For some people, the smell of an apple pie might spark a warm childhood memory. For others, a loud sound may bring back strong battlefield images. New research in rats may help explain these connections by suggesting that emotional memories like these are stored in parts of the brain linked to sight, sound and smell.
Scientists have long known that emotionally charged memories tend to be stronger than neutral ones. But researchers haven’t been able to pinpoint where in the brain those memories are stored in the long term.
In the new study, published in the Aug. 6 Science, researchers trained rats to associate sounds, smells and sights with electric shocks. After a month, the researchers damaged an auditory, visual or olfactory part of the brain, called a secondary sensory cortex, in some of the rats. The damage appeared to make the animals lose memories linked to the damaged sense; they no longer froze in their tracks at the sounds, smells, or sights they had previously learned to fear.
Each sense, including sound, smell and vision, has a primary and a secondary sensory cortex area in the brain. The primary cortex sends sensory information to the secondary cortex, which then connects to emotional and memory areas of the brain.
“I think it’s a groundbreaking paper,” says neuroscientist Alcino Silva of the University of California, Los Angeles. “It’s the first time that I know of that someone was able to connect the sensory cortices to a remote memory, and that’s quite significant.”
The rats’ amnesia happened only for emotional memories associated with fear, not for neutral memories. In addition, the brain damage affected only memories that had been formed a month before, not those from the previous day, and rats could still learn new fears associated with that sense.
“It indicates that sensory cortices that commonly are not considered key structures for emotional memories are indeed necessary for them,” says neuroscientist and study coauthor Benedetto Sacchetti of the University of Turin in Italy.
Sacchetti says he is now doing more experiments to determine whether brain-damaged rats can maintain new fear responses after one month. He also wants to investigate if the same brain areas store positive emotional memories such as joy and if similar brain areas are activated in humans one month after fear-conditioning trials.
Storage of emotional memory over large, distributed areas of the cortex may explain why it’s harder to forget painful memories, Sacchetti says, and could have implications for the therapy of fear-related disorders. A sense-based storage method may also have given a survival benefit to our ancestors, he says. It was probably important for them to remember fear-related stimuli in order to prevent or avoid them in the future.
The new results should be interpreted with caution, says neuroscientist Norman Weinberger of the University of California, Irvine. Since the study doesn’t show whether brain-damaged mice could store new memories in the long term, researchers won’t know for sure if secondary sensory cortices are the only areas responsible for storing emotional memories until they do more studies.
“What is the big story of the 21st century is that primary and even secondary cortices appear to be sites that are likely to store memories,” Weinberger says. “And there’s no part of the brain which is immune from memory storage of some kind.”