Sensory Memory

There are only two sources of information available to you. Information from within yourself, and information from outside yourself. The within-yourself stuff, well, that’s for a future chapter. But the outside-yourself stuff is…your senses! That’s it. There is no other way to get information from “out there” into you. And contrary to popular opinion, you do have more than five senses. The “big five” are known as sight, smell, hearing, touch, and taste. But you also have pain, temperature, balance, acceleration, body position…and most of the time, most of these senses are all giving you information at once.

For example, as I write this, I am currently wearing a shirt, pants, and socks. In other words, maybe 80% of my skin is currently touching some kind of fabric. That means that 80% of my skin cells are all constantly shouting at my brain, saying “I’m touching something! I’m touching something!” once every few milliseconds. But am I paying attention to any of that? Of course not! In fact, most of the sensory information I get at any given millisecond, I immediately throw away. The feeling of socks on my feet is important at exactly three moments: 1) when I put them on, 2) when I take them off, and 3) if something interesting happens to my foot. For example, one of my cats may brush up against my foot. In that case, I am aware that there is a sock between my skin and his fur. Other than those three scenarios though, I don’t need to be constantly updated that my sock is touching my foot. I ignore it.

This is the experience of sensory memory—that millisecond-scale awareness you have of everything you sense—you hang on for just enough time to comprehend the world around you, and then you dump most of it and move on. To understand the second half of this sentence, you need to remember the first half. To understand the right side of a room, you need to remember what the left side looked like. This form of memory is crucial in order for you to have a coherent experience of your world, but otherwise, it’s fleeting, and there is no reason to save most of it for future use.

But the miniscule fraction of information you do save becomes…

Short-Term Memory

If you want to save something in your awareness for more than a few hundred milliseconds, you turn it into short-term memory. Short-term memory is your ability to hold on to something in your mind for a limited amount of time. We’ll now break down the two rather vague terms in that definition: “something” and “limited amount of time.”

Items in Short Term Memory

Your short-term memory can contain any kind of information—the sound of a line of a song, the taste of a piece of cake you just swallowed, the contents of a list you’re trying to memorize, and more. To get a sense of what your short-term memory is like, we can start with an exercise: please take about 30 seconds to try to memorize the following numbers:

3 2 7 1 4 1 7 2 3 3

If you’re like me, you might have repeated these numbers “verbally” in your head until they stuck: an internal monologue that sounds like “three, two, seven, one, four…” and so on. This is an example of the auditory component of short-term memory. If hearing is part of your life, it’s usually one of the ways in which you try to keep things in your head. Formally, we call this process rehearsal.

You might have found it a challenge to memorize all 10 of those digits in 30 seconds, or at least that it took some mental effort. Holding something in short-term memory can feel like trying to hold water in your hands—you have to try very hard to keep it there, but you can’t hold it forever.

Limits to Short Term Memory

How long, exactly, can you hold something in your mind before you either forget it, or you remember it forever? While the general time span for short-term memory is anywhere from a few seconds to a few minutes, a different way to describe the limits of your short-term memory is not in time but in pieces of information.

Consider the 10 digits I asked you to memorize above. That was 10 pieces of information. Ten is on the high end for “numbers of things it’s reasonable to ask someone to remember.” If you think about various strings of digits you may use in your life—ID numbers, PIN codes, license plates—the ones that people most often assume you know off the top of your head tend to be shorter than 10 digits. A 10-digit PIN would be much more secure than a four-digit one, but I am much more likely to remember four numbers than ten! In fact, a scientist named George Miller attempted to formalize this phenomenon in the 1950s, landing on the “magic number” seven…plus or minus two¹. In other words, people can remember at most about 5-9 individual items before it all becomes too much.

Have you ever managed to easily remember more than nine things at once? It’s very likely that you have. There are several glaring exceptions to the claim I made above—that most numbers people expect us to have memorized are short—the most glaring of which might be…phone numbers! You might be thinking, I know a lot of phone numbers, and they are all ten digits long! What gives? Ah, but there is something very special about phone numbers and the way they are presented to us, something strange and magical, with a very whimsical name…it’s called a chunk.

Here are those ten digits from above again, but this time, they are chunked:

3 2 7 – 1 4 1 – 7 2 3 3

Hey, it looks a bit like a phone number now! Indeed, splitting items into groups is a very effective way of tricking our brains into accepting much more information. When you try to memorize a number like this, instead of saying to yourself “three, two, seven, one, four…” you might say something like “threetwoseven…onefourone…seventwothreethree.” You combine ten individual items into three groups, fooling your brain into thinking you’re memorizing three things, not ten. This strategy is very effective in increasing the number of things we can remember – though you will ultimately hit a limit of chunks you can hold in your short-term memory too. If you find yourself remembering long strings of information for longer than a few seconds or minutes, congratulations! You’ve successfully transferred something into long-term memory, which is everything you know and remember, and, well, we’ve already talked about it! But there is one more kind of memory we haven’t addressed yet that might help glue all these pieces of memory together. Let’s look at some memory models and introduce working memory to the party.

Working Memory

So far, we have your sensory memory, short-term memory, and long-term memory. You can represent these in a model like in Figure 7.1.

In this model, sensory memory feeds into short-term memory, and short-term memory feeds into long-term memory. Importantly, loss of memory is possible at each of these stages. Sensory memory doesn’t make it to short-term memory unless you pay attention to it, so inattention is what causes memories to leak out of sensory memory. Short-term memory doesn’t make it to long-term memory without effort, so lack of effort causes memories to leak out of short-term memory. As for long-term memory loss…we will discuss that in the next chapter.

Above these three forms of memory is a block that spans all three called working memory. Working memory is a term for when you are consciously accessing memories and manipulating them. You’re not just experiencing something or holding it in your mind; rather, you’re expending some effort to accomplish a task. An example will help make this clear. Consider the following series of letters:

L H B S Q K

Now try the following exercise!

• Memorize the letters, close your eyes, and recite them
• Now, close your eyes and recite them…in alphabetical order!

You may have noticed that the second task took more effort than the first task. That’s because the first task was only asking you to use your short-term memory, but the second was triggering your working memory. To recite the letters from memory in alphabetical order, you need to dip into not only your short-term memory but also your sensory and long-term memory! The process might look something like this:

• Use short-term memory to rehearse the letters: “L H B, S Q K, L H B, S Q K, L H B…”
• Decide to figure out which letter of those six is the earliest in the alphabet
• While the rehearsal is still running, open up long-term memory to access the alphabet, and determine that “B” is first
• Sensory memory helps keep “B” alive as you transition from long-term to short-term
• Hold both the rehearsal “L H B, S Q K,” and the fact that B is first in your mind
• Open up your knowledge of the alphabet again and repeat!

This is what it is like to use your working memory. More broadly, this is what it is like to think with effort and planning – a task very similar to something we’ve already discussed: executive control. Indeed, working memory and executive control have a lot of overlap. Which term we use to refer to the process of “figuring out what to do next based on current information and past knowledge” depends largely on context.

Explicit Memory in the Brain

The Hippocampus

We’ve already seen the hippocampus (in Chapter 5, when we discussed place cells). Spatial memory is not the only function of the hippocampus! It plays an interesting role in memory storage as well.

Long-term memory is not all the same. You likely experience this fact all the time; some things come to mind much easier than others. Memory is also more or less vulnerable to forgetting and loss under different circumstances, because memory is stored differently in different circumstances! Within long-term memory, there is a difference between fresh explicit memories (memories you’ve formed in the last few hours or days) and long-term explicit memories (memories that stick around over time). Figure 7.2 represents the journey of long-term explicit memories into permanent(ish) storage. The fresh, new long-term memories are kept in the hippocampus. But over time, they are distributed all over the brain.

This is one reason why it’s hard to lose very old, very established memories, and nearly impossible to have your personality “wiped” through memory loss. In order to lose all your explicit memories, you’d need to lose a lot of brain mass. Figure 7.3 shows a comparison of autobiographical memory² between people with various types of brain damage. Because autobiographical memory stretches back a long time and is reinforced frequently, it’s hard to disrupt it: people in this study with damage to their hippocampus demonstrated an intact autobiographical memory. People with damage to their medial temporal cortex (in the temporal lobe, close to the hippocampus) also demonstrated an intact autobiographical memory. The only subjects who struggled with their autobiographical memory had damage not only to their medial temporal cortex but also to other regions of their cortex – in other words, more extensive and broad brain damage.

Memory Loss

There are several different forms of memory loss. Amnesia is one of the best-known: in the movies, it’s often a complete “wipe” of everything you know. But there are actually several different types of amnesia! We will discuss two below. They are quite different, but both involve memory loss, and both are usually triggered by some kind of brain injury (often stroke, surgery, or accident). You can compare how each works in Figure 7.4.

Retrograde Amnesia

When we think about memory loss, it’s perhaps most natural to think about forgetting the past. One form of forgetting the past is retrograde amnesia, in which you lose explicit memories that you had before the injury occurred. For example, if you hit your head at 9 PM on your twenty-seventh birthday, you may not remember anything before 9 PM on that day. You may not even remember your whole 26th year of life. You can lose quite a lot of your past memories, but very importantly, you usually don’t lose it all.

This is for two reasons: one, recall our discussion about how long-term explicit memories are distributed throughout your brain. While retrograde amnesia can occur from injury to a variety of brain locations, it’s unlikely that one injury can ruin every single one of your memories—especially your earliest ones. Even if you can’t remember the last decade of your life, you will likely still know your name and understand who you are.

The other reason is that we are focusing on explicit memories here. The term retrograde amnesia generally does not apply to implicit memories, and those tend to remain intact in these cases. So, even if you don’t remember the last month of your life during which you learned how to ice skate, you will still know how to ice skate. You just won’t remember learning it.

Anterograde Amnesia

Recall our discussion of “fresh” versus “established” long-term memories. Retrograde amnesia refers to the loss of some of these “established” memories stored throughout the brain. But what happens if you experience damage to your hippocampus, where the “fresh” long-term memories live?

Hippocampal damage can sometimes result in anterograde amnesia, in which you lose the ability to transfer memories into long-term storage. New explicit memories appear for you, but they don’t stick around for more than a few minutes or hours. In other words, you can’t learn anything new (explicitly). A few times a day, you will forget everything that happened to you so far that day. If your injury happened on August 26th, 2017, you will wake up every morning believing it is August 26^th, 2017.

If this sounds difficult, it is. Living with anterograde amnesia requires a good support system—but it is not impossible. People with anterograde amnesia can still learn new things (implicitly), they just won’t remember that they learned them. There is no way to “cure” amnesia, but your brain is the most flexible thing in your body. With the right kinds of support now and more research in the future, living with brain injuries that affect your cognition will only become more manageable as the years go by.