Scientists at the RIKEN Center for Brain Science (CBS) have made significant strides in understanding how perceptual memories associated with positive emotions, such as joy or happiness, are reinforced during sleep. Their research, detailed in the journal Neuron, sheds light on the neurological underpinnings that could potentially aid in addressing issues like drug or sexual addiction.
The enquiry centres around why emotionally charged events, whether they elicit joy or sorrow, tend to forge robust, enduring memories of external stimuli such as music, scents, and textures experienced during these events. It is well-established that sleep plays a crucial role in memory consolidation, the process by which new experiences are transformed into lasting memories. However, the specifics of how sleep facilitates this memory enhancement, particularly about emotional events, have remained somewhat elusive. Prior experiments have suggested that the way emotional events are processed might vary with the sleep stage, yet it has not been clear which stage of sleep—REM or non-REM—is more pivotal in cementing these memories. The study led by Masanori Murayama at RIKEN CBS aims to clarify this.
In their experiments, the researchers first created scenarios that mimicked neutral and emotionally charged events using mice. On the first day, male mice explored only a smooth texture, serving as the learning phase. They were then exposed to both grooved and smooth textures on the second day, which acted as the testing phase. If the memory of the soft texture persisted until the second day, the mice preferred the grooved texture, which indicates their natural inclination towards novel environments.
The dynamics changed significantly when the smooth texture was paired with a positive emotional stimulus—interaction with a female mouse. This pairing led to a much longer retention of the texture memory, which lasted up to four days later. This experimental setup successfully demonstrated that emotional experiences could profoundly enhance memory retention of sensory details in mice. As Murayama notes, this study is the first to show that emotions can amplify perceptual memories in experimental animals, leading to the identification of crucial neural pathways involved in this process.
The amygdala, a critical part of the brain’s emotional circuitry, was identified as a key player in this memory enhancement mechanism. It is connected to a cortical circuit that extends from the motor to the sensory cortices, which Murayama’s team had previously discovered, and it plays a crucial role in the accurate perception and recollection of texture information. This tri-regional circuit was found to bolster emotionally linked perceptual memories. During the learning phase, these regions were activated cooperatively and reactivated during early non-REM sleep—though not during REM sleep—to reinforce the memory.
The study also tested the significance of non-REM sleep in this process by temporarily disrupting the amygdala’s inputs to the motor cortex, which sends top-down signals to the sensory area, during non-REM sleep. This disruption led to a failure to retain the texture memory until the fifth day despite the emotional experience. In contrast, similar disruptions during REM sleep did not affect memory retention, thereby underscoring non-REM sleep as the crucial phase for strengthening perceptual memories linked to emotions.
Contrary to traditional views that REM sleep is the primary phase for processing emotional memories, Murayama’s findings suggest that non-REM sleep is more essential. This revelation not only provides a deeper understanding of how emotions enhance perceptual memories but also opens avenues for potential treatments for conditions like addiction. These conditions often involve triggers known as flashbacks, where perceptual details are strongly linked to emotional events from the past. By modulating brain activity in the amygdala and associated regions during non-REM sleep, it might be possible to weaken these memory triggers and, thus, prevent or treat addictive behaviours.
Looking ahead, the team plans to extend their research to models of diseases such as age-related memory decline and addiction. Murayama is interested in investigating whether these findings could be applied to enhance or recover memories in aged mice, aiming ultimately to develop treatments that improve mental health and combat memory-related disorders. The implications of this research are vast, promising new strategies for enhancing mental health through targeted sleep interventions.
More information: Masanori Murayama et al, Amygdalo-cortical dialogue underlies memory enhancement by emotional association, Neuron. DOI: 10.1016/j.neuron.2025.01.001
Journal information: Neuron Provided by RIKEN
