Researchers exploring the brain structures involved in recalling an emotional memory a year later have found evidence for a self-reinforcing "memory loop" - in which the brain's emotional center triggers the memory center, which in turn further enhances activity in the emotional center.
The researchers said their findings suggest why people subject to traumatic events may be trapped in a cycle of emotion and recall that aggravates post-traumatic stress disorder (PTSD). They said their findings also suggest why therapies in which people relive such memories and reshape perspective to make it less traumatic can help people cope with such memories.
The paper by Florin Dolcos, Kevin LaBar and Roberto Cabeza, was published online February 9, 2005, in the Proceedings of the National Academy of Sciences. The researchers are in Duke University's Center for Cognitive Neuroscience, Department of Psychological and Brain Sciences, and Brain Imaging and Analysis Center. Their work was sponsored by the National Institutes of Health and the National Science Foundation.
"This study is the first to really test recall of emotional memories after a long time period," said Cabeza. "Previous studies had only allowed a short time interval, for example minutes, between encoding of the memory and retrieval. Hence, they could not distinguish between the process called consolidation -- in which memories are being established -- and retrieval. Also, they did not distinguish between true recollection of a memory and a vague familiarity. In memory studies, it's very important to distinguish between these two phenomena," he said.
In their study, the researchers showed volunteer subjects images that were pleasant, unpleasant or neutral while their brains were being scanned with functional magnetic resonance imaging (fMRI). In this widely used technique, harmless magnetic fields and radio waves are used to image blood flow in regions of the brain, and increased blood flow is a signature of higher brain activity. The pleasant images were of romantic scenes and sports; the unpleasant images involved injured people and violence, and the neutral pictures depicted buildings or other emotionally non-involving scenes. The subjects were asked to rate the emotional aspects of the images they saw.
Then, a year later, the researchers showed the same subjects a combination of images they had previously seen and new images -- pleasant, unpleasant and neutral -- while their brains were being scanned. They asked the subjects to indicate whether they had seen the images before and whether the memory also brought back associated details. Such details indicated the impact of the picture on the subjects.
The researchers found that the subjects did recall the emotional pictures -- both pleasant and unpleasant -- better than the neutral pictures, and this recall was based on specific recognition of the pictures. This recall was associated with a correlated higher activity in both the amygdala -- the region of the brain responsible for processing emotional memories -- as well as the hippocampus, the main memory-processing center. The study also revealed greater amygdala-hippocampal correlation during recollection of emotional pictures than during recollection of neutral pictures, they said. The researchers said that one way of explaining the "co-activation" of these two centers was that they could be part of a "synergistic mechanism," in which each activates the other during recall of an emotional memory.