We spend our whole lives collecting memories. To many of us, these are far more precious than any of our material possessions. Perhaps this is why diseases like Alzheimer’s that rob us of our memories feel especially tragic and frightening.
There’s a lot of brilliant research being done on ways to prevent Alzheimer’s disease or to halt its progress, and each day the field is making great strides toward this goal. Yet many scientists are reluctant to address the elephant in the room: what about the memories that are already gone? Will we ever be able to bring back a lifetime of precious memories to a patient who’s forgotten them all? It’s a difficult question, one that no one can predict with absolute certainty, but here I’ll attempt to describe where our current research stands and the obstacles we must overcome if we’re ever to achieve this goal.
The Hunt for the Engram
Here’s a deceptively simple question for you: what is a memory? You might think you know the answer. Of course, memories are our way of looking pack on our past, the images you recall from your wedding day, the mouthwatering smell of your grandma’s brownies, the lyrics to your favorite song. But what are they really? Do memories exist in physical space, governed by simple chemical reactions like the rest of our bodies? Or are they more ethereal, an untouchable something that’s a part of us yet separate from our physical form? These questions have been a source of philosophical debate for centuries, going back to the time of Plato and Aristotle.
In the early 1900s, neuroscientist Richard Semon hypothesized that our brains undergo an enduring physical or chemical change whenever we form a new memory. He coined a new term, “engram,” to describe this physical manifestation of memory. The nature of the engram was a topic of considerable debate. Some, like William James, believed that each memory is stored within a single neuron. “Every brain-cell has its own individual consciousness, which no other cell knows anything about,” James famously wrote. According to this theory, there’s a cell for all your memories of your grandmother, a cell for memories of macaroni and cheese, a cell for memories of the color blue, and so on. Others took a more holistic approach to memory theory. They believed that memories were stored as a pattern of activity within a particular group of neurons, rather than in a single neuron.
For nearly a century, neuroscience lacked the proper tools to resolve this debate. Finally, in 2007, a revolutionary paper was published in the journal Science. The researchers in the study genetically engineered mice so that any neurons that became activated while learning a new behavior were permanently “tagged.” The researchers observed a particular group of neurons that were activated after learning the new behavior, but not in control mice that received no training. When the mice were exposed once again to the training, causing them to recall their previous memory, the same group of neurons became active. This was seen as the first proof that engrams existed within the brain and could be reactivated when recalling memories.
Subsequent research provided more concrete evidence that these neurons were indeed an engram. In one study, researchers placed rats in a cage where they received a foot shock. Normally, if the rats were placed in that cage again, they would remember the foot shock and display signs of fear such as “freezing.” However, when the researchers selectively destroyed the cells within that engram, the rats seemed to forget the memory and did not freeze when placed back in the cage.
The next breakthrough came in 2011 with the invention of optogenetics, which allows us manipulate the activity of individual neurons without destroying them. Using this technique, it was shown that activating the fear engram caused the rats to freeze, even if they weren’t in the cage where the shock happened. Conversely, inactivating the engram blocked the memory, so that the rats would not freeze when placed back in the cage where they had been shocked.
These studies show that engrams are probably formed by the activity of multiple neurons, casting doubt on James’s theory of one memory per cell. However, the mystery of the engram is not yet completely resolved. Today we are still trying to figure out what kinds of changes are occurring at the cellular and molecular level within these neurons when a memory is formed.
Alzheimer’s Disease: Memory Destroyer or Memory Blocker?
Now that we have a better understanding of what exactly a memory is, I can return to my original question: can the memories lost by Alzheimer’s patients ever be rescued? The answer to this question depends on what is actually happening to the memories in this disease. If the engrams encoding these memories are destroyed, it seems unlikely that we could ever rebuild them. However, there is a more hopeful possibility. What if the memories are still present in the brain, but Alzheimer’s simply prevents us from accessing them?
This is essentially a question of what aspect of memory is lost in Alzheimer’s disease. Memory formation is divided into three stages:
- Encoding. Your brain translates the raw data obtained through your senses into a pattern of neuronal activity, which forms a short-term memory.
- Storage. If the memory is deemed by your brain to be important, it is transferred into long-term storage.
- Retrieval. Whenever you recall that memory, your brain accesses its stored engram and allows you to remember.
It is still not entirely clear which stage of memory is disrupted in Alzheimer’s disease. Are the patients simply unable to encode new memories? Can they form memories but not transfer them to long-term storage? Or maybe the memories are there but simply can’t be retrieved?
A study published last year in Nature took a step toward addressing this dilemma. Researchers used a mouse model of Alzheimer’s disease and selected mice that were 7 months old. This age group is a representation of early Alzheimer’s disease, when short-term memory is normal while long-term memory experiences deficits. Using the same foot-shock protocol I described before, they saw that the mice showed freezing behavior 1 hour after the training but had forgotten 24 hours later.
Next, the researchers used optogenetics to activate the engram associated with the fear memory 24 hours after the training period. This time, the mice showed freezing behavior, indicating that the memory had been recalled using the light stimulation. This result is encouraging because it suggests that, at least in the early stages of Alzheimer’s disease, memories can be consolidated into long-term storage, and the problem is simply an inability to retrieve them.
Will We Ever Rescue Lost Memories?
Our understanding of Alzheimer’s disease, and of the nature of memory itself, remain incomplete. The study I’ve just described does not address whether activating an engram can retrieve a memory formed months or even years ago. It also does not explore whether mice in later stages of the disease can have their memories revived in this way. Furthermore, at present we do not have a noninvasive method for activating particular engrams in humans.
However, it does provide at least of glimmer of hope. If it is the case that the memories of Alzheimer’s patients are still present in their brains, then the possibility of restoring the memories becomes much more feasible than if they were completely destroyed. It also suggests that emerging therapies like deep brain stimulation could one day be used to help restore memory.
Sometimes, people with Alzheimer’s disease will seem to momentarily remember something they had previously forgotten. They are plenty of videos of this online, such as this one, where a man suffers from severe dementia and yet can inexplicably recall the lyrics to his favorite songs; or this one, where a woman with Alzheimer’s recognizes her daughter after having forgotten her. Perhaps those other memories are not gone but just inaccessible. Perhaps the memories are still buried deep inside their brains, just waiting for the right stimulus to bring them back to the surface.