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Fast Fading Visual Image May Signal Retreating Memory

A new study by USC College scientists links a deficit in visual memory to the mild cognitive impairments that may precede Alzheimer’s disease

By Eva Emerson
January 2005

A simple test of visual short-term memory may prove to be a sensitive way to detect early signs of Alzheimer’s disease, according to a new study led by USC College scientists.

Reporting Jan. 21 in the online edition of the Proceedings of the National Academy of Sciences, neuroscientist Zhong-Lin Lu and his team show that seniors with mild memory loss also show marked deficiencies in what’s called iconic memory.

When you see a room, you immediately form a fairly detailed image of the room—the blue couch, the gray armchair, the plant in the corner—in your brain. Turn off the lights, and, for a fraction of a second, you retain that image in your mind, courtesy of iconic memory. Iconic memory is a fleeting visual memory that in most people lasts about 300 milliseconds before fading or being shuttled into more persistent memory areas of the brain.

Working with former graduate student James Neuse and psychologist Stephen Madigan, both of USC College, along with UC Irvine psychologist Barbara Anne Dosher, Lu assessed the iconic memory of 11 older adults (average age of 85) with mild cognitive impairments.

Compared to a control group of 16 older adults (average age of 82) with no cognitive decline and 23 young adults, whose iconic memory duration is about 300 milliseconds, the study participants showed a significant deficit in iconic memory. They were able to hold the iconic image of a visual scene for less than 70 milliseconds, if at all, Lu says.

“Those with a very short iconic memory were most likely to be those with the first signs of cognitive impairments,” Lu says.

“In this study, observers with mild cognitive impairments performed at the same level as the age-matched controls when the visual display was in front of them. They showed the deficit only when they were asked to identify objects in the display after the display was off. This suggests that the observed deficit was related to sensory memory, not to poor vision.”

The participants’ cognitive impairments were slight but consistent, and included reports of frequently misplacing keys, temporarily forgetting the name of a friend or having troubles problem solving. Mild impairment of this type often precedes a diagnosis of Alzheimer’s.

Lu first became interested in the potential link between fast-fading iconic memory and Alzheimer’s risk while part of a team at studying iconic memory in the late 1990s. Among the volunteers for the study was a 58 year-old man whose iconic memory was all but gone.

“We were all surprised, and couldn’t explain the result,” says Lu. “We even tried to make the test easier for him and gave him plenty of practice, but it made no difference. No one could figure out why this man, who held a very high-profile job, had this deficit.”
But three years later, when the man was diagnosed with Alzheimer’s disease, Lu and the others grew suspicious that there could be a link between the two.

The results of the recent study, he says, are “highly suggestive” of a link between the beginnings of the memory-robbing disease and changes in brain function in the visual part of the brain—an area previously thought to be relatively unaffected by dementia.

In 92 percent of the cases, study participants’ performance on the test of iconic memory, used in combination with a test of verbal fluency, successfully predicted whether they were from the memory-impaired group or the control group with no memory loss.

While the findings seem promising, Lu cautions that more research will be need to be done before these tests could reach patients.

“A careful longitudinal study of observers with mild cognitive impairments and their definitive Alzheimer’s diagnosis is necessary to fully assess the potential clinical value of these methods in predicting disease,” he says.

Scientists are actively studying a number of prospects for diagnosing Alzheimer’s disease before the neurodegenerative disease, which afflicts upwards of 4.5 million people in the U.S., takes hold. The goal has become especially critical with the development of a number of new pharmaceuticals that may slow down cognitive changes associated with Alzheimer’s, but cannot reverse damage that’s already taken place.

“A lot of new drugs have been produced, but they cannot cure the disease. However, they do appear to be able to slow progression, so that makes early diagnosis extremely important right now,” says Lu, a professor of psychology in the College. “Right now, there’s really no way to predict who will get Alzheimer’s and who won’t.”

With the Dornsife Neuroscience Cognitive Imaging Center now open at the College, Lu hopes to collaborate with other scientists at USC to combine neuropsychological tests such as iconic memory and sophisticated brain imaging methods to develop a reliable way to detect early signs of Alzheimer’s.

“That would be an ideal use of our faculty expertise and our new fMRI center,” says Lu, who serves as the scientific director of the new facility.

For more information, contact Eva Emerson at