Similar neural mechanisms for emotion-induced memory impairment and enhancement.

C oncerning emotion’s influence on memory, the psychologist Henri Piéron observed that ‘‘a violent emotion may reinforce memory, and give rise to indelible associations’’ (1). Our common experience (typically with emotions less intense) also tells us that emotional events tend to be well remembered, and extensive scientific evidence confirms anecdotal observations that emotional arousal can strengthen memory. In the past decade, neuroscience has witnessed a compelling convergence of evidence from human and animal subject studies regarding the neurobiology of emotion-enhanced memory. Key among the neurobiological players so far identified are endogenous stress hormones (in particular, the adrenergic hormones epinephrine and norepinephrine) and an almond-sized structure deep in the medial temporal lobe called the amygdala. The evidence suggests that endogenous stress hormones, released by emotionally arousing events, feed back directly or indirectly to the amygdala to amplify long-term memory storage of the events that induced their release (2). When it comes to memory, however, emotion is a double-edged sword. It may enhance or impair memory, depending on many factors. Even different aspects of a single emotional experience may be retained very well by the memory, or seemingly lost from it entirely. The amnestic potential of emotional arousal has received less attention from neurobiologists than has its memory-enhancing potential. Into this relative void comes a new report by Strange et al. (3) in this issue of PNAS, which combines in a single tour de force psychological, pharmacological, and neuropsychological evidence to implicate, in emotion-induced amnesia, mechanisms similar to those involved with emotion-induced memory enhancement. Strange et al. began by establishing an experimental procedure producing both memory-enhancing and -impairing effects reasonably attributable to emotional arousal. Subjects viewed brief lists of nouns presented for 1 s every 3 s. The lists were composed mostly of semantically related, emotionally neutral words. ‘‘Emotional oddballs’’ (aversive words such as ‘‘scream,’’ ‘‘murder,’’ and ‘‘morgue’’) determined by independent judges to be significantly more emotional than the neutral words were interspersed within the lists. After a short delay, subjects freely recalled as many words as they could from the just-seen list, before continuing with the next list. Results from the lists were pooled. Subjects recalled the emotional words (‘‘E’’ words) significantly better than they did the neutral words. Furthermore, subjects recalled the words presented immediately before the E words (neutral ‘‘E-1’’ words) significantly worse than they recalled the other neutral words. Control procedures substituting ‘‘perceptual oddballs’’ (words in the same semantic category, but in another font) or ‘‘semantic oddballs’’ (words independently judged to be semantically unrelated to the other