Shocking the Brain: Computer simulations are helping scientists identify the parts of the brain most vulnerable to blast injury. This series of images shows a simulated pressure wave (originating in the right side of the first image) hitting the front of the virtual head (center, shown here cut in half), with the highest pressure levels shown in red. The pressure wave ricochets around the tissue as it’s deflected by different brain structures and continues to propagate inside the brain even after the pressure wave in the air has passed (last two frames).
Across the Potomac River at DARPA, Geoffrey Ling has embarked on a similar quest to determine how blasts injure the brain. But unlike Radovitzky and Moore, whose computer model focuses on the pressure wave and its interaction with brain tissue, Ling and his colleagues are using animals, mostly pigs, to study the damage inflicted by each component of the blast: heat, sound, light, pressure wave. “We want to figure out what in that dirty environment causes [the most] injury,” Ling says. “Say it’s pressure or sound. Then we can go back and look for strategies to defeat them.”
The pigs are immobilized in harnesses and then exposed to an explosion powerful enough to cause moderate to severe brain injuries. Since the animals will not be thrown against a wall or hit with debris, the scientists can study the effects of the explosion in isolation. “When exposed to a survivable blast, they have difficulty walking that lasts for days,” says Ling. The explosions also disrupt appetite–all symptoms that mimic those reported by soldiers with blast-induced concussions.
But another finding is surprising. Most scientists have assumed that blast-related injury comes from the pressure wave. Preliminary studies from the DARPA program seem to contradict that hypothesis. When pigs were put into a specialized wind tunnel that generates shock waves like those accompanying blasts, the scientists did not see the same neurological effects found in pigs exposed to explosions. “We had to ramp up the pressure significantly before we saw [brain-related problems],” says Ling. “That made us step back and say, maybe it’s something else, or not the pressure wave alone.”
Radovitzky and Moore say that Ling’s findings can’t be directly compared with their own. Pigs’ skulls are thicker than humans’, for instance, so the interaction of the pressure wave and the pigs’ brains may be different, too. But the apparent contradiction does illustrate just how difficult it is to understand brain injury.
Ling’s team will soon begin studying other potential causes of injury, such as electromagnetic pulses (EMPs). If the EMP from a blast is powerful enough, it can interfere with nearby electronic devices. “The brain is an electrical organ,” says Ling. “If an EMP pulse can take out a radio, why not short-circuit the brain?”
Meanwhile, the pig studies have shed some light on the biology of blast-related brain injury. Animals subjected to explosions show signs of neurodegeneration: according to Ling, preliminary results suggest that some of the pigs’ neural fibers start to break down, triggering cell death primarily in the cerebellum (a brain structure involved in balance and coördination) and the frontal lobes (which play a role in impulse control, judgment, problem solving, complex planning, and motivation). As with the injured soldiers, however, it is not yet clear how the test pigs will fare in the long run–whether they will heal, whether their walking deficits will continue, or whether their initial injuries will set off a spiral of neural degeneration. And perhaps most important, it remains uncertain whether pigs exposed to repeated explosions will suffer exponentially more harm than those whose exposure is more limited.
Ling is overseeing a study of marines being trained to set controlled explosions, which should provide some evidence of the effects of successive but milder blasts. “Because [they] expose themselves repeatedly to blast, we can determine if, in fact, these repeated exposures cause mild TBI,” says Ling. The marines will undergo cognitive and neuropsychological testing and intensive brain-imaging studies both before and after their training. And because their blast exposure doesn’t occur on the battlefield, they are unlikely to experience the combat stress that can complicate the diagnosis of brain injury.