In his magnum opus, In Search of Lost Time, Marcel Proust wrote that “remembrance of things past is not necessarily the remembrance of things as they were.” That elegant line speaks to a simple truth: There are things you remember, and there are things you remember well. Even if you can recall a past event, your memories will vary considerably in how much detail they contain, and how correct those details are.

In an elegant experiment, a team of neuroscientists led by Jon Simons at the University of Cambridge have shown that these aspects of our memories—our success at recalling them, their precision, and their vividness—depend on three different parts of the brain.

One of these is the hippocampus—the little, seahorse-shaped area in the middle of the brain that has been most famously associated with memory. In 1953, a neurosurgeon named William Beecher Scoville removed the hippocampus from an epileptic patient named Henry Molaison, robbing him of many past memories of events and preventing him from making any new ones. Molaison became known as Patient HM, and his mental woes enshrined the hippocampus as “the seat of memory.”

It doesn’t act alone. Through brain-scanning studies, scientists have uncovered a network of regions involved in remembering what happened to us—the hippocampus, yes, but also regions further back in the brain, like the angular gyrus and precuneus. When volunteers try to bring up old memories, these areas all start buzzing together.  

But that doesn't necessarily mean these regions are all doing the same thing. If people suffer damage to the hippocampus, as Henry Molaison did, they typically can’t remember anything at all. But in 2010, Simons found that people who suffer damage further back in the brain experience subtler problems—they’ll remember things well enough, but not confidently so. A few years later, he used magnetic fields to temporarily disrupt these areas in healthy volunteers and found the same effect: The recruits remembered accurately, but not confidently. That was a strong hint that different parts of the memory experience are governed by different parts of the brain.  

To explore this idea, Simons’s team members Franziska Richter and Rose Cooper devised a new test. They asked volunteers to memorize pictures in which distinctive objects—say, a purple umbrella or an orange flashlight—floated in front of various background scenes, from the Taj Mahal to a lava field. The images disappeared and after a short pause, the recruits had to say how vividly they remembered the scene. Then, they had to recreate the images, adding whichever objects they remembered back to an empty background, and trying to match their color, orientation, and location. And they did all of this inside an fMRI scanner, which gave Richter and Cooper a view inside their heads.

Richter and Cooper found that anytime the volunteers remembered objects, their hippocampus was more active. When their memories were precise, and they nailed the objects’ features, their angular gyrus was abuzz with activity. And when they claimed that their memories were especially vivid, it was their precuneus that lit up in the scanners. Three areas, three different aspects of memory.

This might seem like a deeply counter-intuitive idea. We’re very used to thinking of our memory as a kind of storage vault, where bits of information are recorded and filed away for later perusal. But it’s not like that at all.

Instead, many neuroscientists believe that “our memories are reconstructed whenever we try to remember something,” says Cooper. We build them from scratch each and every time, drawing on bits of information scattered in relevant corners of the brain. Sights come in from the visual centers, sounds from the auditory centers, and all of it gets re-integrated in areas like the hippocampus. Memory isn’t just an act of retrieval, but of reconstruction.

The team suspect that the hippocampus acts as a gatekeeper in this process. If you’re trying to remember something, and activity in your hippocampus builds over a certain threshold, “it indicates to other areas of the brain that there’s a representation there that can be reconstructed,” says Cooper. The angular gyrus then does the hard work of actually piecing the memory’s details together, which is why its activity correlates with the precision of the final product.

What about the precuneus? “We found that the most difficult region to get our head around,” says Cooper. It seems to govern the subjective side of memory, and “the degree to which participants perceived that they could remember an event, regardless of whether they could or not.” So, very roughly speaking, the hippocampus kicks the process of, the angular gyrus does the heavy lifting, and the precuneus gives us that vibrant, first-person sense of actually remembering something.

And of course, these areas are heavily interconnected, so that activity in one affects the function of the others. By their powers combined, we remember.

“I like this study a lot,” says Elizabeth Chua from Brooklyn College of the City University of New York. “One challenge with studying memory success is that there are many factors that can contribute to success.” By teasing those factors apart in a single study, the team could more effectively work out what different brain regions that have been linked to memory actually do.

It’s further evidence that “memory retrieval is not all or nothing,” adds Maureen Ritchey from Boston College. “We can remember some details of an experience but not others, and the details we do recall can be quite specific or disappointingly vague.” And by studying how these subtleties are encoded in the brains of healthy volunteers, Simons’ team might be well-placed to understand what happens in people whose memories are failing.

There are several neurological and developmental disorders where people don’t lose their memories outright, but show subtler problems. “It's possible that the kind of task [the team used] could be more sensitive to changes in memory than other standard memory tasks,” adds Ritchey. And perhaps doctors could use those changes to spot problems at an early stage.