Scents and Reminiscence: Olfactory Influences on Memory Consolidation in the Sleeping Human Brain

Abstract

Most animals sleep, even though the loss of vigilance associated with the sleep state can have serious consequences for survival. Why animals need to sleep is unclear, and the precise functions of sleep are not well understood. Recent studies have converged on the idea that sleep plays an active role in supporting memory consolidation. In the past decade, researchers have developed a unique method to manipulate specific memories during sleep. Known as targeted memory reactivation, this procedure involves presentation of a sensory cue during an initial encoding task, and then re-presentation of the same cue during slow-wave sleep, resulting in memory enhancement for the cue-associated items in the wake state. Odor stimuli have proven to be an effective entry point for reactivating memories in the sleeping human brain: the olfactory pathway projects directly to limbic networks supporting memory and emotion, and odors can be delivered with minimal risk of waking the sleeping subject. Here we review the human literature on olfactory targeted memory reactivation, detailing its initial applications and more recent progress in the field. Although the underlying neural mechanisms remain elusive, the capacity of sleep-borne odors to selectively target memories has important basic and translational research implications for shaping the consolidation of both declarative and emotional memories.