Introduction

This book is supposed to be about Memory and Cognition. It’s Chapter 5, and we haven’t talked about memory yet! What gives?

Actually, we have been talking about memory! We just haven’t been calling it that. We’ve been calling it concepts. Your knowledge of what things are is an example of one of your memory systems and the subject of this chapter: implicit memory. Implicit memory is unconscious: it’s everything you know, and know how to do.

Association

Because implicit memory is everything you know, it includes all the concepts we’ve been discussing so far, like bird and color and plant. With that knowledge comes all the associative links in between those concepts. Recall our discussion of spreading activation as both a mental and neural function. The formal name for this process of memory is priming.

Priming is a term for what happens when thinking of one thing makes it easier for you to think of another thing. For example, consider the following list of words:

• Strawberry
• Blood
• Ruby
• Tomato

Now…think of a color!

Any color at all!

Okay. You might have thought to yourself, I’ll be clever. She obviously wants me to think of red. So, I won’t. I’ll think of blue.

And that’s fine! But odds are, the idea of red at least popped into your mind, unbidden, before I even asked you explicitly to pick a color. That is, in a nutshell, priming. Thinking of red things makes you think of red. See the example below for an illustration of how thinking of the color white and the word silk makes a lot of people automatically think of milk.

Implicit associative memories are created through many avenues. The simplest method is repetition. If every day, you see an apple, and every day, that apple is red, you will associate apple with red. But there are other forms of associative learning; for example, classical conditioning – in which you associate something neutral with something meaningful, until the originally neutral thing ends up having that same meaning for you as well. You might be conditioned, for example, to feel excited when you hear the theme song of a favorite TV show, to feel hungry when you smell hot food on a grill, or even to flinch when you see something associated with someone you don’t like. These are all things you don’t need to consciously think about to know – which makes them implicit memories.

Procedural memory

Another aspect of implicit memory is procedural memory – the things you know how to do. And there are quite a lot of things that you know how to do! If you are reading this book, you know at least one language. You also know how to move yourself around, eat, drink, and sleep. You might add additional skills to your portfolio over time: some people will learn to cook or play basketball, others to knit or play the cello, still others to ride a bicycle, drive a car, or even steer a boat. All these things that you know how to do are part of your procedural memory. While you might initially learn these things explicitly – that is, by consciously going through the steps, once you’ve learned it, you usually don’t need to consciously go through the steps anymore. For example, when I learned to drive, I had to explicitly think, okay, the accelerator is on the right, and the brake is on the left. Now, that information is present automatically, making it a procedural memory.

Implicit Memory in the Brain

The Basal Ganglia

The basal ganglia is a set of structures deep near the center of your brain primarily known for regulating motor functions, but they are also central to many habit and reward-related behaviors. Much of the signaling in the basal ganglia involves the neurotransmitter dopamine.

These functions make the basal ganglia an important spot for your procedural memory. You can think of procedural memories a bit like habits; procedural skills are often learned through repetition. Damage to the basal ganglia can make it harder to learn things through practice or pattern recognition.

The Hippocampus

Please try the following exercise, if you can:

• Close your eyes!
• Point to the nearest door!
• Open your eyes and check – were you correct?

Odds are that (if you were somewhere indoors), you were correct. Why? Did you try to memorize where the door was when you entered this current room? Perhaps, but likely not. And yet, you usually know where all the doors are around you. In fact, you know a lot about what’s around you and how you got there. Your brain is actually tracking your location all the time, building a little map of the world around you. Your hippocampus is one of the structures primarily responsible for this process.

The hippocampus accomplishes this incredible feat in an incredibly organized way. Consider Tolly the rat – you can observe him again in Figure 5.4, exploring a rectangular room. The figure’s top panel shows Tolly’s path: he’s cautious, staying around the edges, but he ultimately can’t resist visiting the cheese in the open space before exiting.

The middle panel is where things start to get interesting. Here we have three neurons from Tolly’s hippocampus to observe: a purple neuron, a green neuron, and an orange neuron. Each neuron only fires an action potential when Tolly is in a certain area of the room. When Tolly is in the upper left of the room, the purple neuron fires (each action potential is indicated by a purple circle). When Tolly is in the top right-center of the room, the green neuron fires (action potentials indicated by green triangles). Tolly is in the lower-central right side of the room, the orange neuron fires action potentials (indicated by the orange rectangles). These neurons are known as place cells¹. The colored zones in the lowest panel of Figure 5.4 represent each place cell’s favorite area of the room; whenever Tolly enters each part of the room, the place cell that likes that part the best fires an action potential! Tolly’s brain does this for every space he enters – it keeps track of where Tolly is in space. This discovery of place cells (and subsequent work that fleshed out the brain’s mental maps even more²) actually earned its discoverers a Nobel Prize in 2014!