Michael: Do you smell that? It smells like... cinnamon? And... evergreen trees. It reminds me of something. It reminds me of... this video Hank made about how smells trigger memories!

Hank: You’re walking through the hardware store one day when all of a sudden you catch a whiff of something you haven’t smelled in years. Somehow, the scent of glue immediately takes you back to your kindergarten classroom, and you spend the next couple of minutes wondering what happened to the kid who used to eat all that paste.

You just experienced what’s known as an odor-evoked autobiographical memory. To put it simply, a smell made you remember something from your past. And it happened because of the way smells and memories are hardwired into your brain.

Lots of different cues, like sights or sounds -- or even just someone describing something or telling a story that's unrelated to your story -- can trigger memories. But memories linked to smells are often stronger and more vivid, and studies have shown that they also tend to be memories of your early life -- often before you were ten years old.

Which is weird, because adults usually experience what’s known as a reminiscence bump, where they don’t remember much from before their adolescence. But smells are really good at bringing those memories back. These memories tend to be more perceptual, rather than conceptual -- so you remember a particular sensation, rather than a bunch of facts about something that happened.

And researchers have come up with some theories why memories triggered by smells are so odd. There’s a big difference between the way your body handles sight, sound, taste, and touch and the way it processes smells. Those other senses are all routed through the thalamus, the part of your brain that sends them off to the appropriate processing sensors.

But smells bypass all that. Once they’re detected by receptors in your nose, the signal heads straight to your olfactory bulb, the smell-analyzing region in your brain. And that area happens to be connected to the amygdala and the hippocampus, which are the parts of your brain that help handle memory and emotion. So it’s possible that when you smelled that glue in kindergarten, the signal got tangled up with memories of building blocks and apple juice. And when you smelled it again later, you remembered not just the glue, but also some of the associated memories -- like that weird, kid who ate the paste.

In 2013, a group of European psychologists tested this whole phenomenon using functional magnetic resonance imaging. First, they presented the subjects with 20 different strong, specific odors, like garlic, whiskey, and leather. Then, for each person, they identified the two that elicited the oldest positive memories. Then it was time to scan their brains.

Each subject was presented with their two experimental smells, plus two generic, control smells -- flowers and citrus. They were also shown verbal cues, which were just the names of the smells projected onto a screen.

The researchers found that both types of triggers tended to activate the regions of the brain associated with memory. But while the verbal cues lit up parts of the brain responsible for processing smells, the smells themselves were more strongly connected to emotional processing centers.

Some of the participants associated the smells with memories from before they were ten, while others remembered things from when they were between ten and twenty. And, depending on which time frame the memories fell into, their brains tended to use different regions to recall them.

The earlier memories lit up the orbitofrontal cortex, which is connected to perception. The later ones, on the other hand, tended to activate the left inferior frontal gyrus, which handles more conceptual memories. So, can you use your nose’s superpowers to help you remember things for your next big exam?

Probably not. Smells tend to evoke early, perceptive memories of events, not concepts. So the scent of glue might make you remember playing with construction paper in kindergarten, but your smell-memory won’t help you memorize Maxwell’s equations.

Michael: Okay, so now we know why smells trigger memories, but that video also brings up so many other questions: I know how you can remember things, but how do you make memories?

And why can I remember kindergarten, but I can’t remember being a baby? And if smells can’t help me study for a test, is there another way I can hack my memory? Check out these three videos from Hank, Olivia, and me to answer these questions.

Hank: How do you make memories? Inspirational websites will tell you to spend time with the people you love. Travel websites will tell you to travel to beautiful places. I’m just going to tell you to watch this episode of SciShow over and over and over again until it actually sticks in there. But if you ask a neuroscientist, they might tell you about a man named Henry Molaison.

In 1953, Molaison had surgery to remove certain parts of his brain, and lost his ability to form most kinds of long-term memory -- changing our understanding of the human mind and memory forever.

Up until the 1950s, we really had no idea how the human brain could convert an experience into a memory that could be retrieved and relived. Scientists had been trying to figure it out, but they didn’t have access to fancy technology like fMRI scanners to let them look inside living human brains.

Based on what they did find out, from animal studies and the brains of people who had died, they thought that memories might be stored throughout the brain. That seemed to make sense, because patients with injuries to different parts of their brains would sometimes develop amnesia. So they had no clue what they were about to do to Henry Molaison.

When Molaison was a kid, he hit his head while riding his bike -- and after that, he started having seizures. A lot of seizures -- and severe ones. His doctors tried all kinds of different treatments, but in 1953, he was 26 years old and none of those treatments had worked. But there was one more procedure that they thought might help: a surgery to remove the part of his brain where the doctors thought they seizures were coming from.

So, they did the surgery, removing two finger-sized pieces of brain tissue from Molaison’s left and right medial temporal lobes. The specific parts they took out? The hippocampus, amygdala, and part of the entorhinal cortex. You might recognize some of those names, because we now know that they’re really important parts of the brain. But at the time, doctors had no way of knowing just how big a deal removing them was.

When Molaison woke up, he could remember his name and things that happened in his childhood, but he had what’s known as anterograde amnesia. Basically, he couldn’t form new memories anymore. The doctors weren’t going to be able to give him his memory back. But for the rest of Molaison’s life, they tried to learn as much from him as they could. The main thing they found out was that the hippocampus plays a big part in the formation and retention of certain kinds of memory.

Researchers also learned that there are multiple kinds of long-term memory, controlled by different parts of the brain. Long term memories are not as long as you might think - basically, anything your brain retains after about 30 seconds. And there are two kinds of long term memories: declarative, or explicit, and non-declarative, or implicit.

Declarative memories are memories that require conscious processing, and the kind that Molaison lost the ability to form. These include episodic memories -- like the memory of your eleventh birthday party -- as well as semantic memories, which are the facts and ideas. Non-declarative memories, on the other hand, are memories of habit, like riding a bicycle or tying your shoes.

After Molaison’s surgery, he couldn’t learn new facts or remember new events. He’d meet a person and forget them as soon as they walked out the door. But his doctors discovered that the different kinds of long-term memory must depend on different brain structures, because Molaison could still form non-declarative memories.

For example, he could learn new motor skills, like tracing a drawing he was looking at in a mirror, and his reaction times improved with practice. For the rest of his life, Molaison was studied by dozens of doctors. To protect his identity, he was referred to as H.M. in publications. But after his death in 2008, his name was publicly released for the first time, and the rest of the world began to understand just how much he’d taught us.

Even in death, Molaison continues to help us learn more about the brain. His brain was donated to science, so researchers could examine his brain more closely and better understand the effects of his surgery. At his death, his brain was removed and flash-frozen before being cut into 2,401 microsections - super thin slices to be mounted on slides for experimentation. These sections were used to make a 3D recreation of his brain in 2014.

From that, we’ve already discovered that Molaison hadn’t actually lost his entire hippocampus removed - just most of it. But because it was cut off from the rest of the memory systems by his injury, this small part of his hippocampus couldn’t help him regain his memory.

So, Molaison may not have been aware of just how important he was to science. But his life and death are still teaching us all about memory and the human brain.

Michael: Remember that one time when you were a baby? No, of course you don’t. Because, if you’re a teenager or older, chances are you can’t remember anything that happened before you were three. The process of forgetting these really early memories is called childhood amnesia. It happens to pretty much everyone, and has to do with the way our brains develop as we grow up.

Childhood amnesia starts to set in between the ripe old ages of eight and nine. Before then, most children can remember things that happened when they were really young, like visiting family or winning a teddy bear from one of those impossible carnival games. But the passage of time by itself isn’t enough to explain childhood amnesia.

After all, when you’re 30, you can remember certain things that happened 20 years ago, when you were 10. But when you’re 20, you can’t remember being an infant at all. Plus, we don’t forget everything from when we were little. Some things, like the language or motor skills that we pick up, stick with us. But we do tend to forget episodic memories -- memories of specific events and details.

So scientists think that childhood amnesia must have something to do with the way our brains change between infancy and adulthood. It turns out, some parts of our brains don’t finish developing until long after we’re born. One of them is the hippocampus, which helps us form and store episodic memories. Even as adults, our brains are always producing new cells, called neurons, in the hippocampus.

But when you were a young, growing child, the brain produces a lot of new neurons a lot faster. So, to see how brain-cell growth affected memory, a research team from Toronto took adult mice and experimentally made their hippocampuses produce more new neurons. And it turned out that the mice became more forgetful.

They seemed to lose memories, just like humans do with childhood amnesia. But when researchers slowed down the growth of new brain cells in young mice, those mice seemed to forget less from their mousey childhoods.

So the question is: Why would making new brain cells be bad for your memory? Well, It’s not, in the long term, which is why we can keep making new episodic memories as adults. But it seems like trying to fit all those new neurons into your hippocampus when you’re young could cause a problem.

The new neurons shuffle around with the old ones to form new memory connections, and this could make it harder for the brain to find where earlier memories were kept. It might even erase them completely. Still, not all of our memories are kept in the hippocampus, so this doesn’t explain everything about childhood amnesia.

There are other parts of the brain involved in memory, including the amygdala and the prefrontal cortex. So scientists are studying these to see if they also make different amounts of new neurons when we’re children, compared to when we’re adults. We don’t fully understand childhood amnesia yet, but we do know it happens to everyone. So if you can’t remember your first birthday party, don’t worry. Neither can anyone else!

Olivia: Y’know those phrases that just seem to be ingrained in your memory from middle school? Like: Please Excuse My Dear Aunt Sally, ROY G. BIV, or thirty days has September, April, June, and November. Sound familiar? These are different kinds of mnemonics -- shortcuts that we can use to help us remember stuff, like the order of operations, or the color of a rainbow.

Turns out, there are lots of strategies to remember information when you need it most. Take the mnemonic, My Very Educated Mother Just Served Us Nachos. The first letter of each word stands for a planet in our solar system, from Mercury to Neptune.

Now, when you first think about it, mnemonics like this don’t seem like a very helpful memory trick, because you have to remember twice as much -- like, a weird sentence plus all of the names of the planets. But that’s actually why they work.

A simple way to think about memory is that we store information -- kinda like sticking a file in a filing cabinet, or those shelves of orbs in Inside Out -- until we recall, or remember it later. And researchers studying how people learn -- like educational psychologists -- suggest that recalling information can be easier when it’s connected to other information you already know. So you can imagine this model of memory like a web of files, where the ones with more connections are less likely to be lost, and easier to recall.

One influential theory, which was published in The British Journal of Educational Psychology in 1976, put learning in terms of different levels of processing. Basically, they suggest learning can fall on a spectrum of surface-level processing -- which is more like rapid-fire memorization -- to deeper processing -- or, linking new information to an information network, which leads to better recall. And with mnemonics, you’re making more of these connections. Sometimes it’s between random bits of information -- like setting the periodic table to the tune of a song you know.

But there are a lot of memory hacks that psychologists have proposed over the years, and tested in research experiments. Not all of them will work for everyone in every learning situation -- there are just way too many variables in real life -- but they can be helpful.

If you’re learning new words, you can try using the Keyword Method -- a term that was coined in the mid-1970s by researchers from Stanford University, and studied frequently in the next couple decades. This mnemonic can help people learn words in new languages, by connecting how a new word sounds to a keyword in English, for example. Then, the English keyword is linked to a strong visual image that helps you recall what the new word means.

So like in Spanish, say you’re trying to learn the word “perro,” which means dog. You might pick the keyword pear, and imagine a dog holding a pear in its mouth to connect the two. The Keyword Method could also help with more complicated vocab in English -- like, when you think of the word “melancholy,” you can picture a sad melon to remember the meaning.

But what if you’re more of a spatial, visual learner? Then, you can use a technique known as the Method of Loci, which was first described by ancient Greek and Roman texts, and studied by psychologists from the 1960s until now. This strategy allows you to create a kind of “mind palace,” where you mentally walk through rooms in a building or some other familiar space -- the loci. Along the way, you can visualize things like symbols that represent key points in a speech you’re gonna give, or meeting the U.S. presidents in order.

So when it comes to school, trying to memorize one fact at a time might not be the best study strategy. Instead, it might help to connect that new information to other things you learned, or even make some kind of story out of it.

And finally there’s chunking, a theory first proposed by a Harvard psychologist in 1956 that’s still studied today. It’s basically when you learn a whole bunch of information and organize it into chunks that make sense: Like, instead of trying to memorize a sequence of 8 separate numbers you can break it into two chunks that sound like years. So it feels like you have fewer individual things to remember, and it’s easier to store and recall more information. And with more and more exposure to the information you’re trying to learn, like when you’re studying, the larger the chunks of connected information can become.

There’s no replacement for paying attention in class, taking good notes, and spending time studying when it comes to learning. But if you’re having a little trouble remembering stuff, you might be able to use some mnemonics. Because, sometimes, we need all the help we can get!

Michael: Mnemonics! I knew there was a way to help remember things! But that reminds me... Can you really “train” your brain with those games? Here’s one more video about that very topic.

Exercising your muscles helps keep your body strong and healthy, which is why lots of people think your brain works in the same way. There are so-called “brain training” games out there that say they’ll improve your memory, attention, and reasoning skills -- and eventually make your brain faster and healthier. Some even claim to help prevent the onset of dementia Problem is, they don’t really work.

Brain training, or cognitive training, claims to rely on neuroplasticity -- the idea that the connections between neurons in your brain are plastic and changeable, and can adapt to new things. For years, scientists thought only the developing brain was flexible that way, but they eventually figured out that even though many connections do become fixed during childhood, the adult brain is still surprisingly flexible. Studies on dementia and the aging brain show that losing that plasticity leads to cognitive decline, so brain training programs claim to stop -- or even reverse -- the loss by flexing your brain like a muscle.

And we’ve known for a long time that practicing a specific task makes you get better at it - like how the different levels of Mario Kart might get easier the more times you play them. The question was whether playing these games can make you better at doing other, real-life things, like remembering names and appointments.

In 2008, a group of scientists from the US and Switzerland published a paper in the journal PNAS, that seemed to show that it could work. In the study, a group of young adults were tested on their ability to solve new problems. 35 of them were assigned to a control group, and had no contact with the scientists, while another 35 had to track a square flashing on a screen while listening to a series of sounds. They were tested on whether each square and sound matched the ones that came before.

After several weeks, the researchers tested all of the subjects on their problem solving again - and those in the treatment group seemed to show a huge increase in their IQ. Lots of people were excited about that paper, which has been cited more than 800 times since then.

Then some scientists started pointing out that it was seriously flawed. For one thing, there may have been what’s known as a placebo effect, where the treatment group knew they were supposed to improve at the tasks after training -- so they did.

And when other researchers tried to replicate the results, they weren’t able to. Studies since then have shown that brain training can have an effect on your brain, but it’s a lot more specific. One paper published in Nature in 2010 had over 11,000 people practice tasks meant to improve their reasoning, memory, and attention, but after six weeks, they’d only gotten better at the games themselves.

Their new skills didn’t translate to other tasks -- not even similar ones. For example, even if someone practiced a card matching game, it didn’t translate to improvements in their score on the paired-associates learning test - a similar kind of matching test that’s used to assess memory impairment. And when scientists have compared other studies on brain training, they’ve also generally found that it doesn’t have a significant impact on cognition.

The consensus is so strong that in 2014, 70 neuroscientists and cognitive psychologists signed a statement saying that there’s “no compelling scientific evidence that... [brain training games] reduce or reverse cognitive decline.” So does that mean brain training doesn’t work at all? Well, not exactly.

The issue is more how these brain training programs are advertised. They’re wrong if they say that brain training improves brain health overall, but that doesn’t mean it can’t be helpful in some specific cases. In one rehab program that included skills training, practicing things like remembering names and counting change helped patients with Alzheimer’s disease get better at those things.

But “Practice Paying Your Bills!” isn’t such an exciting-sounding video game, which might be why brain training companies aren’t making those games. And it’s not like brain training is a terrible thing. Unlike some other kinds of pseudoscience, it won’t actively harm you.

But these games aren’t cheap - the brain training industry brings in over one billion dollars a year, which is a lot of money for people to be paying for ineffective treatments. So, what can you do to protect your brain? For starters, we’re still trying to understand the effects of aging on the brain, and what causes dementia.

We know that dementia and memory loss are related to damaged neurons in the brain, but scientists aren’t totally sure how the neurons get damaged in the first place. So we don’t know any surefire ways to prevent or treat memory loss. Still, research has shown that there are some things that can help -- without an expensive subscription to a brain training program.

More education translates to a decreased risk of dementia, and maintaining a healthy diet and getting lots of exercise can also help keep the aging brain healthy. Scientists might eventually develop an easy, fun way to protect your brain and make you smarter -- but these brain training games aren’t going to do it.

Thanks for watching this memory compilation. If you’ve ever liked a SciShow video or left us a thought-provoking comment, you’ve definitely added to our great SciShow memories this year. Thank you! If you have an idea for a compilation of videos you’d like to see, let us know in the comments below, and if you just want to continue getting smarter with us, go to YouTube.com/scishow and subscribe.