The Role of Sleep in Memory Reactivation

Updated: Aug 22, 2018

By Loret Haas-Hanser

New knowledge and experience is gained every day. Through processes such as encoding and retrieval, the brain can not only retain, but access and reactivate memories. Initially, consolidating memories is dependent on many internal and external factors – different types of memory are processed and stored through various regions within the brain. Emotional memories require a healthy communication-based relationship between the basolateral amygdala (BLA) and hippocampus within the brain's medial temporal lobe. And specific communications regarding the unification of memory within the BLA and hippocampus are still being discovered. In an August 2017 study, three researchers, Gabrielle Girardeau, Ingrid Inema, and Gyorgy Buzsaki, designed an operant conditioning experiment in which neuronal patterns in rats were measured through a schedule of sleep before and after training.


Stabilization of memories and newly-formed spatial representations rely heavily on the sleep-wake cycle. In particular, researchers decided to study REM sleep. REM sleep is characterized by an active brain, low muscle tone and is the stage in the sleep-wake cycle when dreaming occurs. Hippocampal sharp wave–ripples (SPW-Rs) facilitating towards the BLA indicate that context-dependent emotional memory consolidation occurs during this stage of the amygdala-hippocampus circuit. To analyze hippocampal–amygdala interactions, researchers trained rats to run on a linear track for water rewards and added a location-specific air puff to create a classical spatial test.


A 2014 study by Dr. Lovett-Barron exemplified that the placement of the air puff triggered contextual fear-based learning within both the amygdala and the hippocampus. Girardeau et al. implemented this experimental ideology by placing an aversive air puff on the track to try and characterize a physiological response. After rats ran on the track, an aversive air puff was placed in the same location of each lap. Each day consisted of a pre-learning air puff test session, sleep in the home cage (pre-REM sleep), a session with the air puff, a post-learning sleep (Post-REM) and a final test without the air puff. At the end of the test day, the location and direction of the air puff were changed. The placement of the air puff was changed at the end of each day, but the rats ran multiple laps on the track after originally being exposed to the location of the air puff. After introductory exposure, the rats would slow down before running past the location of the air puff. Typically, the rats would reduce speed while approaching the air puff and accelerate once the air puff was out of sight.


By recording the left and right amygdala alongside the dorsal hippocampus and observing SPW-Rs, it became clear that there were reactivations of memory after the addition of the air puff. Neuronal density facilitates the passage of information between the BLA and hippocampus during SPW-Rs. The reactivation of hippocampal-amygdala activity after the air puff was placed was stronger than without exposing the rats to a fear-inducer. The researchers identified that REM sleep is critical for emotional memory storage and reactivation following a threatening task.


Further research will target the relationship of emotionally consolidated memories with the sleep-wake cycle. By examining how deep stages of sleep allow the hippocampus and amygdala to consolidate and process emotional memory, researchers have opened a gateway to further understanding the symbiotic relationship between sleep processes and reactivating memories.


References:


Girardeau, G., Inema, I., & Buzsáki, G. (2017). Reactivations of emotional memory in

the hippocampus-amygdala system during sleep. Nature neuroscience.


Lovett-Barron, M., Kaifosh, P., Kheirbek, M. A., Danielson, N., Zaremba, J. D., Reardon,

T. R., ... & Losonczy, A. (2014). Dendritic inhibition in the hippocampus supports

fear learning. Science, 343(6173), 857-863.

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