The magnetic field around the earth can protect the earth from cosmic radiation. The human brain may also be sensitive to the earth's magnetic field. According to foreign media reports, for some creatures, the earth's magnetic field, like a compass, can help them judge the direction. Migratory birds, sea turtles and certain species of bacteria are all species with "built-in navigation systems". What about humans? Humans can also feel the earth's magnetic field, according to a new study. Studies have shown that humans may be able to sense the presence of the Earth's magnetic field through magnetic particles distributed throughout the brain. The new study, which uses brain scans, provides direct evidence for this conclusion for the first time. The ability to detect a magnetic field is called "magnetic inductance" (magnetoreception). The idea that humans may have this ability was first put forward in the 1980s. But since the 1990s, follow-up studies of the brain have found no evidence that humans have this ability, but an international research team has decided to use new data analysis techniques to reanalyze the problem. To study whether humans can feel the presence of a magnetic field, the researchers asked 34 subjects to sit in a test darkroom surrounded by a large square coil. Current passing through these coils can change the magnetic field in the test room. Connie Wang, a doctoral student at the California Institute of Technology and lead author of the study, points out that the intensity of the magnetic field is about the same as that of the Earth's magnetic field. About one-100000 of the magnetic field produced by an NMR machine. The researchers asked the subjects to close their eyes, relax their bodies, and then adjust the magnetic field around them. During the experiment, they used an EEG machine to record a class of brain waves called alpha waves. Scientists know that once the brain receives a signal, whether visual, auditory, or magnetic, the amplitude of the alpha wave decreases. Brain scans showed a strong response to one of the changes in the magnetic field in four of the 34 subjects. The change is the direction of the magnetic field from northeast to northwest. If the person outside the test quickly turns his head from left to right, the direction of the magnetic field will also change relatively, except that the head is moving relative to the static magnetic field, not the magnetic field itself. The alpha wave amplitudes in the brains of the four subjects were reduced by nearly 60%. But this reaction occurs only when the direction of the magnetic field changes from northeast to northwest, not in any other direction. "We did not anticipate this asymmetric reaction." Wang said. Although it is not clear why this phenomenon occurs, the researchers believe that the reaction may vary from person to person, just as some people are right-handed and some are left-handed. Some subjects also reacted strongly when the inclination of the magnetic field changed. You also experience this magnetic field change when you travel from the northern hemisphere to the southern hemisphere. To ensure that the results were not accidental, the subjects were tested again a few weeks later, and the results remained the same. (Stuart Gilder, a professor of geophysics at the University of Munich who was not involved in the study, said the repeated results made the study persuasive. Gilder said he did not think that "most people are unaware of the presence of a magnetic field," a finding that contradicts the study, because different brains may behave differently about the ability. "just as some people are good at art, others are good at math. The performance or response of different people's organs does not have to be exactly the same. " However, the study did raise some problems. For example, if people lie down, or if the magnetic field slows down, how do they sense the presence of the magnetic field? It is not clear why some people seem to have magnetic perception, but in theory it does help people judge direction or find their way. "many animals use the earth's magnetic field to direct and find their way." "since a variety of creatures have this ability, we think humans have at least some of it, even though we don't need it in our daily lives," Wang points out. " In addition, there are many unanswered questions about magnetic inductance, such as its working mechanism, and so on. Scientists have so far understood only one operating mechanism of biomagnetism, the so-called magnetotactic organism (magnetotactic bacteria). These microbes can use the magnetic particles (Fe3O4) in their brains to move along the earth's magnetic sensing lines. Decades ago, Joseph Kirshvenk, professor of geobiology at the California Institute of Technology and senior author of the new study, discovered that the magnet particles also exist in the human brain. Not only that, a study published by Gilder's team in August 2018 found that these magnet particles are distributed throughout the human brain. The study's authors conclude that magnet particles are widespread in the human brain, suggesting that they may have certain biological functions.