Novel approaches have reinvigorated the search for the elusive memory engram—and energized attempts to control it.

What is a memory? Can it be purposefully erased, implanted, or altered? Such musings have long captured the public imagination, inspiring cinematic tales of memory manipulation, ranging from the political thriller The Manchurian Candidate, to the sci-fi classic Total Recall. In the quirky romance Eternal Sunshine of the Spotless Mind, memory technicians erase painful recollections of failed relationships. In Inception, memory mercenaries slip into a businessman’s dreams to surreptitiously plant an idea.

Can memory be purposefully erased, implanted, or altered? Numerous research groups are keen to find out. Image courtesy of Dave Cutler (artist).

Memory-tweaking experiments in the laboratory, although decidedly more prosaic, are nevertheless yielding dramatic results. In 2013, Susumu Tonegawa at the Massachusetts Institute of Technology in Cambridge, and his colleagues stimulated certain neurons in the hippocampus of mice, demonstrating that the researchers could plant a fearful memory of being shocked on the foot in an environment where no shock had actually occurred (1). This work was a powerful demonstration of the access and control that neuroscientists are gaining over the once-elusive engram, the physical embodiment of a memory in the brain.

Ever since the idea was first formalized in the early 1900s, researchers have struggled to capture and elucidate the engram. Neuroscientists theorized that memory would leave behind some physical imprint, a structural, electrical, or biochemical change by which networks of neurons might retain information for hours, months, or years. Researchers collected pieces of this puzzle over the years, identifying molecules, genes, and cellular processes that are important for learning and memory. Since discoveries in the 1960s and 1970s, many have suggested that memory is supported by long-term potentiation, an effect by which neurons that fire together are able to strengthen their connections.

But whereas many of the low-level mechanisms of memory attracted thorough examination, it would take a convergence of technological advances over the past decade to enable a more holistic, high-level approach to the engram. Now, for the first time, scientists are beginning to identify, label, and even control the specific networks of neurons that help hold memories. Such work is uncovering new insights into where and how memories are stored, and how they might one day be tweaked in humans to treat disorders such as dementia and addiction.

“We’re energizing some very old questions about memory,” says Sheena Josselyn, a neuroscientist at the Hospital for Sick Children in Toronto, who presented a lecture on the engram at last year’s Society for Neuroscience meeting in Chicago. “The purpose of the brain is to allow you to not just breathe, but to inform your future choices by recollecting your past. And this is really getting to the heart of what brains do.”

Chasing the Engram Since it was first introduced, the engram concept has taken on a range of connotations from the psychological to the biological. German scientist Richard Semon, who coined the term “engram” in the first decade of the 20th century, theorized that experiences and external stimuli created a form of memory imprint in the brain that could later be reawakened to recall the memory. Many modern biologists believe the engram involves the neuronal circuits that hold a specific memory and that can recall that memory when stimulated. In principle, the engram forms when learning activates certain neurons and induces long-lasting changes—altered gene expression or strengthened connections via long-term potentiation, for example—that encode memory information. But tracking down the critical neurons that hold this information has proven exceedingly difficult. In the early 20th century, American neuroscientist Karl Lashley famously spent 30 years trying to prove the engram’s existence. Lashley trained rats on various tasks, such as running a maze, then systematically damaged parts of the cerebral cortex looking for where the memory of the task was stored. He never pinpointed specific brain regions critical for memory, although large injuries appeared to hinder the animals’ performances. By 1950, Lashley gave up his search, concluding that memories were not stored in any cells in particular, but rather diffusely throughout the brain. In recent decades, however, researchers have picked up the trail again and succeeded in narrowing the search for the elusive engram. It turns out that memories are indeed distributed across the brain, but specific types or components of memory can be highly localized. (Unfortunately for Lashley, many of these regions lie outside of the cerebral cortex.) The hippocampus plays a key role in spatial memory, for example, and the amygdala mediates fearful memories, whereas the cerebellum is essential for certain types of Pavlovian conditioning (2). Now, researchers have zeroed in on specific networks of neurons that appear to hold memories, or at least key components of memories. In the case of memories involving simple Pavlovian-conditioned fear, only about 10–30% of neurons in certain brain areas are thought to participate in the engram, according to some estimates in animals (3, 4). But modern molecular genetic techniques have enabled scientists to begin finding these neurons. Researchers can now activate engineered genes inside select cells, or in response to certain cellular conditions. When linked to genes that are induced by neural activity, for example, these fluorescent markers and other constructs enable researchers to identify, control, or alter neurons involved in memory. With optogenetics, scientists can use lights to turn on and off specific neurons, engineered to express light-sensitive proteins, thus artificially stimulating and even distorting memories in mice and other experimental animals. Together, the data from this latest wave of engram research are helping scientists dig deeper than ever into how memories work, how they fall away, and how they might be amenable to alterations. Researchers used a mouse with a head-mounted fluorescent microscope to see the hippocampal cells involved in linking two memories. Image courtesy of Alcino Silva and Denise Cai.

Touching Memories To achieve memory “inception” in their 2013 study, Tonegawa’s group relied on a technique they had published one year earlier, which allowed them to label and control neurons involved in learning (5). The group developed mice that would express a light-sensitive protein in neurons of the dentate gyrus (a part of the hippocampus) after they fired, leaving inactive neurons unaffected. Most of the time, the mice consumed a drug that silenced this labeling mechanism. But the researchers withdrew the drug for a brief window while the animals explored a cage, thus tagging active neurons with light-sensitive proteins. Next, the scientists restored the drug, turning off labeling, as they moved the animals to a second cage. There, electrical shocks were paired repeatedly with light stimulation of the neurons that had encoded the experience of the first cage. When the mice were returned to the first (safe) chamber, the animals automatically froze with fear induced in the second cage. By artificially activating neurons that encoded the first cage while delivering foot shocks in the second cage, Tonegawa’s team had attached a false fearful memory to the previously neutral first cage. Plus, fear responses to the actual shock-associated cage appeared slightly weaker in “incepted” mice than in control animals, suggesting neural competition between the real and false memories. Control experiments showed that the incepted fear was specific to the engram-stimulated cage, and did not extend to a separate neutral cage. “Those studies were so important because they had a huge ‘wow’ factor,” says Alcino Silva, a neuroscientist at the University of California, Los Angeles. “They captured the imaginations of so many people.” Others have pushed the boundaries of memory manipulation by leveraging a key memory protein called CREB (cyclic adenosine monophosphate response element-binding protein). Research by Josselyn and others suggests that individual neurons with relatively higher CREB levels at the time of learning are more excitable than other cells, and are therefore more likely to become part of the resulting memory engram (6⇓–8). In a 2009 study in mice, Josselyn’s group used a virus to boost CREB production in about 15% of neurons in the lateral amygdala—an area involved in auditory fear memory—shortly before training the animals to fear a musical tone that was repeatedly paired with foot shocks (8). The infected neurons that overexpressed CREB were three times more likely than their unaffected neighbors to respond to the experience and become part of the memory engram. Next, the researchers delivered a toxin that targeted the CREB-overexpressing cells. By deleting this subset of the engram cells, the researchers effectively wiped out the animals’ fear of the tone. A few months later, Silva’s group demonstrated electrical changes in the CREB-overproducing neurons that could help explain their role in encoding memories (7). The researchers found that the modified cells in the lateral amygdala fired more readily, and displayed greater-than-normal strengthening of electrical signaling following fear conditioning. Manipulating CREB and using optogenetics to control memories in mice is just the beginning, says Silva. “The concept of the engram is a tool, and it’s useful as long as it continues to show us new and interesting properties about memory,” he says.

Built by Association Silva’s studies, for example, have revealed that memories acquired closely in time tend to share many of the same engram neurons in the hippocampus. This effect could help explain why retrieving one memory (and reactivating engram neurons) often triggers other recollections from around the same time. In a study published in June, Silva’s group found that mice that learned to fear foot shocks in one cage also displayed fear in a cage that they had explored five hours before the fear training (9). At the neural level, imaging revealed that many of the same cells that responded to the first, neutral cage also responded to the second, shock-rigged cage, as if they were already warmed up and still primed to fire. This neural overlap and memory linking disappeared, however, for experiences separated by one week. And the linkage between even closely spaced memories seemed to disappear as animals aged, a finding that could help explain certain memory lapses in aging humans, Silva says. Neural excitability plays a key role in linking memories together in time, Silva and others have found. In middle-aged animals that no longer develop fear of the cage visited five hours before fear training, Silva’s group used a chemical and specially designed virus to boost the excitability of a small number of neurons before the exposure to the first cage. The ability of the mice to link the memories was restored when the researchers boosted excitability in the same group of neurons five hours later, just before placing the animals in the shock-rigged cage. By studying adult mice before these memory problems arise, Josselyn’s group has probed deeper into the mechanisms that normally allow closely spaced memories to be encoded by the same sets of neurons. In a study published in July, the team found that neurons compete against each other to become part of a given auditory fear engram (10). Those neurons that fire in response to an initial experience temporarily suppress the activity of neighboring neurons, thus increasing the chances that the same neurons will respond and become part of the engram for a second experience that follows soon afterward. Neurons in the amygdala (in blue) were activated by recall of two different memories (green and red signals). If the memories were formed six hours apart (Upper), they are stored in the same neurons, but not if memories formed 24 hours apart (Lower). Image courtesy of Asim Rashid and Sheena Josselyn.