When a geologist searches for minerals, they look at a map marked with iron deposits and cracks in the rock. A magnetic map of the brain works in a similar way: it shows where iron has accumulated in the tissues and where the sheath of nerve fibers, myelin, has been damaged.
After a blow to the head, the brain shifts inside the skull, and nerve fibers stretch. If the sheath of a fiber tears, the signal on the magnetic map changes. Scientists made such maps for 25 athletes in the first two weeks after a concussion and compared them with maps from healthy people.
In the deep nuclei of the brain, which control movements, iron did not accumulate. But the damage in the hippocampus explains why after a concussion it can be hard to concentrate or remember new things. A few months later, the scientists will take images again to check whether the brain has recovered.
The hippocampus is named after the seahorse because of its shape, and the CA4 subfield sits in its "tail" and contains cells that are among the first to die in epilepsy and injury.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.