Effect Of Sleep On Neural Activity Reactivation And Its Effect On Memory Retrieval

Successful retrieval of memory information requires that the contents of the memory trace match the information in the extraction environment.According to the neural computing model,the memory can be successfully recalled or retrieved because of the recovery of neural activity pattern during the encoding process,that is,the reactivation of neural activity.Hippocampal indexing theory supports the point of neural computing model,which holds that the neocortex activity pattern will be input into the hippocampus during the encoding process of memory,and the index will be formed in the hippocampus.In the memory retrieval stage,the hippocampus projects information back to the cerebral cortex according to the index,which activates the activity pattern of the cerebral cortex in the initial encoding stage.Based on the neural computing model and hippocampal index theory,we can further infer that the higher the similarity between the activity patterns in memory encoding and retrieving,the better the memory performance.Although the hypothesis has been confirmed by many research results,few studies have explored the influence of reactivation of neural oscillations on memory retrieval,and the influencing factors of reactivation of neural activities are still unclear.Therefore,in our study,we applied steady-state visual evoked potential to the memory task to further explore the influence of neural activity reactivation on memory retrieval and the influence of sleep on neural activity reactivation.In the first study,we investigated whether recurrent neural oscillations occur and their effects on memory retrieval.In the memory encoding stage,the target memory words were presented with background flicker.There are two kinds of flicker frequencies,respectively 12 Hz and 20 Hz,and only the target memory words are presented in the memory retrieval stage without background flicker.The EEG results showed that during the memory retrieval stage,when participants retrieval words which were appeared with 12 Hz flicker,the neural activity reactivated in the frontal lobe and occipital lobe,that is,the EEG signal of 12 Hz would be rapidly reappear.However,no reactivation of neural oscillatory activity was found in the words with 20 Hz flicker,whether or not they were successfully retrieved.After excluding the subjects who failed to flicker,we found that the better memory capacity under the flicker background condition of 12 Hz was significantly higher than that under the flicker background condition of 20 Hz.The results show that the reactivation of the neural oscillations induced by the episodic memory information will occur in the retrieval stage,and the reactivation of the neural oscillations will promote the memory performance.In study 2,we investigated the effects of sleep on reactivation of neural oscillations.Based on study 1,we added sleep and memory tests which is after sleep.In study 2,two memory tests were completed after the distraction task and one after the nap.The results showed that test 1 was consistent with study 1,and the reactivation of episodic memory neural oscillations occurred under 12 Hz flicker background.In test 2,however,the reactivation of episodic memory neural oscillations disappeared in the frontal lobe.This result proves that systematic memory consolidation occurs during the nap,especially in the slow-wave sleep period.In this process,the contents related to the target memory are firstly consolidated,while the episodic memory information which is unrelated to the target memory is not fully consolidated,results in the partial disappearance of neural reactivation.In the second study,the behavioral results were consistent with the EEG results.In the first test,the neural oscillations occurred in the frontal lobe region.After excluding the subjects who failed to flicker,the memory performance under the 12 Hz flicker background condition was significantly higher than that under the 20 Hz flicker background condition.In the second test,the reactivation of neural oscillations disappeared.After excluding the subjects who failed to flicker,there was no significant difference in the memory performance under the conditions of 12 Hz and 20 Hz.The results add to evidence that reactivation of neural activity improves memory retrieval.In summary,our study confirmed that the neural activity of episodic memory would be reactivated during the memory retrieval process,and this phenomenon would gradually disappear due to the systematic consolidation in the nap.At the same time,the recurrence of neural activity in the memory retrieval stage will improve the memory.