The New Yorker
Within the brain, memories are formed and consolidated largely due to the help of a small seahorse-like structure called the hippocampus; damage the hippocampus, and you damage the ability to form lasting recollections. The hippocampus is located next to a small almond-shaped structure that is central to the encoding of emotion, the amygdala. Damage that, and basic responses such as fear, arousal, and excitement disappear or become muted.
A key element of emotional-memory formation is the direct line of communication between the amygdala and the visual cortex. That close connection, Phelps has shown, helps the amygdala, in a sense, tell our eyes to pay closer attention at moments of heightened emotion. So we look carefully, we study, and we stare—giving the hippocampus a richer set of inputs to work with. At these moments of arousal, the amygdala may also signal to the hippocampus that it needs to pay special attention to encoding this particular moment. These three parts of the brain work together to insure that we firmly encode memories at times of heightened arousal, which is why emotional memories are stronger and more precise than other, less striking ones. We don’t really remember an uneventful day the way that we remember a fight or a first kiss. In one study, Phelps tested this notion in her lab, showing people a series of images, some provoking negative emotions, and some neutral. An hour later, she and her colleagues tested their recall for each scene. Memory for the emotional scenes was significantly higher, and the vividness of the recollection was significantly greater.
When we met, Phelps had just published her latest work, an investigation into how we retrieve emotional memories, which involved collaboration with fellow N.Y.U. neuroscientist Lila Davachi and post-doctoral student Joseph Dunsmoor. In the experiment, the results of which appeared in Nature in late January, a group of students was shown a series of sixty images that they had to classify as either animals or tools. All of the images—ladders, kangaroos, saws, horses—were simple and unlikely to arouse any emotion. After a short break, the students were shown a different sequence of animals and tools. This time, however, some of the pictures were paired with an electric shock to the wrist: two out of every three times you saw a tool, for instance, you would be shocked. Next, each student saw a third set of animals and tools, this time without any shocks. Finally, each student received a surprise memory test. Some got the test immediately after the third set of images, some, six hours later, and some, a day later.
What Dunsmoor, Phelps, and Davachi found came as a surprise: it wasn’t just the memory of the “emotional” images (those paired with shocks) that received a boost. It was also the memory of all similar images—even those that had been presented in the beginning. That is, if you were shocked when you saw animals, your memory of the earlier animals was also enhanced. And, more important, the effect only emerged after six or twenty-four hours: the memory needed time to consolidate. “It turns out that emotion retroactively enhances memory,” Davachi said. “Your mind selectively reaches back in time for other, similar things.” That would mean, for instance, that after the Challenger explosion people would have had better memory for all space-related news in the prior weeks.
Picture: Brad Shorr (The Straight North Blog) [Public domain], via Wikimedia Commons