New research appearing in the April 9 issue of The Journal of Neuroscience highlights the important role sleep plays in strengthening and maintaining the accuracy of a memory and hints at why the brain shuts out sensory information during periods of deep sleep. The study found that introducing new odor information to an animal while it sleeps compromises its ability to remember the difference between new and previously encountered smells while awake.

During sleep, the brain performs a number of important repair and maintenance duties that are necessary for normal functioning, including moving information from short-term to long-term memory. Previous studies show that the "replay" of recently learned information during sleep plays an important role in memory storage. However, it was unknown whether introducing new information during slow-wave sleep (SWS) -- a phase of deep sleep during which the brain's sensory systems are far less responsive to external stimuli -- affects memory.

In the current study, Dylan Barnes and Donald Wilson, PhD, of the City University of New York, the Nathan Kline Institute for Psychiatric Research, and New York University Langone Medical School exposed rats to new and previously encountered odor information while the animals slept. To precisely control the animals' odor experience during periods of wakefulness and sleep, the researchers delivered electrical stimulation to brain circuits involved in odor processing rather than relying on the delivery of real odors to the animals.

Exposure to new odor information during sleep made it more difficult for the animals to distinguish the learned odor from the other odors. "While previous work has demonstrated the role of sleep replay on memory strength, these are the first data to show that memory accuracy can also independently be influenced during sleep," Wilson said.

Before introducing odor information to the animals as they slept, Barnes and Wilson taught the animals to associate specific odor information with a mild foot shock while they were awake. After the animals learned to make the association between the odor experience and shock (demonstrated by enhanced fearful behavior), Barnes and Wilson recreated the odor experience when the animals were in SWS.

Animals that received a replay of the learned odor experience during SWS demonstrated increased memory for the odor compared with animals that received a similar replay of the odor while awake. In contrast, the animals that received a replay of new odor information as they slept demonstrated greater difficulty distinguishing the learned odor they were previously exposed to from others.

"We know that during slow-wave sleep, the brain's sensory systems are far less responsive to normal inputs," Wilson said. "Our data suggest this sensory isolation may help allow replay of learned information in the absence of external interference, providing strong, precise memory of important information."

Jan Born, PhD, a neuroscientist who studies sleep at the University of Tübingen and was not involved in this research, noted such information may one day prove important for efforts to reduce traumatic memories. "The question of whether cueing memories during sleep, and specifically during slow-wave sleep, can be used to modify specific memories is currently a hot topic due to the potential for such information to lead us to new ways to weaken the unwanted memories commonly found in psychiatric conditions such as post-traumatic stress and anxiety disorders," he explained.

This research was funded by the National Institute on Deafness and Other Communication Disorders.