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For the first time, researchers have seen in action how the "hot" emotional centers of the brain can interfere with "cool" cognitive processes such as those involved in memory tasks. Their functional magnetic resonance imaging (fMRI) images of human volunteers exposed to emotional distraction revealed a "see-saw" effect, in which activation of emotional centers damped activity in the "executive" centers responsible for such processing.
The findings of the Duke University Medical Center researchers provide insight into the basic brain mechanisms responsible for the distraction caused by emotional stimuli that are irrelevant to a task. Moreover, they said, the findings offer a new approach to understanding how people with depression and post-traumatic stress disorder cope with traumatic events and memories. It is known that people with such problems are far more affected by emotional distraction.
Development of new drugs to alleviate, for example, the haunting memories of PTSD sufferers will be aided by the fMRI technique the researchers developed to precisely measure this distraction, they said.
The researchers, Florin Dolcos and Gregory McCarthy, published their findings in the Feb. 15, 2006, issue of the Journal of Neuroscience. Their work was sponsored by the National Institutes of Health and the Veterans Administration. Dolcos is a postdoctoral fellow and McCarthy is director of the Duke-UNC Brain Imaging and Analysis Center (http://www.biac.duke.edu/), where the studies took place.
In their experiments, the researchers asked volunteer subjects to memorize sets of images of three human faces. Next, they exposed the subjects to one of three types of distracters -- emotional images such as injured people or aggressive behavior; neutral images such as people shopping or working; and scrambled images that meant nothing. The subjects were then showed a face image and asked to determine whether it was one of the original "to-be-memorized" faces or a new face.
Throughout the tests, the subjects' brains were scanned using fMRI. This widely used technique involves using harmless magnetic fields and radio waves to scan the brain to detect levels of blood flow, which indicates increased or decreased brain activity.
In earlier studies, the researchers had found that emotional images activated a "ventral affective system" in the brain that encompasses regions involved in emotional processing. In contrast, they found, cognitive tasks involving memory processes activated a "dorsal executive system." To their surprise, the researchers also found that the emotional distracters not only activated the ventral system, but also deactivated the dorsal regions.
In the new study, the researchers observed the same patterns of activation and deactivation of the regions. The emotional images produced greater activation of the ventral system and deactivation of the dorsal system than did the neutral or scrambled images, they found.
But most importantly, they found graded behavioral effects of the images. The emotional distracters produced the most detrimental effect on memory performance, the neutral distracters impaired performance to a lesser extent; and the scrambled images impaired performance very little. "Along with the fMRI results, these findings provide the first direct evidence concerning the neural mechanisms mediating cognitive interference by emotional distraction," said Dolcos.
"The design of these experiments gave us an excellent chance to fill in a missing link in our earlier studies," said Dolcos. "It enabled us to determine whether there was, indeed, a behavioral connection between deactivation of the dorsal system and impaired performance.
"The experimental design mimicked the kind of distraction people experience in everyday life," Dolcos added. "For example someone driving on a highway, attempting to pay attention to the driving task might encounter an emotional distracter such as an accident. As everybody knows, at that moment drivers lose focus on the task."
"Also, the three types of distracters gave us good controls, which allowed us to clearly establish that the observed effects were due to the presence of emotional distraction rather than to the presence of other meaningful (neutral images) or meaningless (scrambled images) distracters."
The researchers also found individual differences among the subjects in their response to the images. Those people who showed greater activity in a brain region associated with the inhibition of response to emotional stimuli rated the emotional distracters as less distracting. Said Dolcos, "One interpretation of this finding is that, because this region is associated with inhibitory process, people who engage that region more could cope better with distracting emotions."
McCarthy said that the results of their study will likely have important implications for understanding of anxiety disorders. "Our hypothesis has been that people suffering from such anxiety disorders such as depression and PTSD, may see the world differently than other people, and that a distracter associated with trauma may grab control of brain processing and essentially take off-line those areas of the brain we use to stay on task. It's as if when you're sad, the world seems sadder and all you see is bad news."
"Our aim is to reverse that with drug treatment, so we're using these kinds of studies to determine whether particular antidepressants influence the response of that ventral system. And what is particularly exciting is that this method allows us to look directly at the neurological target of the drug and not have to try to measure the more nebulous behavioral response. So, we can detect a sub-threshold response to such drugs, which will help us understand whether we're going in the right direction in terms of drug development."
Such studies are being carried out in Veterans Administration-supported Mental Illness Research, Education, and Clinical Center, which McCarthy directs. That center aims at using genetics, brain imaging and neuropsychological and psychiatric techniques to understand PTSD and related disorders.