Summary: In people with post-traumatic stress disorder (PTSD), levels of norepinephrine and serotonin remain high during REM sleep, which undermines the brain’s normal ability to use rhythmic signals between the prefrontal cortex and amygdala to suppress fear-expression neurons. Models indicate that individuals with PTSD require faster neural rhythms to extinguish fear memories. Therapies that restore or induce those higher-frequency rhythms during sleep may help recover restorative sleep and reduce recurrent, emotionally intense dreams.
Source: Virginia Tech
REM sleep—sometimes called “paradoxical sleep”—often shows brain activity similar to wakefulness, and in some respects it can be even more active. Virginia Tech neuroscientist Sujith Vijayan explains that this heightened activity is linked to the vivid, emotionally charged dreams many people experience during REM sleep.
For most people, REM sleep helps rehearse and process emotional memories, gradually reducing their emotional impact. This memory evaluation likely serves an adaptive purpose, allowing the brain to prioritize and integrate important experiences, especially those involving fear. However, for people with PTSD, this nightly processing can become maladaptive: distressing dreams recur and the emotional intensity of traumatic memories persists rather than fading.
To investigate the mechanisms behind these differences, Vijayan and colleagues built biophysically based computational models of REM sleep and the neural circuits involved in fear memory processing. Their goal was to explore how changes in neurotransmitter levels during REM sleep influence rhythmic interactions between the medial prefrontal cortex—particularly the infralimbic region—and the amygdala, areas known to regulate fear expression and extinction.
In healthy REM sleep, levels of norepinephrine and serotonin fall. Using models that simulated this neurochemical state, the researchers found that reduced neurotransmitter tone strengthens rhythmic coupling between prefrontal and amygdala neurons. Those rhythms allow prefrontal activity to inhibit fear-expression cells in the amygdala, helping to suppress the emotional charge of fear-related memories.
The team also identified a particular frequency range that was especially effective. Within the human theta band—roughly four to eight hertz—lower theta frequencies near four hertz produced the strongest enhancement of prefrontal-to-amygdala connectivity and the most reliable suppression of fear-expression neurons in the models representing normal REM sleep.
When the researchers adjusted the models to reflect the neurochemical profile typical of PTSD—specifically, persistently elevated norepinephrine during REM sleep—they observed a marked disruption of these dynamics. Under high norepinephrine conditions, the theta rhythms that normally suppress fear-expression cells (around four hertz) became ineffective. The rhythmic interactions that promote fear extinction were weakened or dissipated, which the authors suggest could explain why traumatic dreams recur and emotional memories remain charged in PTSD.
The ‘Wild West’ of sleep stages
Vijayan refers to REM sleep as a “Wild West” because, compared with non-REM sleep, REM has been less thoroughly modeled and understood in terms of its circuit-level role in memory processing. Many robust frameworks exist for non-REM consolidation and replay, but REM’s contributions—particularly to emotional memory and fear extinction—have been harder to pin down. By creating detailed biophysical models of REM circuitry, his team aimed to clarify how neural rhythms enable the prefrontal cortex to dampen amygdala-based fear responses and why that mechanism fails in PTSD.
Their simulations started with healthy REM conditions, lowering norepinephrine and serotonin to reflect normal REM neurochemistry. Under those circumstances, rhythmic input to an infralimbic cell assembly representing an emotional memory strengthened infralimbic-to-amygdala synapses while weakening reciprocal amygdala-to-infralimbic connections. The net effect was suppression of fear-expression neurons associated with that memory—a mechanistic explanation for REM-related fear extinction.
Crucially, the models showed frequency specificity: low theta frequencies (around four hertz) worked across a broader range of input strengths to produce these synaptic changes, while other frequencies failed to generate the same effects.
When the team simulated PTSD-like REM, with sustained high norepinephrine, the effective low-frequency theta rhythms no longer produced fear-suppressing synaptic changes. “I was a little surprised that the four hertz didn’t work,” Vijayan said, noting that the normal theta rhythm simply could not re-establish the inhibitory prefrontal-to-amygdala influence under PTSD conditions.
Restoring the recuperative power of sleep
The models point to a possible path forward. Although the typical low-theta rhythms failed in PTSD simulations, the researchers tested alternative frequencies and found that higher-frequency input—around 10 hertz—could restore the synaptic changes associated with fear suppression. In other words, PTSD-like networks may require faster rhythmic drive to achieve the same extinction effects that lower-frequency rhythms provide in healthy REM sleep.
This insight opens translational possibilities: if clinicians can identify and safely induce the frequencies that re-enable prefrontal inhibition of amygdala fear cells in sleeping patients, they might reduce the emotional intensity of traumatic memories and improve sleep quality. One noninvasive candidate the team mentions is covert auditory stimulation—subtle sounds delivered during sleep that shift neural dynamics without waking the person. Such approaches could potentially be adapted for PTSD as well as other conditions that disrupt restorative sleep, including traumatic brain injury or neurodegenerative disorders.
The next steps in this line of research include testing whether targeted stimulation during REM can reliably shift oscillatory frequencies in humans with PTSD and whether such shifts translate into reduced nightmare frequency and improved daytime functioning. The modeling work provides a mechanistic rationale for trials that aim to tune sleep rhythms to promote emotional memory processing and fear extinction.
Author: Margaret Ashburn
Source: Virginia Tech
Contact: Margaret Ashburn – Virginia Tech
Image: The image is in the public domain
Original Research: Closed access. “Emotional Memory Processing during REM Sleep with Implications for Post-Traumatic Stress Disorder” by Sujith Vijayan et al., Journal of Neuroscience
Abstract
Emotional Memory Processing during REM Sleep with Implications for Post-Traumatic Stress Disorder
REM sleep plays a crucial role in processing emotional memories, including those tied to fear. Rhythmic interactions—particularly within the theta band—between the medial prefrontal cortex (mPFC) and limbic structures such as the amygdala are believed to contribute to this process, but the circuit- and mechanism-level details remain unclear.
To explore how rhythmic activity supports fear extinction during REM, the authors used a biophysically detailed model that included the infralimbic cortex (IL), a subregion of the mPFC essential for suppressing fear memories. Theta-frequency (4–12 Hz) inputs to a cell assembly in IL representing an emotional memory strengthened IL-to-amygdala connections and weakened amygdala-to-IL connections, leading to reduced activity of fear-expression cells tied to that memory.
Lower-frequency theta inputs near 4 Hz produced these synaptic changes across a wider range of input strengths. Inputs at other frequencies failed to induce the same synaptic modifications and did not suppress fear memories. Under simulated PTSD REM conditions—characterized by elevated norepinephrine—rhythmic activity degraded and 4 Hz inputs became ineffective. However, higher-frequency (10 Hz) inputs to IL could induce changes comparable to those seen with 4 Hz under normal REM, resulting in suppression of fear-expression cells.
These findings offer a potential explanation for the persistent, repetitive nightmares experienced by PTSD patients and suggest neuromodulatory strategies that could help ameliorate PTSD symptoms by restoring effective REM sleep rhythms.