(AP) -- High school athletes who suffer the mildest of concussions -- in which obvious symptoms disappear within 15 minutes -- often have memory problems and other latent difficulties days after the injury, according to a new study.

The study from the University of Pittsburgh Medical Center Sports Medicine Concussion Program shows that coaches and doctors should be cautious in deciding when an athlete can return to a game after a concussion, said researcher Mark Lovell, a neuropsychologist and director of the concussion program.

Now, coaches and trainers often allow an athlete to return to the game or to practice if the obvious symptoms of a concussion, such as disorientation and uncoordinated hand-eye movements, disappear within 15 minutes of the injury.

"Because on-the-field symptoms disappear within a few minutes and the athlete reports he or she is fine and appears to the sports medicine team to be fine, mild concussions often are unrecognized, overlooked or considered a trivial injury," said Dr. Michael Collins, a neuropsychologist and the assistant director of the concussion program.

The study published in the February edition of the Journal of Neurosurgery evaluated 64 male and female high school athletes who sustained mild concussions.

Researchers evaluated the players on the field. Then, using a computer program that administers a battery of memory and reaction tests, researchers evaluated the athletes at periods of 36 hours, four days and seven days after the injury.

Headaches, dizziness and nausea seemed to disappear within four days, but some athletes had memory problems seven days after they received a concussion, the study found.

If coaches and others evaluating an athlete miss subtle signs such as a slow memory, headaches and dizziness, players could return to a sport before their brain has had time to heal, increasing the chance of more serious brain damage, Lovell said.

Successive concussions could cause long-term memory loss and other problems, Lovell said. In rare cases, young athletes can suffer from "second-impact syndrome," in which an impact can cause a fatal brain hemorrhage, he said.

"The concern is that concussion symptoms are not always straightforward and not always reported by the athlete," said Dr. Joseph Maroon, a study investigator. "On-the-field evaluation of the injury's effects and knowing when it is safe to return the athlete to play can be difficult to objectively measure."

The UPMC concussion program's study confirms what "clinicians have known in their hearts," said Gregory O'Shanick, medical director of the Arlington, Va.-based Brain Injury Association of America.

"Clinicians who have seen many people with concussions have believed that a history of concussions of whatever degree does count in the cumulative effect of concussions," said O'Shanick, who did not participate in the study.

But Lovell's study and the UPMC concussion program's research is important and meaningful because researchers are studying student athletes' memories and reaction times before and after they receive a concussion, O'Shanick said.

Many researchers in the past have evaluated people with concussions retroactively, with no idea of their memory or reaction times before their head injury, and the research can be "fraught with error," he said.

When Joseph Perry, the athletic director of Keystone Oaks High School in suburban Pittsburgh, started coaching 35 years ago, he often allowed football players to return to the game minutes after they received a mild concussion, telling them "they just got their bell rung," he said.

Now, his high school uses a computer program called the Immediate Post-Concussion Assessment and Cognitive Testing system, or ImPACT. It was developed by the UPMC concussion program and tests athletes' memory and reaction skills before and after a concussion.

Lovell said neurocognitive testing such as ImPACT is the best way to evaluate an athlete's condition after a concussion.

"We don't leave it to guesswork anymore because my intuition could be wrong," Perry said. "There are speed, eye-coordination and thought-process tests in there. It's pretty hard to fake."