A research team, led by Brendan Depue, a doctoral candidate in psychology at the University of Colorado at Boulder's Center for Neuroscience, trained 16 subjects—none of whom were previously diagnosed with any psychiatric problems—to recognize 40 pairs of visual stimuli. They were shown cards with pictures of a person's face with a neutral expression along with corresponding cards comprising images designed to evoke strong negative emotions such as those of injured soldiers, and victims of car crashes and violent crimes. These "paired-associate memory" cards were shown repeatedly until all the participants coupled each face with its corresponding disturbing image.
Researchers then scanned the brains of volunteers with functional magnetic resonance imaging (fMRI) while showing them 32 of the face cards; with half of them, subjects were instructed to consciously think about the associated harrowing counterpart and with the other half, they were instructed to consciously suppress their thoughts about it. Each image was shown 12 times with the same instructions on whether or not to try to conjure the corresponding emotionally charged memory. (The eight faces left out served as a baseline to determine exactly what effect repeatedly conjuring or suppressing memories had on a subjects' ability to recall them.)
"The brain imaging data show that the areas of the brain that support memory and underlie memory's existence in the brain are down-regulated [meaning their activity is lessened]" Depue says. During memory suppression, he says, activity tapers off in the brain's visual cortex (which regulates visual representation of a memory), hippocampus (responsible for memory formation and retrieval) and amygdala (a region in continuous communication with the hippocampus that formulates emotional responses to memories). At the same time, there's an increase in activity in the prefrontal cortex, the seat of cognitive control and the source of the motivation for taking actions. "What it looks like is that the prefrontal cortex is modulating those other areas and downregulating them," Depue says.
Subjects next were again shown all 40 of the face flash cards and directed to write brief descriptions of the memories evoked by each. The subjects' memories of images they suppressed were almost uniformly below the baseline and the ones they practiced recalling were almost without fail more vivid than those they deliberately tried to forget.
Depue says that if the results can be replicated in psychiatric patients suffering from disorders such as clinical depression and PTSD, they could help scientists "in understanding where a dysfunction lies and specifically, in developing psychopharmaceutical approaches to better target the suppression of [the] emotional memory mechanism."