When long-term memories are reactivated, they can reenter a transient plastic state in which they are vulnerable to interference or physiological manipulations. The present study attempted to directly affect reactivated memories through a stress manipulation, and compared the effects of stress on reactivated and nonreactivated components of a declarative memory in a within-subject design. We presented image pairs that consisted of an image of an animal and an image of an unrelated object. Participants were instructed to memorize the object images. Forty-eight hours later, we presented half of the animal images again in an unrelated task to indirectly reactivate the associated object images. Immediately after reactivation, participants were exposed to cold pressor stress or a warm water control condition. Forty-eight hours later, we assessed memory for the object images with a free recall test. Reactivation boosted memory performance in the control condition, such that reactivated items were better recalled than nonreactivated items. This memory-enhancing effect of reactivation was completely abolished by cold pressor stress. Importantly, stress selectively impacted only the reactivated items while leaving memory for the nonreactivated items unaffected. The present study shows that it is possible to selectively reactivate and modulate specific parts of a declarative memory.
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