When a football player takes a big hit to the head and gets his “bell rung,” it’s more than an expression: Like a real bell, the player’s brain can oscillate at different frequencies, a new modeling study finds. The results bolster the notion that concussions originate not so much from the collision of the brain with the skull, but in the stretching and shearing of tissue that the ringing causes deep within the brain. The researchers suggest that better helmets could be designed to dampen the most damaging low-frequency vibrations.
The work could significantly simplify the modeling of concussions, says Philip Bayly, a mechanical engineer specializing in head impacts at Washington University in St. Louis in Missouri, who was not involved in the study. “To me, that’s the main thing,” he says. “It’s the low frequencies that dominate and you can simulate the brain with just a few low-frequency modes.”
Every year hundreds of thousands of Americans suffer concussions while participating in sports or other recreational activities. However, scientists don’t know exactly how a blow to the head produces the injury. In the popular conception, a concussion occurs when the cranium comes to a sudden stop and the brain crashes into it, like a car driver flying into the dashboard. But research suggests that concussions are more complicated. For example, a violent rotation of the skull—as can happen when a skateboarder’s head slams sideways into the pavement—can cause more damage than a simple sudden stop. Clinical data also suggest that concussions involve more than the surface of the brain, as their severity correlates with damage deeper inside, such as deformations around the corpus callosum, the bridge between the brain’s hemispheres.
To better determine what actually happens during a hit to the head, David Camarillo, a bioengineer at Stanford University in Palo Alto, California, and colleagues collected data on real blows by outfitting 31 Stanford football players with mouth guards equipped with accelerometers and gyroscopes. Using data on 189 in-game collisions, including two that resulted in concussions, the researchers simulated how the brain responded mechanically to each hit, using data on the material properties of the various brain tissues derived mainly from cadavers.
Each blow set the brain wobbling in a complicated way for a few tenths of a second, the team found. Researchers broke that motion down into dynamical modes—short-lived patterns of motion with distinct frequencies. When jolted, the brain oscillates most vigorously at about 30 cycles per second, roughly the same frequency as the second lowest key on a piano, the researchers report today in Physical Review Letters. On average, modes below 33 cycles per second soak up 75% of the total energy imparted to the brain.
Moreover, harder impacts excite more modes, says Mehmet Kurt, an expert in brain biomechanics at the Stevens Institute of Technology in Hoboken, New Jersey, and an author on the paper. That may be key, he says, because the different modes accentuate motion in different parts of the brain, potentially causing neighboring regions to oscillate at different frequencies. For example, the modeling of a hit in which a player lost consciousness shows that in that collision the corpus callosum oscillated at a higher frequency than the surrounding white matter. Even though the oscillations last for just a few cycles, when neighboring brain regions vibrate at different frequencies, the stretching and shearing of those tissues increases, Kurt explains.
The analysis may be both bad news and good news for concussion researchers, Kurt says. “On one hand, we’re saying, “This problem is more complicated than you might think.’ On the other hand, we’re saying that we might have the right tool to study it.” For example, Bayly says, by comparing the motions of different modes, researchers might pinpoint brain areas most susceptible to damage. Moreover, helmet manufacturers might aim for designs that damp out the most damaging frequencies, Bayly says.
However, the work comes with a caveat, says Bayly, who has written a commentary on the study for the American Physical Society’s website. To derive the motion of the brain from the accelerations measured during a hit, the researchers depend on estimates of the mechanical properties of brain tissues such as their stiffness and propensity to soak up energy. For ethical reasons, those properties cannot be measured in living brains under injury-inducing conditions, Bayly notes. So the mechanical properties of brain material “aren’t anywhere as well-known as those of steel and aluminum,” he notes.
To overcome that limitation, Bayly suggests researchers next validate their technique by applying it to lower-impact collisions, which can be safely inflicted on subjects in the lab and for which the material properties of a living brain under the appropriate conditions can be deduced from magnetic resonance imagining. Successfully putting the modeling to that test would increase confidence in its implications for harder hits, he says. “That would be really interesting and probably the very next step.”