By H. Ober, U. of California - Los Angeles, Aug. 9, 2025
https://scitechdaily.com/scientists-discover-the-brains-reset-button-that-separates-your-memories/
Credit: Shutterstock
A brainstem region helps punctuate memories. Stress may block its ability to mark new experiences.
Although life unfolds in a continuous flow, our memories don’t capture it that way. We don’t recall the past as one seamless timeline but rather as a sequence of distinct, meaningful moments—much like how sentences are broken up with grammar and punctuation. This mental structure gives our experiences clarity and helps us understand both what happened and when it occurred.
The brain must devote a lot of space to this herculean task, right?
Wrong! It turns out that a tiny but mighty region pulls far more than its weight.
In a study published in Neuron, psychologists from UCLA and Columbia University used brain imaging and pupil measurements to reveal that a tiny group of neurons in the brainstem, called the locus coeruleus, functions like a “memory reset button” when meaningful changes occur.
“Our key question was: as an experience unfolds, how does the brain ‘know’ when one meaningful memory has ended and the next should begin?” said UCLA psychology professor and first author David Clewett. “Research has shown that remaining in a stable context, such as the same room, binds sequential experiences together in memory. By contrast, experiencing a shift in context, or event boundary, drives memories apart to represent distinct events. In this way, context acts as the grammar of human memory. What we found is that the locus coeruleus is most active at event boundaries when memories become separated. Thus, this small region at the core of the brain’s arousal system may serve to punctuate our thoughts and memories.”
Auditory cues shape context and memory
Clewett, along with co-authors Ringo Huang from UCLA and Lila Davachi from Columbia, conducted an experiment with 32 participants who viewed images of neutral objects while undergoing MRI scanning. To simulate changes in context, the researchers played simple tones in either the left or right ear. When eight identical tones were played repeatedly in the same ear, it created the feeling of a continuous event. A shift in tone pitch, and ear location signaled a change, creating the perception of an event boundary. This alternating pattern continued, generating the impression of four distinct auditory events.
Next, the team evaluated how these context shifts affected memory. They hypothesized that the ability to recall the correct order of events would reflect whether the experiences had been stored as a single episode or separated into distinct memories. When events are encoded together, sequence recall should be easier; when they are stored apart, it becomes more difficult.
Credit: Semel Institute/UCLA
As expected, increased activity in the locus coeruleus during event boundaries was linked to poorer recall of the order of item pairs that crossed those boundaries, suggesting the memories had been stored separately. To validate this, the researchers compared their fMRI readings of locus coeruleus activity with measurements of pupil dilation taken at the same time, since pupil size tends to increase during new events and locus coeruleus activation. The consistency between these data confirmed that the fMRI signals were accurately capturing activity in this small brain region. Functional magnetic resonance imaging, or fMRI, monitors brain activity by measuring changes in blood flow while participants are inside the scanner.
How hippocampus responds to boundary signals
The impact of this neural “reset” signal extended well beyond initial memory segmentation. Higher levels of locus coeruleus activity at event boundaries were linked to more pronounced shifts in activation within the hippocampus—a brain region crucial for encoding new memories and tracking contextual information such as location and timing.
“Part of the job of the hippocampus is to map the structure of our experiences, so it has an index of the beginning, middle, and ends of events. We found that the locus coeruleus may provide the critical ‘start’ signal to the hippocampus, as if saying, ‘Hey, we’re in a new event now,’” said Davachi. “Prior work had shown that bursts of locus coeruleus activity help reconfigure brain networks to direct attention to new and important experiences. Our findings suggest that this updating signal is even more widespread, also reaching memory-related regions that carry representations of ongoing events.”
The researchers also examined how brief bursts of locus coeruleus activation are influenced by background levels of locus coeruleus activity. This matters because locus coeruleus neurons operate in two distinct modes: a burst-like mode that flags significant events and forms new memories, and a background mode that regulates general alertness and stress.
“The locus coeruleus is like the brain’s internal alarm system,” Clewett said. “But under chronic stress, this system becomes overactive. The result is like living with a fire alarm that never stops ringing, making it difficult to notice when a real fire breaks out.”
Although the dynamic interplay between these firing patterns has been studied in the context of decision-making, perception, and learning, its relevance for how we perceive and remember events has, thus far, been unclear. So, the authors set out to test whether bursts of locus coeruleus activation at event boundaries, the neural signals that segment memories, might be weakened or lost under conditions of chronic stress. This question posed a challenge, as fMRI alone cannot measure absolute levels of stress or locus coeruleus activation. To address this, they used an imaging method that indirectly measures neuromelanin, a pigmented neurochemical that accumulates in the locus coeruleus with repeated activation over time.
Stress weakens the brain’s event detection signals
As predicted, participants with a higher neuromelanin-related signal, thought to indicate chronic stress, showed weaker pupil dilation responses to event boundaries. Stronger low-frequency fluctuations in locus coeruleus activation, a proxy for background levels of activity, also predicted weaker spikes in locus coeruleus activation and pupil responses to boundaries during the task. Together, these findings suggest that chronic hyperarousal may blunt one’s sensitivity to change, disrupting the cues that anchor and organize new episodes in memory.
Identification of the locus coeruleus as the gateway or conductor for memory formation may lead to better ways to treat PTSD and other memory-related disorders, such as Alzheimer’s disease, where the locus coeruleus is unusually hyperactive. There are potential ways to quiet an overactive locus coeruleus, whether pharmacologically or through slow-paced breathing or even hand-squeezed stress balls. But good long-term solutions require further research and will take time to discover and bring to market. Perceiving events in the “right” way is directly linked to better memory, suggesting that improving locus coeruleus function is an effective target for either protecting or recovering memory function.
Clewett said that the sophisticated tools necessary to look into the brain require the kind of funding that only the federal government can provide. Clewett said that several NIH grants that funded this research paid for the scanning and facilities they used to do the experiments, for example.
“Conducting basic science and clinical research is critical for opening new doors for treating debilitating disorders,” Clewett said. “Recent legislative actions threaten this future, not only for scientific research but for breakthroughs that can improve the lives of patients and their families. It is perhaps ironic that at a time when legislation promises ‘big and beautiful change,’ it turns out one of the brain’s smallest players may have the biggest impact on how we understand and remember our lives.”
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