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‘Hidden’ Echoes Play Role in Memory
Posted By Vanderbilt Magazine On August 5, 2009 @ 2:51 pm In Bright Ideas, Summer 2009 | No Comments
Vanderbilt researchers have discovered that early visual areas, long believed to play no role in higher cognitive functions such as memory, retain information previously hidden from brain studies. Researchers made the discovery using a new technique for decoding data from functional magnetic resonance imaging (fMRI). The findings, published Feb. 18 online by Nature, represent a significant step in understanding how we perceive, process and remember visual information.
“How do people maintain an active representation of what they have just seen moments ago? This has long been a conundrum,” says Frank Tong, co-author of the research findings and an associate professor of psychology.
“Before, we knew that early visual areas of the cerebral cortex that are the first to receive visual information were exquisitely tuned to process incoming visual signals from the eye, but not to store this information. We also knew that the higher-order brain areas responsible for memory lack the visual sensitivity of early brain areas, but somehow people are able to remember a visual pattern with remarkable precision for many seconds—actually, for as long as they keep thinking about that pattern. Our question was, Where is this precise information being stored in the brain?”
The researchers found that fine-scale activity patterns in early visual areas reveal a trace of “something like an echo of the stimulus that the person is actively retaining,” says Tong, “even though the overall activity in these areas is really weak after the stimulus is removed.”
Before this discovery the visual cortex had not been implicated in cognitive processes such as memory or active maintenance of information, says Stephenie Harrison, lead author of the research and a graduate student in the Vanderbilt psychology department. “By using a neural decoding technique, we were able to read out what people were holding in their visual memory. We believe this sustained visual information could be useful when people must perform complex visual tasks in everyday life.”
Research subjects were shown two examples of simple striped patterns at different orientations. They were then told to hold one of the orientations in their mind while being scanned using fMRI. Orientation has long been known to be one of the first and most basic pieces of visual information coded and processed by the brain.
“Through both evolution and learning, the visual system has developed the most efficient ways to code our natural environment, and the most efficient way to code any basic shape or contour is orientation,” Tong explains. “We used a decoding method to see if the activity patterns contained information about the remembered orientation, and we found that they do.”
Analyzing responses over several trials, researchers could tell accurately which of the two orientation patterns a subject was holding in his or her mind more than 80 percent of the time. Their predictions held true even when overall activity in these visual areas was very weak—no different than looking at a blank screen. This suggests that the act of remembering an image leaves some sort of faint echo or trace in these brain areas. The activity traces are weak but quite detailed and rich in information.
“By doing these pattern analyses, we were able to find information that was hidden before. We do not know for sure, but it’s possible that a lot of information in the brain may be hidden in such activity patterns,” Tong says.
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