The distinctive role of NREM and REM sleep in the consolidation of fear memory

Abstract

Studies suggest that one-night sleep sufficiently evokes qualitative changes in the emotional memory systems and helps in remodeling the amygdalar and prefrontal neural circuitries after learning. REM sleep also potentiates the conditioned neural responses to fear mainly in the lateral amygdala, hippocampus, and medial geniculate nucleus. In addition, the coherence of oscillatory theta waves increases in the amygdala and hippocampus after conditioning, which presumably helps encode fear memory. Some of the symptoms of post-traumatic stress disorder (PTSD) such as hyperarousal reactions, flashing, and nightmare are developed as a result of over-consolidation of negative memories. Subjects falling into sleep soon after experiencing a life-threatening event may keep such negative emotional memories alive for years. Therefore, sleep deprivation following trauma can be one of the interventions to help prevent the development of PTSD. On the other hand, sleep architecture changes after the consolidation of fear memory. Within sleep, NREM sleep significantly increased, and REM sleep significantly decreased only after the consolidation of fear memory but did not change if the memory was impaired. Using the Bayesian law of conditional probability, we have observed that the consolidation of fear memory requires facilitated NREM sleep. In this chapter, we have reviewed the beneficial effects of sleep loss in the consolidation of fear-conditioned memory and sleep loss-mediated alteration in the possible underlying molecular mechanisms. We have also discussed that rats trained during the active phase exhibited better conditioning response compared to the rats trained during the inactive phase. In addition, we have discussed the correlation between augmented NREM sleep manifestations with the induced freezing response and Bayesian probabilistic theorem to predict, if within sleep, there is (a) a high probability of NREM sleep augmentation after the consolidation of fear memory and (b) a low probability of NREM sleep augmentation after impairment of fear memory.