Previous studies have shown that an undisturbed short plastic period after cueing is needed for TMR to be effective. Therefore, researchers from Université libre de Bruxelles in Belgium looked at the effect of verbatim or interfering auditory presentation during wakefulness or non-rapid eye movement (NREM) sleep and its impact on memory consolidation.
The researchers recruited 96 participants for the study. The participants were tasked to study a list of 40 word pairs that were presented both visually and aurally. During a consolidation interval of 90 minutes spent either awake or asleep, the researchers provided the first word of 20 of the pairs aurally, succeeded by its correct (verbatim) or incorrect (interference) association. After that, they assessed memory for all pairs.
Results revealed that the participants in the wake conditions showed improvement in their memory after verbatim cueing; however, their memory weakened after an incorrect association was presented, when compared with uncued word pairs. On the other hand, the targeted memory reactivation or sleep did not affect the memory consolidation of the participants in the nap conditions.
Moreover, time-frequency assessments indicated that increased spindle-related sigma activity succeeds the presentation of the first word during NREM sleep. This indicated that the reactivation of the related memory content was instigated. On the other hand, the immediate presentation of the second word, either correct or incorrect, led to the elimination of sigma activity.
The researchers found a successful memory reactivation during sleep requires a sensitive silent plasticity period after the presentation of learning-related cues during sleep to be effective.
How the brain consolidates memory during sleep
Sleep, according to numerous studies, is important for learning and forming long-term memories. However, how long-term memories are formed remained unclear, until this study. Researchers from the University of California – Riverside explained how deep sleep promoted the consolidation of recent memories. (Related: Memory and sleep: Researchers finally discover why getting sufficient sleep dramatically boosts brain performance.)
The brain stays active during sleep, exhibiting electrical activity in the form of sharp-wave ripples in the hippocampus and large-amplitude slow oscillations in the cortex, which reflects changing periods of active and silent states of cortical nerve cells during deep sleep. During deep sleep, traces of episodic memory gained during wakefulness and initially stored in the hippocampus are transported to the cortex as long-term memory.
The researchers used a computational model to provide a link between electrical activity in the brain during deep sleep as well as synaptic connections between neurons. They showed that the hippocampal sharp-wave ripples influence patterns of slow oscillations in the cortex, which also determine synaptic changes in the cortex. In turn, synaptic changes influence the patterns of slow oscillations, which stimulate a kind of reinforcement and replay of particular firing sequences of the cortical neurons. This represents a replay of a particular memory.
The findings of the study, which were published in the Journal of Neuroscience, suggest that the input from the hippocampus, the sharp-wave ripples, arrives in the cortex during deep sleep and affects how the slow oscillations are established and promoted in the cortical network.
“By influencing the nature of these oscillations, this hippocampal input activates selective memories during deep sleep and causes a replay of specific memories. During such memory replay, the corresponding synapses are strengthened for long-term storage in the cortex,” explained Yina Wei, the first author of the study.
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