Summary: Our memories don’t flow like a continuous stream—they’re divided into meaningful events, thanks to a tiny brain region called the locus coeruleus. New research shows this region acts like a “reset button,” activating at event boundaries to signal the hippocampus to store distinct memories.
Using fMRI and pupil measurements, scientists found this mechanism is weakened by chronic stress, which blunts the brain’s sensitivity to change and disrupts memory organization. Understanding this process could lead to better treatments for memory-related disorders like PTSD and Alzheimer’s disease.
Key Facts:
- The locus coeruleus helps segment life into distinct, memorable events.
- Chronic stress overactivates the region, impairing memory organization.
- Targeting this system could improve therapies for PTSD and Alzheimer’s.
Source: UCLA
Life may unfold as a continuous stream, but our memories tell a different story.
We do not recall the past as one long, unbroken text. Instead, we remember it as a series of meaningful events, like how sentences are structured with grammar and punctuation.
Like any narrative, this organization gives our experiences shape and coherence, helping us make sense of what and when things happen.
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 paper published in the journal Neuron, psychologists at UCLA and Columbia University combined brain imaging and pupil measurements to show that a small collection of neurons in the brainstem, known as the locus coeruleus, acts like a “memory reset button” during meaningful changes.
“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.”
Clewett and co-authors Ringo Huang at UCLA and Lila Davachi at Columbia recruited 32 volunteers who looked at pictures of neutral objects while inside an MRI scanner. To manipulate whether the surrounding context was stable or changing, simple tones were played in either the right or left ear.
Eight pure tones were repeated in the same ear to create a sense of a coherent event, then the tone switched to the other ear and changed in pitch to elicit perception of an event boundary. This repeat-switch pattern continued throughout the remainder of the sequence, creating the perception of four different auditory events.
The researchers then tested how these tone switches influenced memory. They reasoned that time provides a window into how events are formed: when people successfully reconstruct the order of a sequence, it suggests these items are linked within a single memory. In contrast, it is harder to remember the precise sequence of events when they have been stored in separate, distinct memories.
As they predicted, locus coeruleus activation at event boundaries predicted later memory separation, as shown by worse ability to remember the order of item pairs that spanned boundaries.
The researchers also cross-checked their measurements of locus coeruleus activation against eye pupil dilation measurements taken at the same time, because pupils are known to dilate slightly both as new events occur and when the locus coeruleus is active.
These measurements confirmed that observations during the fMRI were indeed capturing activation in this small brain area. Functional magnetic resonance imaging, or fMRI, measures brain activity by detecting changes in blood flow while a person is inside the scanner.
The consequences of this neural and memory reset signal were far-reaching. Stronger locus coeruleus activation at boundaries between events predicted larger changes in activation patterns within the hippocampus, a brain region that tracks contextual details like place and time and is central to the formation of new memories.
“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.
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.”
About this memory and neuroscience research news
Author: Holly Ober
Source: UCLA
Contact: Holly Ober – UCLA
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Locus coeruleus activation “resets” hippocampal event representations and separates adjacent memories” by David Clewett et al. Neuron
Abstract
Locus coeruleus activation “resets” hippocampal event representations and separates adjacent memories
Memories reflect the ebb and flow of experiences, capturing distinct events from our lives. Using a combination of functional magnetic resonance imaging (fMRI), neuromelanin imaging, and pupillometry, we show that arousal and locus coeruleus (LC) activation segment continuous experiences into discrete memories.
As sequences unfold, encountering a context shift or event boundary triggers pupil-linked arousal and LC processes that predict later memory separation.
Boundaries, furthermore, promote temporal pattern separation within the left hippocampal dentate gyrus, which correlates with heightened LC responses to those same transition points.
Unlike transient LC effects, indirect structural and functional markers of elevated background LC activation correlate with reduced arousal-related LC and pupil responses at boundaries, suggesting that hyperarousal disrupts event segmentation.
Our findings support the idea that arousal mechanisms initiate a neural and memory “reset” in response to significant changes, fundamentally shaping the episodes that define episodic memory.
