ST. LOUIS, Mo., Aug. 20, 1998--The birth of a memory, the split second when the human brain encodes an event for future reference, has been captured through sophisticated neuroimaging and used to predict accurately whether a specific experience will later be remembered or forgotten, according to research published in the Aug. 21 issue of the journal Science.
Based on collaborative research by scientists at the Massachusetts General Hospital (MGH)-NMR Center in Boston, Washington University in St. Louis and Harvard University, the article describes how levels of activity in certain brain structures involved in processing verbal information can predict whether that information will be retained in memory.
"This study marks the first time we've been able to peer inside someone's brain and predict on average whether or not you will later forget something you are now experiencing," said Randy L. Buckner, senior author of the article and assistant professor of psychology in Arts and Sciences at Washington University in St. Louis. "Now, we can actually see areas of the brain as they go about the process of memorization."
Although psychologists have long suspected that how we process information into memory is critically important to later remembering and forgetting, this study is the first to capture images of specific memories as they are being formed within the brain.
"This study provides a firmer biological underpinning for the concept that how we encode information is key to whether or not it is remembered," said Daniel Schacter, chairman of psychology at Harvard and a co-author of the study. "It is the first work to tie the creation of a specific verbal memory to specific levels of activity in certain areas of the brain."
Anthony Wagner, a postdoctoral research fellow at the MGH-NMR Center and the Department of Psychology at Harvard University, spearheaded the research and is the article's first author. His study is one of two articles on memory in this issue of Science that are based on advanced functional magnetic resonance imaging (fMRI) techniques that were developed by Buckner and Anders Dale, a researcher at MGH and another of the co-authors on Wagner' study.
"This new technique is what allows us to specify how brain activity during learning differs between experiences later remembered and those later forgotten," Wagner said.
Wagner and colleagues used event-related fMRI to measure small, but significant differences in brain activation as young adults completed various verbal tasks. The experiments were structured to test whether participants are more likely to remember tasks that require a deeper, more meaningful level of consideration, as opposed to more routine tasks, such as identifying obvious differences in physical appearance, a theory supported by earlier psychological research.
While the participants' brain activity was being scanned, they were asked to complete a word processing task that required them to think semantically, to distinguish between words that are concrete, such as dog or cat, and words that are abstract, such as love or hate. Subjects were later asked to recall whether specific words had or had not been included in the tests.
When researchers compared the level of brain activity during processing of words remembered with activity for words later forgotten, they found that increased activity in specific structures in the left frontal and temporal lobes predicted whether participants would accurately and confidently remember whether a particular word had been part of the test. The study also confirms that a person seems to remember something better when he or she pays attention to its meaning, rather than to more superficial attributes such as physical appearance.
One of the structures showing increased activity in verbal memory formation is the left parahippocampal gyrus, a main input pathway to the hippocampus, a part of the brain that has long been recognized as crucial for storing and retrieving memories. The authors note that, while this area previously had been implicated in processing unfamiliar experiences, this study suggests the parahippocampal gyrus plays a broader role in memory formation. Even when experiences are similarly novel, differences in parahippocampal activity were seen that predict future memory.
While people used the same general brain regions to process similar verbal experiences, whether or not these experiences were remembered hinged on subtle differences in the magnitude of activity within those regions.
"It's interesting that the people in our study ostensibly had very similar experiences," said Wagner. "They all did the same test, yet something about the brain processes they recruited for some experiences as opposed to others was slightly different. It was these small differences that made a big difference in how well they remembered."
Previous brain research using positron emission tomography (PET) and other relatively slow forms of brain imaging required the use of "block trials," in which subjects were asked to repeat a string of similar short tasks so that data could be collected over a longer period.
In 1996, when Buckner was at the Harvard Medical School, he and colleagues capitalized on the speed of fMRI to collect brain image data from "individual trials," pioneering a research method now known as "event-related" neuroimaging. Images that took about one minute to capture using PET technology can now be captured by fMRI for events as brief as 30 milliseconds.
"The split-second thoughts that people have about incoming information play a huge role in whether that experience will be remembered or forgotten, something that happens in a second can have consequences for the rest of our life," Schacter said. "This study gets us a step closer to understanding what is going on in the brain when this crucial step is taking place in the brain."
Other co-authors of the paper are Michael Rotte, MD., and Bruce Rosen, MD, of the MGH-NMR Center, and Wilma Koutsaal, Ph.D., and Anat Maril of the Department of Psychology at Harvard University. The study was supported by grants from the National Institute on Aging, the National Institute on Deafness and Communications Disorders, the Human Frontiers Science Program and the Deutsche Forschungsgemeinschaft.