THURSDAY, Feb. 19 (HealthDay News) -- Middle-age people whose parents had Alzheimer's and who carry the so-called Alzheimer's gene might very well have the memory of someone 15 years older, a new study has found.

This memory decline was not detected in people of middle age whose parents had Alzheimer's but who do not carry the gene, known as ApoE4, according to the study.

About 20 percent to 25 percent of the population have at least one copy of the ApoE4 gene, but not all people with the gene develop Alzheimer's, said study co-author Dr. Sudha Seshadri, an associate professor of neurology at Boston University School of Medicine.

The study involved 715 participants in the ongoing Framingham Heart Study, including 282 whose parents, one or both, had been diagnosed with Alzheimer's or other dementia. The participants averaged 59 years old and were healthy, with no memory complaints, said Seshadri, who is also a senior investigator with the Framingham study.

But when given a battery of cognitive tests, those who logged the lowest scores on verbal and visual memory tasks were people who were carriers of the ApoE4 gene and had parents with dementia.

Seshadri stressed that the neuropsychological tests and brain imaging conducted as part of the study offered a sensitive measurement of memory. The participants were "performing older than they're expected," she said, but added that there were "no memory symptoms associated with this." Participants still tested within the normal range for memory and were living normal lives, she said.

The results suggest that the Alzheimer's gene is facilitating the expression of some other gene, Seshadri said. "It's just giving us a clue that whatever [other] gene we find is going to have an interaction with ApoE," she said.

Finding other genes will require a sample of 10,000 to 20,000 people and the collaboration of several research groups, Seshadri said. But, she predicted that "within the next year or two, I think we are going to find more genes."

The study, which was funded by the U.S. National Institutes of Health and released Wednesday, is to be presented at the American Academy of Neurology's annual meeting, from April 25 to May 2, in Seattle.