Why We Remember

1

Where Is My Mind?

why we remember some things and forget others.

. . .

Maybe the reason my memory is so bad is that I always do at least two things at once. It’s easier to forget something you only half did or quarter did.

—Andy Warhol

Over your lifetime, you will be exposed to far more information than any organism could possibly store. According to one estimate, the average American is exposed to thirty-four gigabytes (or 11.8 hours’ worth) of information a day. With a near-constant stream of images, words, and sounds coming at us through our smartphones, the internet, books, radio, television, email, and social media, not to mention the countless experiences we have as we move through the physical world, it’s not surprising we don’t remember everything. On the contrary, it’s amazing that we remember anything. To forget is to be human. Yet, forgetting is one of the most puzzling and frustrating aspects of the human experience.

So it’s natural to ask, “Why do we remember some events and forget others?”

Not long ago, Nicole and I celebrated the thirtieth anniversary of the year we met. To mark the occasion, we pulled out old family videos that had been gathering dust over the years and had them digitized. I was particularly fascinated by the footage of our daughter Mira’s birthday parties. As we watched the videos of Mira growing up, I expected them to trigger a flood of memories. Instead, I discovered that almost all of them seemed new to me. I was the one shooting the videos, yet I did not have the experience of recollecting these parties as individual events—except for one.

For most of her early childhood, we organized Mira’s birthday parties at such places as the Sacramento Zoo, the local science museum, a gymnastics studio, or an indoor rock-climbing gym. These venues ensure that the kids can be entertained and contained, with a steady stream of food, sugary drinks, and activities provided during the two-hour reserved window. At these birthday parties, I would participate in the festivities, but for the most part I focused on documenting these precious moments so that Nicole and I could revisit them later.

The year Mira turned eight, I decided to try something different. When I was a kid, my brother, Ravi, and I celebrated our birthdays at home. We had a lot of fun, and our parents didn’t need to spend a lot of money. So, that year I followed my do-it-yourself punk-rock ethos and organized Mira’s party at our house. Anyone who has ever hosted a children’s birthday party knows the number one goal is to keep the kids busy. Mira was always into art, so I found a shop in a nearby town that provided premade cat-shaped ceramics that the kids could paint with glaze and later have fired to take home. Between the craft activity and the SpongeBob SquarePants piñata I had hung up in the backyard, I figured I had it covered.

I couldn’t have been more wrong. Roughly fifteen minutes into the activity, all the cats were painted. With hours left to fill before cake time, the children were getting restless, and I was beginning to panic. I herded the kids out to the backyard, where they lined up to take turns whacking a piñata that refused to burst. Eventually, I took matters into my own hands, got out a golf club from the garage, and bashed a hole in it. Candy went flying everywhere and the kids descended on that papier-mâché SpongeBob like a scene from The Walking Dead. I saw one kid launch herself like an Olympic gymnast across the yard to get to a Snickers Mini she’d spotted in the grass.

It was still too early to bring out the cake, so I came up with the bright idea of having them play tug-of-war with an old rope I found in the garage. It had rained the day before, and the kids kept slipping and sliding around on the muddy grass. I remember looking around the backyard—some of the kids were chasing one another around in a sugar frenzy, one or two were complaining about rope burn, and a couple were taking turns beating the SpongeBob carcass to death with the golf club—and thinking how quickly an eight-year-old’s birthday party can go from painting ceramics to Lord of the Flies. It was not my finest moment, but it is one I remember in excruciating detail.

Not all our experiences are of equal importance. Some are utterly unremarkable; others are moments we hope to treasure forever. Unfortunately, even priceless moments can sometimes slip through our fingers. At the time, I could have sworn I would vividly remember each of Mira’s parties, so why is it that this one stands out and the other birthday videos seem like reruns from a distant TV show?

How can an experience that feels so memorable while we’re living it ultimately be reduced to little more than a vague fragment of what transpired?

Although we tend to believe that we can and should remember anything we want, the reality is we are designed to forget, which is one of the most important lessons to be taken from the science of memory. As we will explore in this chapter, as long as we are mindful of how we remember and why we forget, we can make sure to create memories for our most important moments that will stick around.

making the right connections

The scientific study of memory as we know it today was pioneered in the late nineteenth century by German psychologist Hermann Ebbinghaus. A cautious and methodical researcher, Ebbinghaus concluded that, to understand memory, we must first be able to quantify it objectively. Rather than asking people subjective questions about events such as their kid’s birthday parties, Ebbinghaus developed a new approach to quantify learning and forgetting. And unlike modern psychologists, who have the luxury of enlisting college students to voluntarily participate in their studies, poor Ebbinghaus worked alone. Like a mad scientist in a Gothic horror novel, he subjected himself to mind-numbing experiments, in which he memorized thousands of meaningless three-letter words called trigrams, each consisting of a vowel sandwiched between two consonants. His idea was that he could measure memory by counting the number of trigrams—e.g., DAX, REN, VAB—he was able to successfully learn and retain.

We should pause a moment to appreciate the painstaking work that went into Ebbinghaus’s studies. In his 1885 treatise, On Memory: A Contribution to Experimental Psychology, he writes that he could only memorize sixty-four trigrams in each forty-five-minute session because “toward the end of this time exhaustion, headache, and other symptoms were often felt.” In the end, his Herculean efforts bore fruit, as his experiments revealed some of the most fundamental aspects about the way we learn and forget. One of his most important achievements was to construct a forgetting curve, allowing him to graph, for the first time, how quickly we forget information. Ebbinghaus discovered that only twenty minutes after memorizing a list of trigrams, he had forgotten nearly half of them. One day later, he had forgotten about two-thirds of what he originally learned. Although there are some caveats to Ebbinghaus’s findings, his bottom line holds: Much of what you are experiencing right now will be lost in less than a day. Why?

To answer this question, let’s begin by breaking down how a memory is formed in the first place. Every area of the human neocortex, the densely folded mass of gray tissue on the outside of the brain, consists of massive populations of neurons—86 billion, according to one estimate. To put this number into perspective, that’s more than ten times the human population of the earth. Neurons are the most basic working unit of the brain. These specialized cells are responsible for carrying messages to different areas of the brain about the sensory information we take in from the world. Everything we feel, see, hear, touch, and taste, every breath we take, every move we make (sorry, couldn’t help myself), happens because of communication between neurons. If you feel yourself falling in love, if you’re angry, or if you’re slightly hungry, that’s the outcome of neurons talking to one another. Neurons can also work in the background to handle important functions we’re not even aware of, such as keeping our hearts pumping. They even work while we’re sleeping, filling our heads with crazy dreams.

Neuroscientists are still working out exactly how all these neurons work together, but the knowledge we have so far is enough to build computer models that capture some of the basic principles that govern brain function. In essence, neurons function like a democracy. Just as one person has only one vote to influence the outcome of an election, a single neuron plays only a small role in any kind of neural computation. In a democracy, we form political alliances to advance our individual agendas, and neurons form similar alliances to get things done in the brain. The Canadian neuroscientist Donald Hebb, whose work was influential to our understanding of how neurons contribute to learning, called these alliances cell assemblies.

In neuroscience, as in politics, it’s all about having the right connections.

To get a better sense of how this works, let’s consider what happens as a newborn baby is exposed to human speech. Before a language is learned, babies can hear differences between sounds, but they don’t know how to parse those sounds in a linguistically meaningful way. Fortunately, from the moment we are born, our brains get to work making sense of what we are hearing, trying to break up a continuous stream of sound waves into discrete syllables. What the baby ultimately perceives will depend on an election taking place in areas of the brain that process speech sounds. Perhaps the baby hears a sound, but there is some noise in the room, so it’s unclear whether that sound was bath or path. Somewhere in the brain’s speech centers a large coalition of neurons casts votes for the sound bath, a smaller coalition votes for path, and an even smaller minority votes for other candidates. Within less than half a second, the vote is tallied, and ultimately the baby picks up that it is time for a bath.

Here’s where learning kicks in: In the aftermath of the election, the winning coalition works to strengthen its base. Neurons that only weakly supported the winning sound might need to be brought into the fold, and the ones that didn’t need to be purged. The connections between the neurons that supported bath are strengthened, and connections with neurons that voted for the wrong sound are weakened. But at other times, the baby might hear someone loudly say the word path. The connections between the neurons that supported path will be strengthened and become less connected from the neurons that voted for the wrong word. Through these postelection shake-ups, the parties become more polarized; neurons will become even more strongly affiliated with the assemblies they already supported and pull further away from the ones they were lukewarm about. That leads the elections to become more and more efficient, so that the outcome of an election becomes apparent early in the voting.

Children’s brains, in particular, are constantly in flux, reorganizing to optimize their perception of the environment. During their first few years, babies make dramatic progress at learning how to differentiate syllables, so that a continuous stream of sound can become sensible speech through the constant reorganization of connections between neurons. But as those neurons settle into coalitions that differentiate between the sounds the baby is hearing, they are becoming less sensitive to sound differences that don’t exist in that language. It’s as if the neurons are choosing between a small number of candidates based on a few key issues.

The baby’s ability to change connections in the neocortex in response to new experiences is called neural plasticity. The reduction in neural plasticity as we transition to adulthood is well-known, though the science has been a bit distorted by news articles and TV shows conveying a bleak message that our capability for plasticity slips away as we get older. This message has been exploited by companies selling products that purport to stave off the inevitable decline. It’s true that, past the age of twelve, the neural alliances formed around familiar sounds become more entrenched and it becomes harder to learn new kinds of syllables as quickly. This is why it can be more difficult to start learning Mandarin or Hindi in your forties than if you had been exposed to those languages as a child. Fortunately, adult brains still have plenty of plasticity without the need for any pills, powders, or supplements. The connections in your brain are constantly being reshaped with the goal of improving your perception, movement, and thinking as you gain more and more experiences. Moreover, as you go past simple perception (what we see, hear, touch, taste, and smell) and move into higher-order functions (e.g., judgment, evaluation, and problem-solving), the brain is remarkably plastic, and the neural elections are highly contested.

So, suppose you’ve spent a week in Delhi learning Hindi, and you’d like to ask for water at a restaurant. You memorized that word only an hour ago, but now you can’t find it. Unfortunately, until you become more proficient, many Hindi words might sound alike to you. The cell assembly for the word you’re looking for (paani) is not yet strongly connected, and a lot of neurons have divided loyalties, torn between competing possibilities. This is the same challenge we face when trying to remember more complex experiences such as my daughter’s well-organized birthday party at the Sacramento Zoo. To get to what we want to remember, we must find our way to the right coalitions of neurons, but in many cases, there is an intense competition between the coalition that has the memory you’re looking for and coalitions representing other memories you don’t need at that moment. Sometimes, the competition isn’t so bad, but if you have a lot of coalitions representing similar memories, the battles can be intense, and there might not be any clear winner. In memory research, this competition between different memories is called interference, and interference is the culprit behind a lot of our everyday forgetting. The key to escaping interference is to form memories that can fight off the competition, and fortunately, we have the capability to make that happen.