Why Do We Remember—or Forget—the Details?

Mara Mather
Professor Mara Mather

When someone is walking down the street and is startled by a car accident, what determines whether they remember the details of what they were doing prior to the crash clearly?

Paradoxically, such alarming or exciting moments—known as “emotionally arousing” events—can either impair or enhance memories of the moments surrounding them. With the car crash example, this can result in fuzzy memories for some witnesses and vivid recollections for others.

A new model of how chemicals in the brain work during these moments, proposed by a University of Southern California-led research team, makes sense of this paradox, illustrating that it all comes down to whether the witness is actively paying attention to something prior to the event.

In the car crash example, this means that the witness most likely to accurately remember the details of what they were doing prior to the crash would be the person actively paying attention to some train of thought, activity, or part of their surroundings as they traveled down the street. Conversely, the person idly walking without paying any particular attention to anything might have little memory of what they were specifically thinking or doing prior to the accident.

When someone is focusing on a task or piece of information, neuron cells in their brain release the neurotransmitter glutamate, a chemical that passes across the gaps, or synapses, between the cells in order to transfer nerve impulses through the brain. Paying more attention to something means releasing more glutamate in the specific applicable region of the brain.

According to the proposed model, glutamate molecules interact with norepinephrine, a hormone that’s the precursor to adrenaline and is released during emotionally arousing events. When released into the same region of the brain, norepinephrine and glutamate enter into a positive feedback loop, with each chemical increasing the release of the other.

This results in “hot spots” of improved processing and better memory for information to which attention is being paid. Conversely, less processing power is given to background information that’s not being focused on, said Mara Mather, USC Leonard Davis School of Gerontology professor and first author of the paper.

The culmination of nearly a decade’s work, including reviews of literature and studies of how human subjects’ memories were affected by emotionally arousing images or noises, the model is called Glutamate Amplifies Noradrenergic Effects, or GANE. It not only explains the paradox on why memory can be impaired or enhanced in emotionally arousing situations but also shines light on how a region of the brain called the locus coeruleus influences attention and memory.

The locus coeruleus region of the brainstem produces most of the brain’s norepinephrine and plays a role in attention and memory as well as cognitive control and stress. It’s also suspected of being involved in memory-robbing diseases such as Alzheimer’s, Mather added.

“[The locus coeruleus] is like a mixing table at a recording studio that’s increasing the volume on what matters, but turning other things down at the same time,” Mather said. “It’s modulating how active something is; it can control what signal you get from each of the many things that are processed by the brain simultaneously.”

The study, “Norepinephrine ignites local hot spots of neuronal excitation: How arousal amplifies selectivity in perception and memory,” first appeared online in Behavioral and Brain Sciences in July 2015. Mather’s coauthors include USC PhD in Neuroscience student David Clewett; Michiko Sakaki, Senior Research Fellow at the University of Reading; and Professor Emeritus Carolyn W. Harley of the Memorial University of Newfoundland. Their research was supported by National Institute on Aging grant RO1AG025340.

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