Stress Exerts Differential Effects on the Recall and Recognition of Emotional Words, Depending on Proximity to the Learning Experience
Extensive work has shown that stress exerts a profound, yet complex, influence on learning and memory and can enhance, impair or have no effect on these processes. Two factors that mediate this complexity are the temporal proximity of the stressor to the learning experience and the emotional nature of the to-be-learned information. We have examined the differential effects of brief stress on the learning and memory of emotional and non-emotional words when the stress was administered immediately before learning versus when it was administered 30 min before learning. Seventy-two undergraduate students were randomly assigned to stress or no stress conditions. Participants in the stress conditions were exposed to a socially evaluated cold pressor test; they submerged their dominant hands in a bath of ice cold (0-2°C) water for 3 min while being stared at by a confederate of the opposite sex and believing that they were being videotaped. Participants in the no stress conditions submerged their dominant hands in a bath of warm (35-37°C) water for 3 min. All participants rated the painfulness and stressfulness of the water bath manipulation. Either immediately (Exp 1) or 30 minutes (Exp 2) after water bath exposure, participants were presented with a list of 30 words (10 neutral, 10 positive, 10 negative); they read each word aloud and rated its emotional valence. Saliva samples and cardiovascular measurements were collected throughout the session to verify the induction of a physiological stress response. Twenty-four hours following exposure to the list of words, participants returned to the laboratory and completed free recall and recognition tests. In both experiments, stressed participants exhibited significantly greater blood pressure, salivary cortisol levels, and subjective pain and stress ratings of the water bath manipulation. Stress applied immediately prior to learning (Exp 1) significantly enhanced the recognition of positive words, while stress applied 30 min prior to learning (Exp 2) significantly impaired free recall of negative words. In contrast to the effects observed in Exp 1, the deleterious effects of stress observed in Exp 2 were associated with participants’ cortisol and blood pressure levels. These findings support previous human research by demonstrating that stress has a greater effect on the acquisition and consolidation of emotional information, implicating the involvement of amygdala-induced modulation of hippocampal function in such effects. They also lend insight into the differential effects that acute stress may exert on learning and memory depending on the temporal proximity of the stressor to the learning experience.