Even in the daily bedlam of Iraq, life for Jason Ehrhart and Larry Perry had a measure of clarity. But that was before roadside bombs blew them out of their Humvees and into a fog that has yet to lift.
It's not clear whether either suffered a direct blow to the head, but like many brain-injured comrades, they have lingering memory problems. What is clear is that invisible blast waves slammed into their skulls and shook their brains like gelatin.
As many as one in five combat troops in Iraq and Afghanistan have suffered traumatic brain injuries, and military medical experts believe the concussive force of blast waves has contributed to more than half of those. The damage can accumulate - the result of repeated exposure to explosions at a distance.
Long-term effects can include headaches, slow thinking, poor attention and difficulty carrying out plans.
Many of the injuries have multiple causes, such as blast waves combined with the impact of flying shrapnel or a crash landing.
But doctors have also treated patients who weren't hit by anything except blast forces. In some cases, they walked away without realizing they had suffered concussions.
"Blast injuries have unfortunately been a hallmark of the recent wars," said Dr. Paul Fishman, chief of neurology at the Baltimore Veterans Affairs Medical Center. "We now have a growing body of people who have traumatic brain injury but with no apparent head injury."
Although the heavy toll from improvised explosive devices (IEDs) in Iraq and Afghanistan has grabbed 21st-century headlines, blast injuries are not a new phenomenon. From the trenches of World War I to the jungles of Vietnam, combat troops have long been exposed to the brain-altering forces of exploding bombs.
The same is true for victims of suicide bombs in the Middle East, some of whom were standing far enough from explosions to escape obvious physical injury. Israeli doctors in particular have discovered serious and sometimes fatal damage to victims' brains and abdominal organs - the result of blast waves penetrating tissue.
"Blast is a bit difficult to understand," said Fishman, but he offered a simple analogy:
"With ultrasound, we can break up kidney stones. If you have energy directed into tissue and the tissue absorbs that energy, something's got to give."
Blast waves are accompanied by rapid changes in temperature and pressure that can damage the brain, spinal cord and other organs bathed in fluids, as well as air-filled organs like the ear and lungs.
"The brain," Fishman said, "is basically a computer whose parts are microscopic gelatin globs. You rearrange the pattern of those globs and you've destroyed their function."
The effects can be subtle or severe. And, although the military tries to make sure bomb victims receive prompt treatment, experts are learning that some troops sustained multiple blasts whose symptoms can be cumulative.
"It's sort of like a football player," said Alison Cernich, a neuropsychologist at the Baltimore VA. "You shake it off. They think, 'Today I was in a firefight, and it was very stressful.' That's why they are tired, but they don't attribute it necessarily to brain injury."
The cases of Jason Ehrhart and Larry Perry are more complex - and perhaps more typical - than those of patients who survived blast injuries looking perfectly intact.
Ehrhart, who is 22 and lives with his parents in the Howard County community of Dayton, suffered burns over 60 percent of his body, lost a leg and spent three months in a coma before waking up in a stateside military hospital.
But his head appeared unscathed, except for a pressure sore that his parents say developed over months of hospitalization.
Perry, who lives in North East, sustained so many elbow fractures and bone infections that doctors ultimately removed the entire joint. Now, he's learning to function with a "flail elbow," one bound only by soft tissue.
Along with their physical struggles, both are working hard to recover memory through occupational therapy and psychotherapy - with the help of electronic devices such as computers and Palm Pilots. All are provided directly or indirectly by the VA.
The servicemen have improved, but they have far to go. Perry, 26, still has trouble remembering tasks without checking his hand-held organizer. Ehrhart, more severely injured, often forgets what he was told minutes earlier and is relearning basic skills such as brushing his teeth and writing his name.
"I'd like to go to college," the former Howard Community College student said recently, sitting in the furnished basement of his family's home, where an aide helps him relearn the tasks of daily living. "I want to go to the University of Maryland, where my mom went."
Upbeat as she is about her son's recovery, his mother is setting modest goals.
"I can't see you going to school and trying to think during a lecture and retaining any of that," said Pam Estes, who along with her husband, Michael Estes, has been a tireless advocate for his care.