Neuroscientists at Stanford and Harvard have been able to show that neural activity in certain brain regions predicts what experiences will be remembered later. The study at Stanford involved memory for scenic photos while the study at Harvard involved memory for words. Both are reported in the Aug. 21 issue of the journal Science, along with a commentary article about them. Scientists have long suspected that people remember some things better than others partly because of differences in the way the initial experience is encoded into the brain. These are the first studies to show which parts of the brain determine whether a specific, current experience is fated to be remembered or forgotten.
Both studies took advantage of new developments in functional magnetic resonance imaging to obtain measures of the neural activity of volunteers as they viewed words or photographs, one at a time. (Only recently has the temporal resolution of imaging been sufficient to allow brain activity to be measured separately for each picture or word.) Previous studies had indicated more neural activity when volunteers were presented with novel photos than when they were presented repeatedly with the same ones, but the earlier work had not determined the quality of their later memories for the photos.
The research at Stanford was conducted by James Brewer, a graduate student in neurosciences and in medicine; associate psychology Professor John Gabrieli; radiology Professor Gary Glover; and research associates Zuo Zhao and John Desmond.
Gabrieli said that people with memory difficulties as a result of brain injuries or diseases like Alzheimer often have vivid memories for events early in their lives but do not remember recent experiences.
"This gives us the tool to ask whether such selective problems in memory for recent events occurs due to poor initial encoding -- the memory is never formed in the first place -- or later difficulties of storing or using the memories," he said.
In the Stanford study healthy volunteers were shown a group of photos of indoor and outdoor scenes and then a larger group of photographs about a half hour later, Brewer said. They were asked to identify which photos they clearly remembered seeing earlier, which ones looked familiar but that they weren't certain they had seen earlier, and which ones they did not recall seeing at all.
"Several regions of the brain are active when one processes a photograph, such as visual areas and areas that allow a person to make sense of the picture, but the level of activity in most of those regions does not predict whether the photograph will be remembered or not," Brewer said.
Activity in one region of the right prefrontal lobe and several areas on the right and left inner aspects of the temporal lobes -- called the medial temporal lobe system -- did predict whether the individuals would later remember the picture very well, somewhat or not at all, he said. The area in the right frontal lobe is one that is known to be associated with making sense of spatial relationships.
The largest region of activation occurred in the parahippocampal cortex, a major structure in the medial temporal lobe that was known previously to play a role in memory, based on studies of animals and of people who suffer brain lesions or Alzheimer disease, Brewer said. Scientists did not know before, however, if this region was involved in encoding information into the memory, or if it was involved in storing it or retrieving it later.
The individuals in this study varied greatly on which pictures they remembered well, but the activity levels in each person's brain at the time a picture was viewed predicted if he or she would remember that picture later, Gabrieli said.
"It's interesting to speculate about what could be driving the difference in the activation and the correlating differences in memory," Brewer said. "Perhaps one person would see a photograph of Zion National Park and think, 'Hey, I just visited that place on my way to California!' while another would think, 'Outdoor desert scene. . . What's the next scene going to be'" ? The first person's memory of the picture would likely be stronger, he said.
In the Harvard study, led by Anthony Wagner, a recent Stanford graduate who worked in Gabrieli's lab, activity in several regions of the left prefrontal cortex predicted subsequent memory for words. In their Science report, the Harvard researchers said they found some of these areas also showed greater activity when people analyzed words on the basis of their meaning rather than their appearance, such as whether the letters were in upper or lower case. This suggests that part of the brain is encoding for meaning at the time of the experience.
The research might lead someday to better treatment of Alzheimer disease, Brewer said. "Alzheimer disease ravages the medial temporal lobe. We hope that these findings might give us a tool to examine the very earliest effects of Alzheimer disease so that any treatment that is employed will merely have to spare the neurons that are at risk, rather than replace the ones that have been destroyed."
Gabrieli said one of the next steps is to "apply these new techniques to older people at risk for Alzheimer disease and see if they predict who will and will not get the disease."
The research might be applied in other ways, he said. "This may give us a new tool to objectively measure what is memorable for the brain, and that could inform everything from curriculum development in schools to advertising."
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