Here’s the first map of the magnetic mineral magnetite in the human brain. Turns out that our brain stem may be full of it.

Scientists have mapped magnetic materials in human brains for the first time, revealing that our brains may selectively contain more magnetic material in their lower and more ancient regions.

Researchers used seven specimens donated in Germany to measure brain tissue for signs of magnetite, Earth’s most magnetic mineral. Scientists have known that other types of life, such as special kinds of bacteria, contain magnetite. But the distribution of magnetite in human brains has been unclear because no systematic study had mapped the mineral in human tissue before.

The results could shine a light on why humans have magnetite in their brains to begin with, which remains an open question. Stuart Gilder, the lead author of the study and a scientist at Munich University, said that their results show that magnetic particles exist in the “more ancient” part of the brain. “We thought from an evolutionary standpoint, that was important,” Gilder said.

Magnetic Minds

Scientists discovered the first hints of magnets in human brains in 1992: A paper reported that tiny crystal grains, some barely wider than a DNA strand, were found in human brain tissue from seven patients in California. The crystals looked just like the tiny magnets in magnetotactic bacteria that help them navigate along geomagnetic field lines in lakes and saltwater environments.

Scientists are not sure why or how magnetite gets into human brains. Magnetite could serve some physiological function, such as signal transmission in the brain, but scientists are only able to speculate. A further mystery is how magnetite arrives in the brain in the first place: One study of the frontal cortex of 37 human brains suggests that we breathe in magnetite from the environment. But other researchers, like Gilder, think magnetite comes from internal sources.

From Rocks to Brains

To find out some answers, Gilder and his team dissected seven brains and measured their magnetic strength and orientation. The brains had been preserved in formaldehyde since the 1990s, when relatives and guardians of the deceased donated them to science. The brains came from four men and three women between the ages of 54 and 87.

Gilder typically studies rocks in his lab to ascertain their geologic history, but his latest study was not so different, he says. “I could essentially apply everything that I do to rock to brains,” Gilder said. The scientists cut the preserved brains into 822 pieces and ran each sample through a magnetometer, a machine used to measure records of Earth’s magnetic field in rocks.

When Gilder studies a rock, he measures its magnetism in two steps: First, he tests the rock’s natural magnetic strength, which will typically be low because rocks are bad at creating orderly magnets. (Even if the rock contains magnetic particles, their dipoles point in random directions, potentially canceling each other out.)

Second, Gilder uses an electromagnet to apply a strong magnetic field to the sample, and this aligns the tiny magnetic particles so that they all face the same direction. When he tests their magnetic strength a second time, he sees the full strength of the magnetic signal from the rock. “If I measure something that is more magnetic after I’ve applied a very big magnetic field, that’s proof that this material contains magnetic recording particles,” Gilder said.

Gilder applied the same two-step technique to the brain samples. The comparison revealed that the human brain had a detectable magnetism after a magnetic field had been applied to the samples. The results showed that magnetite was in “almost every piece” of the specimens, said Gilder.

“The Exact Same Pattern”

The latest study reveals that the lower regions of the human brain, including the cerebellum and the brain stem, had 2 or more times the magnetic remanence of the upper regions of the brain. The upper regions of the brain compose the cerebrum, which is responsible for reasoning, speech, and other tasks, whereas the lower regions handle muscle movement and automatic functions like heart rate and breathing.

Gilder said that the pattern emerged in each of the seven brains, and it showed no difference depending on the person’s age or sex. The brain stem had consistently higher magnetization than any other region, although only five of the seven brains had brain stems intact.

Joseph Kirschvink, a professor at the California Institute of Technology in Pasadena not involved in the study, said that the work “confirms the biological origin of the brain magnetite.” Kirschvink said that the results in the study closely matched research he had performed in his lab, but the latest research has “100 times more data.”

The scientists took pains to limit contamination, cutting the samples with a ceramic knife and staging the experiment inside a magnetically shielded room in a forest far from urban pollution. They removed samples with high levels of natural magnetic strength that could have been polluted with fragments of the saw cutting into the donors’ skulls many years ago. Even with the potentially contaminated samples removed, the data still showed an anatomical pattern.

Gilder presented the research this month at AGU’s Fall Meeting 2019 in San Francisco, Calif.

—Jenessa Duncombe (@jrdscience), News Writing and Production Fellow