The Persistence of Memory?

"What color hat was the bank robber wearing?" the police officer asks the eyewitness. "Red, no, black, definitely black," the witness insists. The question seems innocent, but it can prompt the witness to vividly remember a black hat, when in fact the robber wore no hat at all.

Human memory is notoriously unreliable, especially when it comes to details. Scientists have found that prompting an eyewitness to remember more can generate details that are outright false but that feel just as correct to the witness as actual memories.

In day-to-day life, this isn’t a bug; it’s a feature. We can’t possibly remember every tiny detail we see, but our memories would feel incomplete if there were big swaths of gray running through them. So the brain fills in the details as best it can, borrowing from other memories and the imagination in order to build what feels like a complete picture.

“A key rule about memory change over time is what we call fade-to-gist,” explained Dr. Charles Brainerd, a professor of human development at Cornell University, in an interview with Healthline. “That is, we lose the details of experience rapidly but retain our understanding of its gist much longer. After attending a baseball game, we may quickly forget what the score was, who pitched, and what we had to eat, but not that our team won and we had a fun evening.”

According to the American Bar Association, of the 21 wrongful convictions overturned by the Innocence Project in 2011, 19 involved eyewitness testimony. More than three-quarters of wrongful convictions that are later overturned by DNA evidence were based on eyewitness reports.

The legal system finally acknowledged this problem last year, when the New Jersey Supreme Court instructed judges to tell jurors that “human memory is not foolproof” when considering eyewitness testimony in a case.

This change comes just in time, as science is finding new ways to modify memory even further.

Now You Remember, Now You Don’t

Sometimes, the process by which memories fade-to-gist doesn’t happen properly. Addiction and post-traumatic stress disorder (PTSD) both occur when the brain forms a powerful association between two things that doesn’t fade over time.

This inability to fade makes addiction and PTSD incredibly difficult to treat. Even if the person can stop using a drug, powerful cravings can be easily triggered and are hard to resist. To find out why this is, Dr. Courtney Miller at the Scripps Research Institute teamed up with Dr. Gavin Rumbaugh and others.

They found that with memories of addiction and trauma, brain cells don’t form memories normally. Homing in on a brain region called the amygdala, which processes fear and other emotions, they discovered an important difference. In order to form new connections, proteins called actins inside the brain cell push the edges of the cell outward, growing new branches to reach other cells.

When healthy memories form, the actins stabilize and stop growing within a few minutes. But with addiction or trauma memories, the actins stay active, causing the connections to constantly strengthen and refresh.

Miller’s team developed a drug that targets the misbehaving proteins and shuts them down. Actins that are working properly remain unaffected. And even better, unlike other treatments in development, the patient wouldn’t have to actively access the memories in order to edit them.

“This is exciting because substance abusers have many, many associations with drug use, so targeting every single one in a clinical setting by retrieving and disrupting them may not be practical,” explained Miller, an assistant professor of neuroscience at Scripps, in an interview with Healthline.

This would also help people with PTSD, for whom recalling traumatic events can be re-traumatizing on its own. “The potential benefit would be that we would be able to administer these inhibitors to drug addicts and PTSD patients at any time, and it would only affect the ability of these unwanted memories to influence their behavior,” Miller said. Patients wouldn’t have to worry about becoming amnesiacs, but would be free of the compulsive drug-seeking or fear-based behaviors that their memories were causing.

The Power of Total Recall

Striving in the other direction, a team of scientists at the University of California, Irvine have discovered how to create a new memory in rats using direct brain stimulation. Team director Norman Weinberger worked with colleagues Kasia Bieszczad and Alexandre Miasnikov to investigate how auditory memories form in rats and whether they could initiate this process themselves.

Weinberger played a certain sound for the rats, which they ignored. Then, he electrically stimulated a deep brain region that is involved in memory formation and played the tone again. This time, the rats recognized and paid attention to the tone.

“The rats now had a ‘created memory,’ as they acted like the paired tone was now important,” said Weinberger in an interview with Healthline. “Such created memory has all of the major features of ‘natural’ memory, including long-term retention.”

His team was even able to pinpoint how the new memories formed. They scanned the rats’ brains, homing in on the auditory cortex, the area that processes sound. They found that once the artificial memory had been formed, extra cells in the rats’ brains attuned themselves to the particular sound that had been played. “The more cells, the stronger the memory,” Weinberger explained.

This study is one of the first to find the exact physical basis by which a memory is formed and stored. “Previously, research has neglected the neural representation of the ‘stuff’ of memories,” Weinberger says.

Weinberger stresses that this false memory creation technique can only occur with the help of a deep brain implant.

“The take-home message about memory is that, like intelligence, it is not a simple ability,” says Brainerd. “It is rich and complex. There are different types of memories that differ in reliability, that involve different brain areas, and that behave differently when we test them."

Photo courtesy of the University of California, Irvine.

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