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.
Said Dolcos, "One way to interpret our result is that emotion can trigger recollection, and vice versa. The synchronicity between activity in the amygdala and hippocampus could go either way. The emotion enhances recollection, but at the same time by recollecting those events, you would also remember the emotional response. It could be like a loop in which the amygdala interacts with the hippocampus." According to Dolcos, this memory loop could help understand the searing recall of traumatic memories in people with post-traumatic stress disorder.
"In such people, an emotional cue could trigger recall of the event, which would then loop back to a re-experiencing of the emotion of the event," said Dolcos. "Or, remembering the event may trigger the emotional reaction associated with the event, which in turn could trigger more intense recall, in a continuous loop."
Such insights into the nature of emotional memory support a therapeutic process that can affect "reconsolidation" of traumatic memories, said Cabeza. "Some studies have suggested that when you retrieve a memory it can not only be re-encoded, or reconsolidated, but you also put it into a labile state in which it can be transformed. While in such labile state, either the memory itself or the person's perspective of it may be altered." According to Cabeza, therapists working with people suffering from PTSD as a result of the 9/11 terrorist attack have used this technique to alleviate its symptoms.
In further studies, the researchers plan to manipulate the degree of emotion experienced by the subjects, as well as how much detail is remembered, to explore the specific interactions among brain structures in processing emotional memories. They also plan to analyze activity of other regions -- such as those that process spatial, auditory, or visual information -- during emotional memory processing to understand their role. Such studies would yield insights into how emotional memories involve integrating multiple brain regions, they said.