REM sleep is a brain state with desynchronized activity, often linked to dreaming. Dreams during REM sleep are typically bizarre, emotionally intense, and fragmented, yet feel story-like and personally meaningful. Elements of real-life events, especially trauma, tend to reappear, but the dream plot is distorted. This review presents a new testable model, the Trace-Spur-model, based on Hebbian neural network theory. It proposes that dream bizarreness results from changes in synaptic strength, allowing hyper-associative mental activity that may help consolidate or extinguish waking learning. Dream recall offers only a fragmentary glimpse into these processes, biased by autobiographical memory.
The traditional view that non-REM sleep is a uniform state of unconsciousness is being challenged by recent findings. Slow oscillations, once considered exclusive to sleep, have been observed during wakefulness, and neural dynamics typical of conscious states have been detected during non-REM sleep. Additionally, the disconnection in cortical effective connectivity, long thought to be a hallmark of unconsciousness in non-REM sleep, appears less general than previously assumed. These findings complicate the search for neural correlates of consciousness (NCC) and suggest that the boundaries between conscious and unconscious brain activity may be more fluid than standard models propose.
During REM sleep, rapid eye movements (REMs) may scan dream images, but evidence has been mixed. In a study of 56 patients with REM sleep behavior disorder (RBD), who enact dreams due to absent muscle paralysis, REMs were found to be directed toward the patient's goal-oriented actions (e.g., reaching, greeting) 90% of the time, though REMs were absent in 38% of such behaviors. This directional coherence between eye, head, and limb movements suggests that when REMs occur, they imitate scanning of the dream scene. Because REM characteristics were similar in RBD patients and controls, the finding likely extends to normal REM sleep, supporting a brainstem generator that coordinates images, sounds, limb movements, and REMs in parallel, like a virtual reality.