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and sinuses, and to a lesser extent through the skull, an observa-
tion that could influence the design of protective gear. Radovitzky
and Moore are testing a new version of the model that includes
a helmet, to evaluate how well it shields against the blast wave.
"Blast protection for the head has not been a consideration in the
design of body armor," says Radovitzky. "Maybe a small change
to the armor could mediate the damage."
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 environ-
ment 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 e ects of the explo-
sion in isolation. "When exposed to a survivable blast, they have
di culty 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 hypothe-
sis. 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 e ects found in pigs exposed to explo-
sions. "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 di erent, too. But the apparent contradiction does
illustrate just how di cult 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 struc-
ture 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 con-
tinue, or whether their initial injuries will set o a spiral of neural
degeneration. And perhaps most important, it remains uncertain
whether pigs exposed to repeated explosions will su er exponen-
tially more harm than those whose exposure is more limited.
Ling is overseeing a study of marines being trained to set con-
trolled explosions, which should provide some evidence of the
e ects 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.
On May 20, 2004, Jerry Pendergrass's convoy was ambushed.
The National Guard sergeant was standing outside his Humvee
when a rocket-propelled grenade landed a few feet behind him and
exploded, launching him 15 feet in the air. A few moments later,
Pendergrass found himself lying on the ground, shrapnel lodged
in his leg and his helmet several yards away. He was conscious
but unsure of where he was, classic signs of concussion. Another
member of his unit pulled him behind the protective barrier of
the disabled Humvee, where they awaited evacuation to a medi-
cal checkpoint in a secure zone down the road.
Pendergrass soon returned to duty, ignoring the persistent
headaches and the sleep, memory, and balance problems that
plagued him after the blast. When his tour was up and he returned
home to North Carolina, he took prescription painkillers and
drank, trying to wash away both his memories of war and the real-
ity of his health problems. It wasn't until he began a second tour---
and was evacuated two months later for spinal damage linked to
the earlier blast---that he realized the full extent of his injuries. He
was diagnosed with both mild traumatic brain injury and post-
traumatic stress disorder (PTSD)---a condition, first defined in
Vietnam veterans, that can develop after exposure to a terrifying
event. "Big bangs scare the living fart out of me," says Pendergrass,
in a conference room at the Lakeview Virginia NeuroCare cen-
ter in Charlottesville, VA. He seems startled by even small noises,
jumping as a nearby copy machine is jostled into action.
Watch video interviews with Harvard s Kit Parker and MIT s Raul
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