Introduction

You go to school every day, but you probably remember some school days better than others. What makes 1 day more memorable, and how does your brain save that experience to be remembered for a long time? Much like a computer has a “save” button for when we want to store a picture or a movie, our brains do, too! Although we usually forget most of our daily experiences, if something funny, scary, or strange happens, our brains have a way of “clicking save” on that event so we can remember it later. This ability is essential for remembering important things that happen in our lives that might influence how we act in the future.

Many parts of the brain are important for memory, but the amygdala is at the center of the saving process. When something eventful happens, the amygdala functions as a sort of alarm system that tags the highly emotional experience as worth remembering [1, 2]. Regardless of what happened or where it happened, when the amygdala is highly active after an event, you are more likely to form a strong, long-lasting memory. However, these memories are not like making a video recording of the experience. It is more like a recording that was chopped up into the sounds, smells, feelings, and sights you experienced during the event. Later, when you are reminded of the experience, your brain puts these chopped-up details back together into a story that feels like a video you can remember or share with someone else (and when you cannot remember certain details, your brain fills in the gaps with what might have happened!). The amygdala makes it more likely you will later remember those sounds, smells, feelings, and sights when you experience them.

The “Save” Button

The amygdala is one small but influential part of a larger network of brain areas involved in learning and memory. When the amygdala gets activated, it tells a neighboring part of the brain, the hippocampus, to make note of the many brain regions that were activated by that experience. All those pieces of the memory can be saved and put back together later, when you remember the event. This is like recording a small snapshot of a virtual class for school (Figure 1). The virtual meeting is underway, and your teacher starts talking about something really important, like a lesson about the brain, that you know you will want to remember later. You hit the big red “record” button on the screen. As the meeting continues, you can see the other kids in your class in their video bubbles on the side of your screen. You can also see students typing questions into the chat box while your teacher talks. Occasionally, a student will use the thumbs-up reaction emoji to let the teacher know they understand what is going on. You stop the recording after a few minutes, and it gets saved to a folder on your computer that contains each piece of the recording. The video, the audio, the chat thread, the emojis, and the list of participants, each get saved as individual files to this folder. This is similar to the way the hippocampus stores each part of an experience, so you can remember it later. You can think of the amygdala as important for telling the brain when to save an experience, and the hippocampus as the part that is important for knowing where the memory is saved and how to stitch these details back together when recalling the memory later. Without this amygdala “save” feature, your virtual class meeting would be lost or forgotten. But hitting the “record” button tells your hippocampus and the rest of your brain to make a memory for the specific pieces of that class experience.

Activating the Amygdala

The almond-shaped amygdala is a brain region deep behind the ears, near the middle of the brain. Part of the amygdala, called the basolateral amygdala, gets activated during emotional experiences. When something exciting happens, like getting surprised by your friends for your birthday, the adrenal glands on top of your kidneys release a substance called epinephrine, also known as adrenaline (Figure 2). The adrenaline then causes another part of the brain, the locus coeruleus, to release another substance, norepinephrine, directly into your basolateral amygdala, causing it to be activated [3]. The activation of your amygdala by this rush of norepinephrine clicks “save” on this experience, making it more likely you will remember this birthday party for a long time.

Discovering the Brain’s “Save” Button

Rat brains and human brains have many of the same brain regions and similar connections between those brain regions. This similarity makes rats a great model for figuring out how the brain works. For example, scientists know the amygdala is important for saving your memories based on decades of research using artificial ways of activating the amygdala in the rat brain. In some experiments, researchers showed that injecting norepinephrine into a rat’s basolateral amygdala caused the rat to better remember the layout of a room, so it could navigate better in the future [4]. Similarly, when scientists surgically placed metal electrodes into a rat’s amygdala, applying a tiny amount of electrical stimulation to those electrodes improved the rat’s memory for the objects it previously sniffed and saw [5]. Sometimes humans who have seizures also have electrodes implanted into their brains to figure out why they are having seizures. Researchers found that low levels of electrical stimulation of the basolateral amygdala in these patients (about 1/1,000th the strength of a flashlight) increases memory for pictures the people were shown on a computer screen (Figure 3) [6]. This suggests that, by artificially pressing the “save” button in both rodents and people, we can make the brain more likely to remember experiences.

Researchers can also learn a lot about the function of a particular brain region by preventing that region from working. For example, rats given an injection of a drug that stops norepinephrine from activating the amygdala had a difficult time remembering a prior encounter they had with a set of objects [7]. These studies highlight the importance of the amygdala in saving memories, and suggest that, without this brain region, experiences are less likely to be remembered for very long.

Save Vs. Storage

Turning down the activity in the amygdala can worsen memory, and turning up activity in the amygdala can improve memory. However, researchers do not think this is because memories are stored in the amygdala. Instead, the amygdala is known as a modulator or influencer of memory because it affects a variety of other brain regions that handle storing the memories. The striatum, for instance, is an important part of the brain for forming new habits (that is right, making a new habit is a type of memory). When you learn to tie your shoes and no longer have to think very hard to do so, this type of memory is stored in the striatum. When you smell pumpkin pie wafting out of the oven and suddenly remember being with your family during the holidays last year, this odor memory is stored in your olfactory cortex. When you are at a really cool place and listening to some music you love, those memories are stored in your visual and auditory (seeing and hearing) brain areas. When all these different brain regions become activated during complex experiences, the amygdala tells the hippocampus to keep a record of those regions that were involved during the event. These regions can then be reactivated later, when you remember.

Scientists have even figured out how this communication between the amygdala and other brain regions works at the most basic level! Each brain region is made up of small cells called neurons, which can send signals to each other. Much like you can send texts to communicate with family or friends that live in other cities and states, neurons in the amygdala can send electrical messages to the neurons in the hippocampus, telling them to save an experience. The amygdala can also communicate with the hippocampus to save a memory by influencing proteins that are important for memory. When your amygdala is activated by an emotionally intense experience, the amygdala’s electrical signals can activate chemical signals causing memory proteins to be produced in the hippocampus. These memory proteins strengthen the connections between neurons [8]. Connections between neurons are what make a strong memory that you can easily recall. We think that both the electrical and protein communication between the amygdala’s neurons and other parts of the brain are what helps these memories to be saved and not forgotten. However, because these experiments are difficult to do in humans, more research is needed to explore this idea.

Conclusion

Some things that occur in our daily lives are more memorable than others, and the amygdala is essential for clicking “save” on all the pieces of these experiences. When something eventful happens to us, norepinephrine gets released into the basolateral amygdala, activating it and causing the neurons there to send messages to other brain regions, like the hippocampus, to remember that experience for later. Even though many brain regions are activated by an experience, the amygdala can tell the hippocampus to keep a record of those regions, so they can be reactivated later when you need or want to remember the event. Scientists are still studying the ways the amygdala is important for saving our experiences, so they can better understand how the brain makes long-lasting memories. This kind of research is not only important for understanding how the brain works, but might also help us develop treatments for people with memory problems who have lost the ability to make new memories or remember old memories. After all, memory is a critical part of how we know who we are and how we define the world around us. The brain’s “save” button, the amygdala, allows us to capture the unique and important experiences that happen in our lives, so we can make good future decisions.

Glossary

Hippocampus: ↑ A seahorse-shaped part of the brain that is important for activating other brain regions during memory retrieval, to help put the pieces back together so you can remember.

Adrenal Glands: ↑ Small, hat-shaped organs that sit on top of each kidney and release a hormone, called epinephrine, when something scary or exciting happens.

Epinephrine: ↑ Also known as adrenaline, this hormone is released by the adrenal glands and is part of the body’s quick response to danger, excitement, fear, and even stress.

Norepinephrine: ↑ A chemical released by a brain region called the locus coeruleus and that activates the amygdala during emotional experiences to help create memories.

Striatum: ↑ A brain part important for movements and motor memory. It allows complex movements like kicking a soccer ball or tying your shoes to eventually become almost automatic.

Olfactory Cortex: ↑ The brain region essential for sense of smell. It can store memories related to different scents.

Neurons: ↑ Cells in the brain that act like messengers; communication between neurons helps us learn, remember, move our bodies, see, hear, smell, and other important functions.

Protein: ↑ A molecule that makes up many body tissues and is also important for forming memories. Memory-related brain proteins can make the connections between brain cells stronger to help strengthen memories.

Acknowledgments

This work was supported by NIH NIMH R01 MH120194.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.