, essentially tiny iron magnets. Biogenic magnetite crystals were first observed in the teeth of a group of molluscs, then in bacteria, then in various other organisms ranging from protists and animals such as insects, fish and mammals, including in the tissues of the human brain.
Nevertheless, scientists have not regarded humans as magnetically sensitive organisms.
Manipulation of the magnetic field
In our new study, we simply asked 34 participants to sit in our test chamber while we were recording electrical activity directly in their brains with electroencephalography (EEG). Our modified Faraday cage included a set of 3-axis coils that allowed us to create controlled magnetic fields of great uniformity via the electrical current flowing through its wires. Since we live in the mid-latitudes of the northern hemisphere, the environmental magnetic field of our laboratory tilts north about 60 degrees from the horizontal.
In a normal life, when someone turns his head – for example by nodding or turning his head from left to right – the direction of the geomagnetic field (which remains constant in the space) will move through compared to his skull. This is not a surprise to the subject's brain, as it required the muscles to move their head in the proper way in the first place.
In our experimental chamber, we can move the magnetic field silently to the brain, but without the brain triggering any signal to move the head. This is comparable to situations where your head or trunk is shot passively by someone else or when you are a passenger in a rotating vehicle. In these cases, however, your body will always record vestibular signals regarding its position in the space, as well as magnetic field changes. In contrast, our experimental stimulation was only a magnetic field shift. When we moved the magnetic field in the room, our participants did not feel any obvious feelings.
EEG data, on the other hand, revealed that some rotations of the magnetic field could trigger strong and reproducible brain responses. A known EEG scheme of existing research, called alpha-ERD (event-related desynchronization), usually occurs when a person suddenly detects and processes a sensory stimulus. The brains were "preoccupied" by the unexpected change in the direction of the magnetic field, which triggered the reduction of the alpha wave. The fact that we have observed such alpha-ERD models in response to simple magnetic rotations is irrefutable proof of magnetoreception.
The brains of our participants responded only when the vertical component of the field was down at about 60 degrees (in horizontal rotation), as is the case here in Pasadena, California. They did not react to the unnatural directions of the magnetic field – as when it was directed upwards. We suggest that the response is adjusted to natural stimuli, reflecting a biological mechanism that has been formed by natural selection.
Other researchers have shown that animals' brains filter magnetic signals, reacting only to those of environmental interest. It is wise to reject any magnetic signal that is too far from natural values, as it is most likely a magnetic anomaly, such as a light strike or limescale deposit in the ground. A first report on birds showed that blackbirds stopped using the geomagnetic field if the force was more than 25% different from the one they were used to. Perhaps this is why previous researchers have struggled to identify this magnetic sense: if they increased the strength of the magnetic field to "help" subjects to detect it, they might have rather what their brain does not ignore.
In addition, our series of experiments shows that the receptor mechanism – the biological magnetometer in humans – is not an electrical induction and can distinguish north from south. This last characteristic completely excludes the so-called "quantum compass" or "cryptochrome" mechanism that is popular today in animal literature on magnetoreception. Our results are consistent only with functional magnetoreceptor cells based on the hypothesis of biological magnetite. Note that a magnetite-based system may also explain all behavioral effects in birds that have favored the hypothesis of quantum compass rise.
Brains record magnetic changes, unconsciously
Our participants were not aware of changes in magnetic fields and their brain responses. They felt that nothing had happened during the whole experience – they had sat alone in a dark silence for an hour. Below, however, their brains revealed a wide range of differences. Some brains almost did not react, while others had alpha waves that were halved compared to their normal size after a shift in the magnetic field.
It remains to be seen what these hidden reactions could mean for the behavioral abilities of man. Do weak, strong brain responses reflect some kind of individual difference in the ability to navigate? Can those with a weaker brain response benefit from a workout? Can those with strong brain reactions be trained to actually feel the magnetic field?
A human response to magnetic fields of land might seem surprising. But given the evidence of magnetic sensation in our animal ancestors, it may be more surprising that humans have completely lost every last element of the system. So far, we have found evidence that active magnetic sensors transmit signals to the brain – a sensory capacity hitherto unknown in the subconscious human mind. The full extent of our magnetic heritage remains to be discovered.
Shinsuke Shimojo, Professor Gertrude Baltimore of Experimental Psychology, California Institute of Technology; Daw-An Wu, California Institute of Technologyand Joseph Kirschvink, professor of geobiology Nico and Marilyn Van Wingen, California Institute of Technology
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