Their new study provides clear evidence from humans that the brain's emotional center, called the amygdala, interacts with memory-related brain regions during the formation of emotional memories, perhaps to give such memories their indelible emotional resonance.
The researchers said their basic insights could contribute to understanding of the role that the neural mechanisms underlying emotional memory formation play in post traumatic stress disorder and depression.
The study by Florin Dolcos, Kevin LaBar and Roberto Cabeza was reported in the June 10, 2004, issue of the journal Neuron. Dolcos is a research associate in the Brain Imaging and Analysis Center and LaBar and Cabeza are, respectively, assistant and associate professors of psychological and brain sciences. They are also faculty in the Center for Cognitive Neurosciences. Their research was supported by the National Institutes of Health.
According to Dolcos, in their experiments the researchers were seeking evidence for the "modulation hypothesis," which holds that the brain's emotional and memory centers interact during the formation of emotional memories.
"The basic idea was simple: to find evidence supporting the notion that the brain's emotional region modulates activity in the memory regions to form an emotional memory," said Dolcos. "This idea was supported by animal research, but the evidence from neurologically intact humans was scarce and indirect. So, our goal was to find the right method that would allow us to demonstrate that this phenomenon happens in human, too," he said.
In their study, the researchers sought to establish that the memory-enhancing effect of emotion is due to interaction between emotion- and memory-related brain regions. Thus, they first exposed volunteer subjects to a collection of pictures that evoked both positive and negative emotions and those that were neutral. Emotional pictures depicted such negative events as aggressive acts or injured people and such positive events as romantic scenes or sporting triumphs. Neutral pictures included such subjects as buildings or scenes of routine shopping.
While viewing the emotional and neutral pictures, participants' brains were scanned using functional magnetic resonance imaging. Such imaging involves the use of harmless magnetic fields and radio signals to measure blood flow to individual brain regions, which reflects greater activity in those regions. Then, following the scanning session, the researchers tested participants' memory for the images they viewed during the scanning.
Two important features distinguish the new study from previous functional neuroimaging studies, said the researchers. First, to identify the brain regions associated with the memory-enhancing effect of emotion, the study identified the brain regions whose activity during memory formation predicts what items would be remembered or forgotten (encoding success). Thus, it was possible to compare successful encoding activity for emotional and neutral items.
Second, to delineate the contribution of the emotion and memory-related regions during emotional memory formation, the study used precise anatomical methods, which involved tracing of these regions on each subject's brain image. Thus, it was possible to precisely localize the signal coming from anatomically proximal brain regions. As expected, analysis of the behavioral data revealed evidence that the memories of emotional images were more strongly encoded than the neutral ones. And importantly, the brain scans showed that the emotional memories evoked activity in the amygdala as well as the "medial temporal lobe memory" structures. Specifically, these structures include the hippocampus and associated regions. Moreover, according to Dolcos, the analysis revealed a significant correlation between the strength of activity in the emotion- and memory-related brain regions.
"We found evidence that the interaction between the emotional and memory regions occurred more systematically and consistently during the formation of emotional memories than during the formation of neutral memories," Dolcos said. "More specifically, we found that the subjects showing greater successful encoding activity in the emotional region also had greater activity in the memory regions," said Dolcos.
Said Cabeza of the findings, "Other studies have focused on the general enhancing effects of emotion on memory, and the evidence for the modulation hypothesis was disparate and inconclusive. Thus, this is the first direct evidence for the modulation hypothesis in humans."
What's more, said Cabeza, "We also found indications that some regions within the medial temporal lobe may actually be more specialized for encoding neutral information. We don't know exactly what the processes involved are, or why these regions are engaged. But we speculate that the regions that were more activated for emotional stimuli are involved in semantic processing of the meaning of the images, whereas those that are more activated by neutral stimuli reflect perceptual processing."
Thus, said Cabeza, the findings not only establish the functional link between the emotional and memory areas; but also hint at differences within the memory areas that should be explored with further studies. As part of their research, the authors are now exploring the role of these brain regions during the retrieval of well-consolidated emotional memories.
While such studies are basic in nature, said Cabeza, better delineation of the role of the amygdala in emotional memory could aid understanding of post traumatic stress disorder -- especially such phenomena as flashbacks of traumatic memories. Said Dolcos, "Also, people who suffer depression ruminate obsessively on negative or unpleasant memories. This problem could reflect a pathology in how their memory systems have processed emotional memories."
Thus, said Cabeza, he and his collaborators are now exploring the nature of emotional memory encoding in people with depression, before and after therapy.