Everyone wants a good memory but we do not often ask ourselves what memory is for. Let us begin our exploration of why memory is important and how it works with a little memory test: Do you remember what you learned in class last Wednesday? How about what you got for your last birthday? One of the greatest mysteries of the human mind is why we remember some things, but not others. Why is it that we might have perfectly vivid memories of what we did on vacation 2 years ago, yet we struggle to remember details from what we learned in class just last week? What happens in our brain that turns some moments into lifetime memories, while others are lost, forever forgotten?

How Does Memory Work?

It turns out that there is one brain structure that appears to be a ‘memory maker’. Without it, we cannot remember anything that has happened to us, even from only a few minutes ago, let alone from last summer. Indeed, there was a very famous case of a patient named Henry Molaison (often referred to as H.M.), who had damage to his brain and was completely unable to form new memories. This kind of conditions, in which a person cannot make new memories, is called anterograde amnesia. The brain structure that is needed to form these kinds of memories, and which was damaged in Henry’s brain, is the hippocampus, a seahorse shaped brain region buried deep inside your head between your ears, as shown in the Figure 1.

Perhaps what is even more amazing is that patients like Henry can lose their ability to create new memories, yet they are still perfectly capable of bringing to mind old memories. Moreover, Henry could hold a conversation, do math, maintain friendships and do almost everything normally … except build new memories. This is because other aspects of the way we perceive and think about the world remain intact after the hippocampus is damaged.

But other than the rare patients with specific damage to the hippocampus, how do we study how memories are formed in a healthy brain? In the lab, we can study how memories form in a person’s brain by using functional magnetic brain imaging (or fMRI for short). This tool allows us to measure which brain regions are most active during new learning.

We bring in healthy volunteers and ask them to play a “computer game” that tests their memory. The game is pretty easy from the perspective of the participant. They come in to the lab and we put them in the fMRI scanner while they are able to view visual images on a computer screen – much like watching a movie (sometimes we do show movies!). While they are watching images on the screen, we take pictures of their brains every few seconds. This gives us a changing measure of activity in the whole brain. Then, we take them out of the scanner and they go home. The next day, they come back to the lab and we test their memory for the previous day’s experiences. Once we know what each person remembered or did not remember, we are halfway there: now we can see if there was something special that happened in the brain the day before for things that people remembered. We do this by sorting brain activity from the first day into two piles – brain activity that happened when people later remembered the event and brain activity that happened when people later forgot the event. When we do this, we are able to clearly see that the hippocampus is much more active when people are forming memories! We can also put people in the fMRI scanner during the memory test, and we see that the hippocampus is again more active when people are remembering more details. Altogether, then, by simply showing people new things and getting them to form new memories in the laboratory, we can peer into the brain to learn about how it remembers.

Why Do We Remember Some Things Better than Others?

You might be wondering: if we all have a hippocampus whose job it is to help us build new memories, then why do we forget so many things? Of course, we cannot remember everything (and we probably would not want to!). Indeed, an important difference between our brain’s memory and a computer’s memory is that our brains filter our experiences. Some experiences turn into lasting memories, others decay over time and are forgotten, making room for new ones.

One interesting aspect of this process is that which memories stay and which are forgotten is not random. Our brain has several ways of making sure that the memories it holds on to are the particularly important ones to each of us: things that are essential for survival, such as food (have you ever forgotten which cupboard the cookies are in?) or things that we care about or enjoy, such as everyday events related to our family, friends, and interests.

One way that the brain makes sure to remember the events that are important to us is through specialized chemicals called neurotransmitters, which, when released, help build stronger memories. One such neurotransmitter is dopamine. Dopamine is released in the brain when something surprisingly good happens, such as walking into class and finding a box of chocolates on your desk. When dopamine is released, it strengthens memories. It does so by tagging these memories when they are created in a way that makes these memories slowly strengthen over time. This means that just as memories for unimportant things start fading away, memories for important things grow stronger over time. In the end, memories for these surprisingly good events can have greater influence over our behavior, making it easier to use these memories to get the things we really want or need.

A similar process takes place when especially bad things happen, such as eating something that tastes terrible, encountering a spider, or other frightening events. This kind of fear learning depends on a specialized part of the brain, the amygdala, which works together with the hippocampus to strengthen memories. As with good events, it is easy to understand why we would need to have strong memories for bad things that happen to us. After all, an important part of survival is learning how to find the chocolates and avoid the spiders.

If We Do Not Remember What Happened, How Can We Still Remember What to Do?

So far, we have discussed vivid memories for specific events or episodes, a kind of memory sometimes referred to as episodic memory. But past experiences – whether good or bad – can shape our behavior even when we do not have a vivid memory of the experiences themselves. This kind of unconscious influence of past experiences on behavior is often referred to as habit learning, and it exerts a greater influence on our behavior than most of us realize (and maybe greater than we would like). Have you ever reached for a cookie, only to realize that you probably should have had an apple, or found yourself playing a video game for hours when you meant to do your homework? These are just two examples of how habits – whether we like it or not – can shape our behavior.

Whereas episodic memory depends on the hippocampus, habit learning depends on a different part of the brain – the striatum, shown in the figure below. But similar to episodic memories, specific habits are not formed randomly. They are shaped by the outcomes of our actions and are strengthened with dopamine. You already know that when we encounter unexpectedly good outcomes, our brain releases dopamine. In addition to strengthening memories in the hippocampus, dopamine also strengthens habit learning in the striatum. Except that, in the striatum, dopamine reinforces memories for actions, instead of for episodes. This way, when something good happens, it not only strengthens memory for what happened, but also reinforces the action that led to it. If a particular action (e.g., opening the cookie box) repeatedly leads to a good outcome (enjoying the cookie), the repeated reinforcement biases our brain to take that same action again next time, even if we have no conscious memory of these events at all. This is why, sometimes, we do not remember what happened, but we remember what to do. Habit learning promotes behaviors that have been successful in the past. Moreover, behaviors that are habitual can happen quickly and automatically, so that they can adaptively guide our behavior even when we are distracted, stressed, or tired. This may be useless on an exam, but it is crucial for survival.

At the beginning of this article, we asked you to remember two things. Our guess is that most of you were probably able to remember what you got on your last birthday but struggled to remember what you learned in class last Wednesday. If you want to tip the balance ever so slightly and remember more of what you are learning in school, we put together a few suggestions (see Box 1). In the end, if you can find a way to turn everyday moments into something more personally relevant, more meaningful, and pair it with reward (did you say chocolate?) – you are helping your brain work for you!