UCI Study Ties Memory to Emotion
In the first study of its kind on human subjects, scientists from UC Irvine and a Long Beach veterans hospital have determined that people tend to remember emotional events best because of stress hormones such as adrenaline activated during and after the experiences.
The research, to be published today in the scientific journal Nature, showed that drugs known as beta blockers, which block the adrenaline system’s response, impaired people’s long-term memories of emotional stories.
But the drugs had no effect on the memories of other people who were told emotionally neutral stories, suggesting that the stress hormones selectively enhance retention of emotional experiences.
The research, an outgrowth of years of animal studies, was described as novel by memory researchers across the country because it neatly demonstrates in humans what specific neurochemical processes are involved in the recall of emotional experiences.
“Everybody knows that we tend to remember exciting things,” said study co-author James L. McGaugh, a neurobiologist at the Center for the Neurobiology of Learning and Memory at UCI. “Now we’re beginning to understand why.”
While the results of the study are scientifically compelling in their own right, McGaugh said, the research may have broader practical implications. For example, it raises the question of whether beta blockers might be used in advance to blunt the memories of people who know they are about to undergo traumatic experiences.
And it suggests the possibility that patients who take beta blockers as medication for heart disease or high blood pressure, for example, might suffer mild impairment in their long-term recall of emotional events.
But “no one should consider reducing the dosage of beta blockers without consulting their personal physician,” McGaugh said.
The study, whose primary author was UCI’s Larry Cahill, makes a crucial link between human and animal research, confirming much of what memory researchers had suspected based on animal models.
“We never really knew what we were studying when we gave a foot shock to a rat,” said Dr. Larry Squire, a neuroscientist and professor at UC San Diego School of Medicine. “Now we have this contact point with humans. It really encourages the whole research enterprise in how the brain stores and retrieves memories.”
It was essential to make the connection to humans because “we don’t know really know what’s going on in (animals’) heads,” said Joseph LeDoux, a neuroscientist at New York University who studies the mechanisms of emotional memory.
Humans have a specific kind of conscious memory called “declarative memory,” whereas animals do not. But the UCI/Long Beach Veterans Affairs Medical Center study shows that the release of certain chemicals such as adrenaline acts on receptors to enhance memory in both humans and animals.
The study involved giving 36 healthy adult men and women either a placebo or a beta blocker, propranolol hydrochloride, before showing them a set of slides. About half the subjects were told an emotionally arousing story accompanying the slides--about a boy injured in a car accident--and the other half a neutral story about a boy who witnesses a disaster drill.
A week later, researchers tested subjects’ memories of the events, comparing those who had received placebos to those who had received a beta blocker. Their findings: Beta blockers made no difference in the recall of subjects who heard the neutral story; they recalled it as well as the comparable placebo group.
But the memories of those who heard the emotional story and received propranolol hydrochloride--and thus had their stress responses diminished--were significantly impaired. The placebo subjects who heard the emotional story recalled more slides than the group that received propranolol and performed better on a subsequent multiple-choice test.
In particular, the placebo group had superior recall of elements of the story that were emotionally arousing.
Researchers found the beta blockers did not dull subjects’ emotional responsiveness to the material they saw--as assessed immediately after it was presented--only their long-term recall.
And although the propranolol, as expected, significantly reduced subjects’ heart rate and blood pressure, that effect was not responsible for the subsequent memory impairment, according to the study. If that had been the same, the subjects who heard the neutral story and received the beta blocker would have shown memory problems as well.
The fact that the stress response plays a role in long-term memory makes sense from an evolutionary standpoint, McGaugh and other scientists said. In the standard stress response--often described as the fight-or-flight reaction--hormones such as adrenaline, noradrenaline and cortisol act to increase the heart rate, blood pressure and metabolism, leading to an improvement in the body’s overall performance.
The UCI/Long Beach study suggests that though the hormones stay in the system only about 30 minutes, they may have lasting protective effects in the form of useful memories.
“It’s adaptive to remember where you got hurt in the past . . . which rock you were standing on when you came across a leopard,” LeDoux said.
But the study opens the door to many more questions.
It still is unknown precisely where the hormones are acting--whether it is in the peripheral nervous system or in the brain, McGaugh said.
In addition, others said, it is unclear what effect more emotionally arousing experiences than the one used in the study would have on memory and whether there are limits to the enhancing effect.
Interestingly, McGaugh said there is abundant evidence that extremely high doses of adrenaline administered in animal studies can create the opposite effect--amnesia. But, he said, it is unknown whether those amounts can be produced naturally during emotional arousal.
