We often think of memories like the contents of a museum: static exhibits that we view to understand the present and prepare for the future.

The latest research, however, suggests they are more like well-thumbed library books that wear and change a little bit every time they’re pulled off the shelf.

Think of one of your happiest memories. For real. Sit with the recollection. Let your mind’s eye wander around the scene. See if you can feel a spark of the joy or hope you felt at the time. Let a minute pass. Maybe two.

If you played along with this experiment, you are physically different now than you were a few minutes ago.

When you began to reminisce, brain cells dormant just seconds before began firing chemicals at one another. That action triggered regions of your brain involved in processing emotions, which is why you may have re-experienced some feelings you did at the time of the event.

Chemical and electrical signals shot out to the rest of your body. If you were stressed before you began this exercise, your heart rate probably slowed and stabilized as levels of cortisol and other stress hormones decreased in your blood. If you were already calm, your heart rate may have quickened with excitement.

In either case, regions of the brain that light up when you get a reward jittered with dopamine.

The memory changed you. But by pulling this memory to mind, neuroscientist Steve Ramirez says, you also changed the memory.

Some elements of the memory heightened in importance. Others receded. Your brain snipped out and inserted details without your conscious knowledge. The mood you were in at the time of reminiscence left emotional fingerprints on the memory, as neurons activated by your mental environment synced up with those activated by the recollection.

Every time you revisit this heartwarming scene you change it a little bit, both as a subjective experience and a physical network of cells.

Humans have engaged in this two-way operation of memory revision for as long as we’ve been conscious. But over the last two decades, neuroscientists have found mind-bending ways to control this process (in mice, at least): implanting false memories, deleting real ones, resurrecting memories thought lost to brain damage, detaching the memory of an emotional reaction to one event and attaching it to the memory of another.

“It is all part of a larger revolution brewing in science to make memory manipulation a commonplace practice in the lab,” Ramirez writes in his recent book, “How to Change a Memory: One Neuroscientist’s Quest to Alter the Past” (Princeton University Press). “A memory may transform me entirely, but I have the power to transform it as well — both with my mind and with my science.”

In movies about stuff like this, there’s often a sinister air around the memory-tweaking scientist character. Ramirez, a Boston University professor, is friendly, earnest and keeps a giant inflatable T-rex named Henry in his office.

He sees this research not as the next frontier of coercive mind control but as another way to alleviate mental suffering, alongside medications and cognitive therapies.

“It’s amazing that we can do these things in contemporary neuroscience,” Ramirez said recently from his lab in Boston. “But the real-life, overarching goal of all of this is to restore health and well-being to an organism. … Memory manipulation is another antidote [that] can be part of our toolkit in the clinic.”

Memory is the reason Ramirez exists at all.

His father was once kidnapped at gunpoint by soldiers in his native El Salvador and falsely accused of being a left-wing guerrilla. (Their “evidence”: He had a beard.) He was spared execution when one of his captors took a second look at his face and recognized him as the generous schoolmate who used to share his lunch.

Both of Ramirez’s parents emigrated to the U.S. before his birth, and raised him and his older siblings in Boston. Ramirez got a bachelor’s in neuroscience from Boston University in 2010 and his doctorate from MIT in 2017. As a graduate student he joined the lab of Nobel laureate Susumu Tonegawa, where he was paired with a postdoc fellow named Xu Liu.

Both Ramirez and Liu were drawn to the study of memory as a possible therapeutic tool, and instantly hit it off as friends and lab partners.

Their first major breakthrough together came in 2012.

Three years earlier, a University of Toronto team identified the neurons that lit up when a mouse was exposed to a scary stimulus — in this case, a sound that earlier accompanied a shock. The Toronto researchers then injected the mice with a toxin that killed only those brain cells that lit up when they heard the sound.

The result: The treated mice no longer demonstrated a fear response when the sound was played. Essentially, the scientists had erased a specific memory.

If a memory could be deleted in the lab, Ramirez and Liu reasoned, one could be implanted.

For their experiment, the pair identified brain cells in a mouse hippocampus that activated when the animal received a startling shock. Then they took the mouse out of the enclosure where the shock occurred and placed it in a new box with no sights or other sensory cues associated with the memory of its old environment. Next, using millisecond-long pulses of light, they activated those same brain cells — without the physical shock of the earlier stimulus.

The mouse acted exactly as it had when the shock happened, even though no shock occurred.

You can’t interview a mouse about its memories. Researchers base their conclusions on the animal’s behavior. And in this case, it appeared that they’d turned a memory on.

“It just blew everyone away,” said Sheena Josselyn, a University of Toronto neuroscientist who led the 2009 work on erasing fear memories. “When you can do those sorts of things to memories, you know you have found the neural basis of a memory.”

In 2013, Ramirez and Liu set a mouse loose in a box — let’s call it, as Ramirez does in his book, Box A — and took note of the brain cells that activated as it explored the environment.

They then scooped it up and placed it in a second box, Box B. With minuscule pulses of light, they reactivated the cells that lit up in Box A, triggering a memory of that earlier environment as it explored the new one. At the same time, they gave the mouse a shock.

When they put the mouse back in Box A, a place where it had never been harmed, it froze in fear.

The mouse’s negative memory of being shocked in Box B had, essentially, been remapped to what was previously a neutral memory of Box A. The scientists had created a false memory, another seminal feat.

For their final project together, they put a mouse in an enclosure with other mice and took note of the neurons that fired as it responded positively to the social interaction.

Then they moved that mouse to a smaller cage than usual, where it was alone.

At first, this rodent equivalent of downsizing dimmed the mouse’s mood.

Given the choice between plain and sugary water, healthy mice prefer the latter. But when stressed or depressed, mice show no preference. That’s how Ramirez and Liu’s lonely mouse acted initially.

But when the scientists activated neurons associated with the memory of hanging out with other mice, the mouse’s behavior suddenly changed. It enthusiastically slurped the sweet water. Remembering better times had changed its behavior to resemble that of a healthy mouse.

The paper was published in 2015 in the prestigious journal Nature. But unlike their past shared achievements, this one couldn’t be celebrated together. As it was going through the review process, Liu died suddenly at the age of 37.

Grief, Ramirez writes, is not so different from memory: “Both endure across the entire lifespan, forever changing us, helping us to decide what matters most.”

Ramirez, now 37, opened his own lab at Boston University in 2017. In the years since, memory researchers have made impressive strides: restoring memories lost to amnesia, activating a memory while suppressing the emotions attached to it, detaching the emotional reaction to one memory and attaching it to another. The tools now exist to erase whole events and corresponding emotions from mouse brains, or to artificially jump-start memories and all the feelings that go with them.

But there is no expectation in the research community that laser-wielding doctors will one day artificially reshape human patients’ memories.

For one, these experiments are possible only with mice that have been genetically modified to have brain cells that light up when exposed to lasers. Genetically altering a human in this manner, researchers interviewed for this story said, is neither ethical nor practical.

It’s also not necessary.

“We don’t need to generate technophobic fears of a digital future where our memories will be distorted — our memories can already be distorted very effectively by nondigital means,” memory scientists Ciara Greene and Gillian Murphy wrote in “Memory Lane: The Perfectly Imperfect Ways We Remember,” published earlier this year.

Humans are suggestible creatures with extremely pliable memories. Armed with little more than a few leading questions, researchers have found that most humans can be easily manipulated into believing that they did or saw something they didn’t. We don’t need lasers to activate our memories, which can be summoned at will or triggered by any number of sensory cues, or to edit their contents, which our brains do constantly without any conscious input from us.

The real goal of research like his, Ramirez said, is to establish the biological mechanisms of memory and apply that knowledge to noninvasive therapies.

If researchers understand exactly how to retrieve a memory from a mouse hippocampus that brain damage has rendered inaccessible, for example, that information could be the basis for a drug that helps preserve or strengthen certain types of memory in people suffering from dementia or other cognitive disorders.

Understanding how an animal brain encodes memories and the emotional responses they evoke could lead to better cognitive therapies for post-traumatic stress disorder.

The obvious dark side of this line of research is that someone who understands how to boost well-being through memory manipulation could just as easily use the same knowledge for pernicious ends.

“The idea of artificially changing our own memories might elicit uneasy feelings of a dystopic future where relationships are erased, identities are replaced, and governmental powers implant thoughts in our heads to mind-control society,” Ramirez writes in his book. But, he said, any tool in existence can be used to harm or help, and he’d rather make well-intended progress than none at all.

“The idea of memory manipulation, to me, makes sense if we have an ethically bounded goal, and that ethically bounded goal is to restore health and nourish human well being,” he said. “Exercise is an antidote for the brain, and social enrichment is an antidote [and] a good night’s sleep is an antidote. What if toggling with memories in a therapeutic manner can also be an antidote? Then we’re in business.”

Fueled by a new viral strain, flu is hitting California early — and doctors are warning they expect the season may be particularly tough on young children.

Concentrations of flu detected in wastewater have surged in the San Francisco Bay Area, and the test positivity rate is rising in Los Angeles County and Orange County, according to state and county data. Hospitalizations and emergency room visits for flu are also rising in L.A. and Orange counties.

“We are at the point now where we’re starting to see a sharp rise in flu cases. This is a few weeks earlier than we usually experience, but very much akin to what was seen in the Southern Hemisphere’s experience with flu during their winter,” said Dr. Elizabeth Hudson, regional physician director of infectious diseases at Kaiser Permanente Southern California.

At Kaiser, flu cases are primarily being seen in clinics so far, but hospitalizations typically rise after Christmas. “We expect to see the same this year, too,” Hudson said.

“The number of cases appears to be higher at an earlier time in the usual flu season than we’ve seen in years past,” she added.

Flu levels are high in San Francisco’s sewage as well as in wastewater across San José, Sunnyvale and Palo Alto, according to WastewaterSCAN and the Santa Clara County Public Health Department.

One area of concern this winter has been the rise of a relatively new flu subvariant, known as H3N2 Flu A subclade K, which appeared toward the end of summer. That was months after officials decided which strains this fall’s flu vaccine would target.

Subclade K “is causing an active, early flu season, with more cases occurring in some countries within the Northern Hemisphere,” the California Department of Public Health said.

It remains unclear whether subclade K will reduce the efficacy of this year’s flu shot. Data recently released in Britain showed this season’s vaccines were 70% to 75% effective against hospitalization for children from the flu, and 30% to 40% effective in adults, which is within expectations, the U.S. Centers for Disease Control and Prevention noted.

This suggests “that influenza vaccination remains an effective tool in preventing influenza-related hospitalizations this season,” according to the agency.

However, the intended effectiveness of the flu vaccine against symptomatic disease caused by the new subvariant remains uncertain, the World Health Organization said.

Overall, flu rates in L.A. County remain relatively low, but are on the rise. Across California, flu hospitalizations are likewise low but increasing.

On a national level, severity indicators remain low, according to the CDC.

But the experience in other nations have led some experts to worry another severe flu season could be on deck for California.

Australia’s flu season, which is seasonally opposite from California’s, came far earlier than usual, hit with record strength and was particularly hard on the nation’s children.

Japan, Taiwan and Britain have also reported early spikes to their flu seasons.

“Whether or not this season will be more severe, only time will tell,” Hudson said of California. “We know that we have a mutation … which may make the flu vaccine work less well. But the vaccine still offers excellent protection against hospitalization and death, even with the mutated strain in circulation.”

Based on what happened in the Southern Hemisphere — particularly in Australia — “we are expecting this season to have a disproportionate impact on children under the age of 10,” Hudson said.

Already, three flu-associated pediatric deaths have been reported nationally this season, including two Friday.

During the flu season that ended in September, 280 children died from flu — the most since the swine flu pandemic season of 2009-10.

Overall, the 2024-25 season was considered the worst flu season since 2017-18, and hit adults hard as well. At least 38,000 people died from the flu last season, health authorities estimate.

Only a little more than half of the children who died from flu had an underlying medical condition, and 89% of those who died were not vaccinated, according to the CDC’s National Center for Immunization and Respiratory Diseases.

Among the children who died from flu last season, the most common complications experienced before death were shock or sepsis, pneumonia, acute respiratory distress syndrome, seizures and damage to the brain.

Early diagnosis of flu can help stave off the worst by giving those who are sick time to take antiviral medications like Tamiflu. Three out of five children who died from the flu during the 2024-25 season never received antiviral medication.

Emergency warning signs of flu complications in children include trouble breathing; bluish lips or face; ribs pulling in with each breath; chest pain; severe muscle pain, in which a child may refuse to walk; dehydration, signs of which include no urine for eight hours or no tears when crying; seizures; fevers above 104 degrees that are not controlled by medication; fever or cough that improve but return or worsen; and any fever in newborns younger than 12 weeks.

Since the official start of the respiratory virus season Oct. 1, the CDC estimates there have been at least 1,900 flu-related deaths, 49,000 hospitalizations and 4.6 million illnesses nationwide.

Doctors have been urging everybody to get the flu vaccine — the CDC recommends it for everyone age 6 months and up.

But vaccination rates have been lagging. Among children age 6 months to 17 years, an estimated 40.8% had been vaccinated as of the first week of December, according to the National Immunization Survey. In the last season before the COVID-19 pandemic, flu vaccination rates were notably higher by this time of year, at 51.2%.

At the end of last flu season, only 49.8% of children and teens had been vaccinated, the survey estimated, down from the 62.4% who had gotten their shots by the end of the 2019-20 flu season.

The decline in flu vaccinations has been seen locally, too. “Notably, fewer influenza vaccines have been administered this year compared to the same period last year,” the Orange County Health Care Agency said.

It takes about two weeks for protection to build, but getting vaccinated as soon as possible before travel or seeing friends and family “helps keep you and your loved ones safer,” the L.A. County Department of Public Health said in a statement to The Times.

California milk cartons may lose their coveted recycling symbol, the one with the chasing arrows, potentially threatening the existence of the ubiquitous beverage containers.

In a letter Dec. 15, Waste Management, one of the nation’s largest waste companies, told the state the company would no longer sort cartons out of the waste stream for recycling at its Sacramento facility. Instead, it will send the milk- and food-encrusted packaging to the landfill.

Marcus Nettz, Waste Management’s director of recycling for Northern California and Nevada, cited concerns from buyers and overseas regulators that cartons — even in small amounts — could contaminate valuable material, such as paper, leading them to reject the imports.

The company decision means the number of Californians with access to beverage carton recycling falls below the threshold in the state’s “Truth in Recycling” law, or Senate Bill 343.

And according to the law, that means the label has to come off.

The recycling label is critical for product and packaging companies to keep selling cartons in California as the state’s single-use packaging law goes fully into effect. That law, Senate Bill 54, calls for all single-use packaging to be recyclable or compostable by 2032. If it isn’t, it can’t be sold or distributed in the state.

The labels also provide a feel-good marketing symbol suggesting to consumers the cartons won’t end up in a landfill when they’re discarded, or find their way into the ocean where plastic debris is a large and growing problem.

On Tuesday, the state agency in charge of waste, CalRecycle, acknowledged Waste Management’s change.

In updated guidelines for the Truth in Recycling law, recycling rates for carton material have fallen below the state threshold.

It’s a setback for carton manufacturers and their customers, including soup- and juice-makers. Their trade group, the National Carton Council, has been lobbying the state, providing evidence that Waste Management’s Sacramento Recycling and Transfer Station successfully combines cartons with mixed paper and ships it to Malaysia and other Asian countries including Vietnam, proving that there is a market. The Carton Council persuaded CalRecycle to reverse a decision it made earlier this year that beverage cartons did not meet the recycling requirements of the Truth in Recycling law.

Brendon Holland, a spokesman for the trade group, said in an email that his organization is aware of Waste Management’s decision, but its understanding is that the company will now sort the cartons into their own dedicated waste stream “once a local end market is available.”

He added that even with “this temporary local adjustment,” food and beverage cartons are collected and sorted in most of California, and said this is just a “temporary end market adjustment — not a long-term shift away from historical momentum.”

In 2022, Malaysia and Vietnam banned imports of mixed paper bales — which include colored paper, newspapers, magazines and other paper products — from the U.S. because they were so often contaminated with non-paper products and plastic, such as beverage cartons. Waste Management told The Times on Dec. 5 that it has a “Certificate of Approval” by Malaysia’s customs agency to export “sorted paper material.” CalRecycle said it has no regulatory authority on “what materials may or may not be exported.”

Adding the Sacramento facility to the list of waste companies that were recycling cartons meant that the threshold required by the state had been met: More than 60% of the state’s counties had access to carton recycling.

At the time, CalRecycle’s decision to give the recycling stamp to beverage cartons was controversial. Many in the environmental, anti-plastic and no-waste sectors saw it as a sign that CalRecycle was doing the bidding of the plastic and packaging industry, as opposed to trying to rid the state of non-recyclable, polluting waste — which is not only required by law, but is something state Atty. Gen. Rob Bonta is investigating.

Others said it was a sign that the Truth in Recycling law was working: Markets were being discovered and in some cases, created, to provide recycling.

“Recyclability isn’t static, it depends on a complicated system of sorting, transportation, processing, and, ultimately, manufacturers buying the recycled material to make a new product,” said Nick Lapis, director of advocacy for Californians Against Waste.

He said this new information, which will likely remove the recycling label from the cartons, also underscores the effectiveness of the law.

“By prohibiting recyclability claims on products that don’t get recycled, SB 343 doesn’t just protect consumers. It forces manufacturers to either use recyclable materials or come to the table to work with recyclers, local governments and policymakers to develop widespread sustainable and resilient markets,” he said.

Beverage and food cartons — despite their papery appearance — are composed of layers of paper, plastic and sometimes aluminum. The sandwiched blend extends product shelf life, making it attractive to food and beverage companies.

But the companies and municipalities that receive cartons as waste say the packaging is problematic. They say recycling markets for the material are few and far between.

California, with its roughly 40 million residents, has some of the strictest waste laws in the nation. In 1989, the state passed legislation requiring cities, towns and municipalities to divert at least 50% of their residential waste away from landfills. The idea was to incentivize recycling and reuse. However an increasing number of products have since entered the commercial market and waste stream — such as single use plastics, polystyrene and beverage cartons — that have limited (if any) recycling potential, can’t be reused, and are growing in number every year.

Fines for municipalities that fail to achieve the required diversion rates can run $10,000 a day.

As a result, garbage haulers often look for creative ways to deal with the waste, including shipping trash products overseas or across the border. For years, China was the primary destination for California’s plastic, contaminated paper and other waste. But in 2018, China closed its doors to foreign garbage, so U.S. exporters began dumping their waste in smaller southeast Asian countries, including Malaysia and Vietnam.

They too have now tried to close the doors to foreign trash as reports of polluted waterways, chokingly toxic air, and illness grows — and as they struggle with inadequate infrastructure to deal with their own domestic waste.

Jan Dell, the founder and CEO of Last Beach Cleanup, released a report with the Basel Action Network, an anti-plastic organization, earlier this month showing that the Sacramento facility and other California waste companies were sending bales of carton-contaminated paper to Malaysia, Vietnam and other Asian nations.

According to export data, public records searches and photographic evidence collected by Dell and her co-authors at the Basel Action Network, more than 117,000 tons or 4,126 shipping containers worth of mixed paper bales were sent by California waste companies to Malaysia between January and July of this year.

Dell said these exports violate international law. A spokesman for Waste Management said the material they were sending was not illegal — and that they had received approval from Malaysia.

However, the Dec. 15 letter suggests they were receiving more pushback from their export markets than they’d previously disclosed.

“While certain end users maintain … that paper mills are able to process and recycle cartons,” some of them “have also shared concerns … that the inclusion of cartons … may result in rejection,” wrote Nettz.

Dell said she was “pleased” that Waste Management “stopped the illegal sortation of cartons into mixed paper bales. Now we ask them and other waste companies to stop illegally exporting mixed paper waste to countries that have banned it.”