Older adults with mild cognitive impairment may have problems retaining a mental picture of objects they’ve just seen - a subtle memory problem that could serve as an early indicator of Alzheimer’s disease risk, researchers reported Monday.

Their study of elderly and young adults found that older men and women with mild signs of mental decline had problems with a test of so-called “iconic” memory. This type of memory refers to the visual image a person holds onto after briefly looking at an object.

If someone walked into a room, quickly scanned it, then turned off the lights and tried to recall the objects in the room, that would draw upon iconic memory, explained lead study author Dr. Zhong-Lin Lu, a professor of psychology at the University of Southern California in Los Angeles.

Iconic memory is by nature fleeting, regardless of a person’s age, and it is distinct from short-term memory, which draws upon information deposited in the brain’s temporary storage bins - such as when someone remembers a message to pass on later that day.

In their study, Lu and his colleagues found that elderly men and women with mild cognitive impairment performed more poorly on a test of iconic memory than either young adults or older men and women with no signs of mental decline. This was despite the fact that the two groups of elderly participants performed similarly on tests of short-term memory.

The findings are published in the advance online edition of the Proceedings of the National Academy of Sciences.

Mild cognitive impairment - which involves the type of “benign” forgetfulness in which a person frequently misplaces the car keys, for example - is considered a risk factor for Alzheimer’s disease. Indeed, scientists estimate that about 80 percent of people with mild cognitive impairment develop Alzheimer’s within 10 years, according to Lu.

Right now, Lu told AMN Health, mild cognitive impairment is diagnosed through standardized interviews rather than specific, sensitive tests. If a decline in iconic memory is indeed an objective marker of mild impairment, he said, then testing for it may allow doctors to detect the earliest stages of Alzheimer’s.

First, however, long-range studies will need to look at how well a score on an iconic-memory test predicts the risk of developing Alzheimer’s, Lu said.

Lu and his colleagues decided to study the relationship between iconic memory and mild mental impairment after a chance discovery in an earlier study. In that study, Lu said, a healthy, fully functioning 58-year-old man inexplicably performed poorly on tests of iconic memory. About two years later, the man was diagnosed with Alzheimer’s disease.

The current study included nine men and women ages 74 to 98 who were mildly impaired, based on standard interviews. Sixteen healthy elderly adults and 23 college students served as comparison groups. Iconic memory was gauged with a test in which participants saw eight letters flashed on a computer screen in a circular arrangement. After the image disappeared, they were asked to name the letter that had been in a particular spot.

The group with mild cognitive impairment had significantly shorter iconic memory than either of the other two groups, the researchers found.

Finding new, objective and sensitive tests for early Alzheimer’s is important, according to Lu, because current drug therapies designed to stabilize symptoms are most effective when started as early as possible.

SOURCE: Proceedings of the National Academy of Sciences, online January 17, 2005.