No direct impact caused Paul McQuigg’s brain injury in Iraq three years ago. And no wound from the incident visibly explains why Mr. McQuigg, now an office manager at a California Marine base, can get lost in his own neighborhood or arrive at the grocery store having forgotten why he left home.

But his blast injury — concussive brain trauma caused by an explosion’s invisible force waves — is no less real to him than a missing limb is to other veterans. Just how real could become clearer after he dies, when doctors slice up his brain to examine any damage.

Mr. McQuigg, 32, is one of 20 active and retired members of the military who recently agreed to donate their brain tissue upon death so that the effects of blast injuries — which, unlike most concussions, do not involve any direct contact with the head — can be better understood and treated.

The research will be conducted by the Sports Legacy Institute, a nonprofit organization based in Waltham, Mass., and by the Boston University Center for the Study of Traumatic Encephalopathy, whose recent examination of the brains of deceased football players has found damage linked to cognitive decline and depression.

Whether single, non-impact blasts in battle can cause the same damage as the years of repetitive head bashing seen in football is of particular interest to researchers. The damage, primarily toxic protein deposits and tangled brain fibers, cannot be detected through noninvasive procedures like M.R.I.’s and CT scans.

“We don’t know much about the medium- or long-term effects of head trauma experienced by our military,” said Robert Stern, co-director of the Boston University center as well as its Alzheimer’s Disease Clinical and Research Program. “We know that there are some immediate effects in terms of blast injury on cognition and behavior. But we do not yet know whether there are any long-term effects.”

“Does that single blow result in something that doesn’t go away,” he added, “or perhaps sets off a cascade of events that leads to a progressive degenerative brain disease?”

Mr. McQuigg may be finding out the cruelest way. In February 2006, he was on combat patrol when his Humvee was hit by a roadside bomb, knocking him unconscious, shattering his jaw and damaging his right eye. His helmet could not protect him from a severe concussion that doctors told him was caused solely by the bomb’s force waves, not direct impact.

Now he is experiencing headaches, short-term memory problems and trouble with balance that have only worsened.

“With prosthetics, you can replace an arm or a leg and can still throw a football with your kid,” said Mr. McQuigg, who works at Camp Pendleton, north of San Diego. “If you have a severe brain injury, you might not be able to live on your own.”

“And people don’t know what’s wrong with you,” he added. “People know if you’re missing an arm, something happened. If it happened to your brain, they can’t tell.”

An estimated 320,000 soldiers have experienced some form of traumatic brain injury during their service in Iraq or Afghanistan, according to a 2008 RAND Corp. study. Blast injuries have risen in prominence in recent years because improvements in armor and medical treatment allow soldiers to survive explosions, then experience any delayed effects.

Blast injuries result from waves of air pressure that can travel several times as fast as hurricane winds. Those waves can not only throw a soldier dozens of feet in the air into other objects — causing a conventional concussion as the brain crashes inside the skull — but may also subject brain tissue to sudden pressure variations that can cause similar damage.

Repeated brain trauma among some football players and boxers has been linked to the subsequent appearance of toxic proteins and neurofibrillary tangles in the brain — a disease known as chronic traumatic encephalopathy, or C.T.E. Many athletes who were found after death to have had the disease experienced memory loss, depression and oncoming dementia as early as their 30s, decades before afflictions like Alzheimer’s appear in the general population.

Just as researchers at the Boston University center and elsewhere have linked some athletes’ later-life emotional problems to their on-field brain trauma, the research on military personnel will try to determine whether some soldiers with post-traumatic stress disorder — a psychological diagnosis — actually retain physical brain damage caused by battlefield blasts. Some signs of P.T.S.D., particularly depression, erratic behavior and the inability to concentrate, appear similar to those experienced by concussed athletes.

Such a link could have effects beyond medicine. Disability benefits for veterans can vary depending on whether an injury is considered psychological or physical. And veterans with P.T.S.D. alone do not receive the Purple Heart, the medal given to soldiers wounded or killed in enemy action, because it is not a physical wound.

Dr. Stern, at Boston University, said that blast injuries could be seen as this generation’s version of exposure to Agent Orange, the herbicide used in the Vietnam War.

“During exposure to Agent Orange, it wasn’t known what long-term effects there would be, but through scientific study, long-term study of veterans, those effects have been more clearly understood,” he said. “We need to know if these individuals with blast injuries are going to require long-term care and treatment.”

The Boston University center and the Sports Legacy Institute will compare findings from the brains of military personnel with those from their athlete program, which has signed up more than 120 donors in less than a year, and other brain banks around the world. The two centers, not the military, are paying for the registry, storage and examination of brain tissue.

But Col. Michael S. Jaffee, national director of the Defense and Veterans Brain Injury Center, said the Defense Department supported the spirit of the research and could assist in approaching active and retired soldiers to register for brain donation.

“Having a brain bank to allow us to study what these brains look like will help us correlate this with other emerging research findings,” said Colonel Jaffee, who is a physician.

But he cautioned: “Whenever we’re talking about organ donation for the sake of science, we’re dealing with a lot of sensitive and cultural issues. We ask people to consider and realize that asking family and individuals to remove the brain from the body, many cultures and traditions may not find that acceptable. So it’s always a challenge to balance the benefits, which are real and will come, with a way to maintain the dignity and respect of people who have made the ultimate sacrifice.”

Benefits of the research on military personnel could extend to the general population, said Dr. Daniel P. Perl, director of neuropathology at the Mount Sinai School of Medicine in New York. Even though civilians are rarely subjected to anything close to the devastating waves that burst from battle explosions, the characteristics of blast injuries could lend insight into brain damage caused by single impacts in automobile accidents, for example.

If protein deposits and tangles appear in the hippocampus area of the brain, for instance, then they would affect short-term memory; appearance in the frontal lobes could impair executive function, and in the cerebellum coordination and balance. The researchers will also be looking at possible genetic factors.

“I wouldn’t be surprised if there was a great deal of overlap between examples of this from the sports arena and the military, but we don’t know,” Dr. Perl said. “The forces are different and presumably the mechanisms are somewhat different. If this research and the examinations are done right, they have the potential to contribute significantly. It could tell us what happens, which we’re not going to get otherwise.”