Forgetting to remember: emotion robs memory while reviving it.

Emotionally charged events often seem particularly memorable. But this vivid recall may come at a cost. A new study in England suggests that the same biological process that aids recall of emotional experiences also blocks memories of what happened just before those arousing occurrences took place.

These memory effects appear to depend on a common neurobiological mechanism, says neuroscientist Bryan A. Strange of University College London. Women suffer larger emotionally instigated memory losses than men do, Strange and his coworkers also have found.

Emotion-induced memory gains and losses reflect the activity of stress hormones from the adrenal glands on the amygdala, an inner-brain structure, the scientists assert in an upcoming Proceedings of the National Academy of Sciences.

Prior research suggested that these adrenergic hormones, stimulated by emotionally arousing events or language, induce the amygdala to create long-term memories of those inputs. Those studies tested memory after a delay of several weeks or more.

In contrast, Strange's team examined recall in the immediate aftermath of an emotional language-based event. Lists of neutral nouns presented to 58 male and female volunteers contained a single, randomly placed noun with a disturbing connotation, such as murder or scream. Nouns appeared on a computer screen at a rate of one every 3 seconds and were visible for 1 second. After viewing a list, volunteers tried to remember as many words as possible before moving on to the next list.

Overall, men and women recalled the emotional words much more often than they did the neutral words. Moreover, the poorest memory occurred for neutral words that were presented immediately before the disturbing words. Women forgot those words twice as often as men did.

Emotions are critical to this memory effect, Strange says. Among the same adults, no comparable pattern of memory enhancement and impairment appeared for all-neutral-noun lists that contained a single word in a different font or one word with a meaning unrelated to that of any of the other words.

Moreover, participants who were administered a propranolol pill before viewing lists didn't exhibit the superior memory for disturbing words seen without the drug. Propranolol blocks transmission of beta-adrenergic hormone and thus blunts emotional reactions. Intriguingly, the people who took this drug recalled words that had appeared just before emotional words better than they did other neutral words.

In the same study, a man with extensive amygdala damage due to a rare genetic disease showed no emotion-related memory effects after viewing noun lists.

The new findings "have moved us closer to understanding both the beneficial, and harmful, effects of emotion on memory," comments neuroscientist Larry Cahill of the University of California, Irvine. Although common biological processes may underlie emotion-induced memory enhancement and amnesia, there may be differences between short-term and long-term memory, Cahill adds.

For instance, the brain may activate adrenergic hormones primarily in response to brief, mildly arousing experiences such as reading disturbing words, thereby fortifying short-term memories of those experiences, he theorizes. However, Cahill notes, adrenergic activation by sensory and motor neurons outside the brain may enhance long-term memories of highly emotional events. The surprising evidence for propranolol's potential as memory-boosting agent also deserves further investigation, Cahill adds.