[MUSIC, Title: "Introduction to the Declarative Learning System"] [Barb] Now it's time to reintroduce you to a friendly part of the brain that we talked about before— the hippocampus, which helps us both learn and remember. [Beth] Now, hold on, Barb— I don't remember us talking about the hippocampus. [Barb] Uhhh—that's right. [Looks embarassed.] We didn't talk about the hippocampus. Terry, over to you. [Terry] Thank you, Barb. Now, the hippocampus is an important part of the brain that helps us remember information like dates, faces, and concepts. That is, if we've heard about them before. But it turns out that if you don't have a hippocampus or it's damaged somehow, the information never gets put into your memory to remember. This is called "anterograde amnesia." Incidentally, we say "hippocampus" as if it's the only one thing. But, in fact, as you can see, we actually have two hippocampi, one deep within either side of your brain. As you can see here, they're each the size of a big lima bean. The hippocampus sits at the top of the declarative learning system. It receives high-level sensory information from the back of the brain and working memory from the front of the brain. But the information eventually ends up in the neocortex. You're mostly conscious of what you're learning when you use your declarative system. In other words, you can explicitly "declare" it. A lot of what students learn in school uses the declarative system. In other words, education makes it possible for students to declare information like facts and events. Now, remember, your neocortex is spread out over most of the brain. It's only a few millimeters thick, and when it's unfolded like this dinner napkin, it's around 24 inches by 24 inches. Although it's thin, it's a vast warehouse of long-term memory, millions of billions of sets of links. These "neuron-dots" are what you've learned and remembered. But with the neocortex being so enormous, how can your working memory ever find any specific piece of information? The solution? An index. [MUSIC, Title: "The Hippocampus Your Friendly Index to Information"] [Barb] If you look at a typical book, all the information is in the book's main body of text. How can you find that information? By using the index at the back of the book. It tells you where to find the information you're looking for. Want to find where the book discusses the beautiful country of Colombia? Look up Colombia in the index. The hippocampus, as it turns out, is a sort of index. When you want to look something up in your long-term memory, you working memory sends a signal to the hippocampus and then the hippocampus retrieves and links together the information that's distributed in the neocortex. That information is then brought back into working memory. What's really cool here is that every time a student retrieves information from long-term memory in the neocortex, the hippocampus helps strengthen links between the information stored around that neocortex. Hip, hip, hooray! Eventually, memories become consolidated— that means strong and firm—in the neocortex. You can think of consolidation as neatening and solidifying the links, a process that can take days, months, even years. It's a little like hot Jell-O cooling down and firming up. It takes time to do that. Once consolidation has happened, working memory is able to retrieve the information directly from the neocortex without using that hippocampus as the index. [Beth] Here's the challenge. Indexes aren't as big as the book itself for a very good reason. Indexes are supposed to only give you a link to where the information is. They're not supposed to carry as much information as the main body of the book itself. This means that indexes can only hold so much information. If an index—that is, the hippocampus— gets too much thrown at it in one session, it starts overflowing. Nobody's quite sure how much indexing information an index can hold before it starts overflowing, but it looks like about 10 or 15 minutes or so for adults might be a good guess. Although, of course, people vary. Variables like interest in the topic or the engagement in the materials certainly affect how much information can be absorbed at one time. When the hippocampus gets full, though, it will need at least a tiny break to help it offload information. The process of offloading new information in the brain is a little complex. So let's create a story to allow us to better illustrate what's going on. Come with us to the next video, where we will hear the songs of a VERY unusual choir, and you'll begin to understand what happened to me when I lost the ability to read and write. [Beth] I'm Beth Rogowsky. [Barb] I'm Barb Oakley. [Terry] I'm Terry Sejnowski. [All] Learn it, link it, let's do it!