8.4 Short-Term Memory: How STM Works
Close your eyes. I’m going to say eight words out loud. When I say so, pause the lesson, and try to remember as many of them as you can. Ready? Close your eyes; here we go. Brush. Star. Horse. Table. Lemon. Bottle. Ship. Book. Now pause the lesson, try to remember the words, and when you’re ready, push play again.
How many words did you remember? Which ones did you remember? Here’s the list again: brush, star, horse, table, lemon, bottle, ship and book. If you tried to follow along in this exercise, you were using your short-term memory to complete the task. In another lesson, you can learn about the difference between sensory memory (sometimes called the sensory register), short-term memory and long-term memory. This lesson focuses exclusively on short-term memory and how it works.
An Auditory Process
Theoretically, short-term memory could work in a variety of ways, but for most people, it seems to be tied to the sense of hearing. In other words, it’s an auditory process. Why does research seem to think that short-term memory is auditory? We have two pieces of evidence for this conclusion.
The first piece of evidence comes from what people say about what they do when they’re trying to remember something. If you tried to remember the list of words from the beginning of this lesson, what strategy did you use? Most people report that for a task like this one, their memory strategy is to repeat the words silently to themselves over and over. So, if you were trying to remember the word brush, you’d close your eyes and say brush, brush, brush to yourself, in your head, as many times as possible. The fact that you’re mentally saying the words means that you’re using the sense of hearing. Compare this to other options you had. You could have tried to visualize what a typical brush looks like. Or, you could try to imagine holding a brush in your hand, and what it feels like, or even what a brush smells like or tastes like! We have all five senses available to us in a task like this, but most people choose the mental repetition of what the words sound like. This means that our basic orientation for short-term memory is auditory. People report using this strategy even when the words they’re supposed to remember are presented visually, like on a computer screen. So, that’s the first piece of evidence that short-term memory is auditory.
What’s the second piece of evidence? Imagine that you were asked to remember a list of letters, instead of words, and that one of the letters was C. Now you’re asked to identify the list of letters using a multiple-choice test. For the letter C, you are given three choices: C, P, or O. If you couldn’t remember and you made the wrong answer choice, do you think you’d be more likely to choose O because it looks like C? Chances are you would not. Instead, you would choose P, because P sounds like C; P and C rhyme. So again, even if the letters were presented visually, it seems that people form memories based on how the letters would sound.
Based on these two pieces of evidence, researchers have concluded that short-term memory is an auditory system. Why does this matter? In another lesson, we’ll discuss strategies for improving your memory, and we’ll talk about how many of these strategies use this information to help you remember things later. But a quick tip for now is for when you first meet a new person and you hear his or her name. To help you remember that name later, be sure to repeat the name out loud when you say hi. This auditory repetition will work right in to your short-term memory.
Storage Capacity of Short-Term Memory
Now, let’s talk about how big short-term memory is. When you were trying to remember the list of words at the beginning of this lesson, was it pretty easy or did you find it fairly difficult? How many pieces of information can you keep in short-term memory at any given time?
Research on this question has come up with what we call the magic number of seven plus or minus two as the number of pieces of information capable of being used in short-term memory at any given time. What does seven plus or minus two mean? Another way to say it would be that you can keep between five and nine pieces of information in your head at once. So, it ranges from five (which is seven minus two) up to nine (which is seven plus two).
Asking you to remember the eight words at the beginning of this lesson should have been possible, but right on the edge of what you can easily remember using your short-term memory. Of course, we have more than nine memories in our heads! We can keep an unlimited number of items in our long-term memory, which is where we permanently store information. But short-term memory is a temporary place where we keep information that is new before it’s been made permanent. Sometimes people call short-term memory working memory because it’s basically what your mind is working on at any given moment.
What accounts for the range in the number? Why isn’t the number just seven? We have a range due to several variables that might affect whether any particular person’s memory is relatively better or worse. These variables are things like intelligence, whether the person is paying attention, if the person is motivated to remember and so on. So, if you only remembered a few things from the list at the beginning, maybe that’s because you were distracted, tired or simply didn’t care if you remembered the words or not.
The Serial Position Curve
The last idea in this lesson is something called the serial position curve. In order to understand this concept, again think about the list of eight words from the beginning of the lesson that I asked you to remember. Which particular words did you remember? Most people will have better luck remembering the words at the beginning of the list and at the end of the list. The words in the middle are the ones most likely to be forgotten. That tendency is what the serial position curve is all about.
If we wanted to graph how likely it is that each word on the list would be remembered, we could do that pretty easily. The y-axis could be the percentage or proportion of words that are remembered, anywhere from zero to 100%. The x-axis could be each word in order of where it appeared on the list, from the first word to the last word. The term for where the item occurred on the list is called the serial position of the word. If we put a dot on the graph to represent how likely it is that each word in the list is remembered, we’d end up with a graph like the one you see here. The words at the beginning of the list are more likely to be remembered, as are the words at the end of the list, with the words in the middle being least likely to be remembered. So, the pattern of dots on the graph looks like the letter U. This standard shape of the letter U is what we call the serial position curve. There are two parts to the serial position curve.
The top left part of the serial position curve shows that the words from the beginning of the list were very likely to be remembered. This tendency is called primacy. Primacy is the tendency for items at the beginning of a list to be remembered. Why are the words at the beginning easier to remember? It’s probably because when you start rehearsing the words, or practicing them in your head, you have the most time to practice these words that you heard first.
The top right part of the serial position curve shows that the words from the end of the list are also very likely to be remembered. This tendency is called recency. Recency is the tendency for items at the end of a list to be remembered. Why do we remember the words at the end? The answer here seems to be something we call interference. The words at the end didn’t have anything happening afterward, so no other pieces of information could interfere with your memory for these words. So, the lack of interference distracting you and the fact that they were simply more recent both combine to make these words likely to be remembered.
The combination of primacy and recency means that the words in the middle are the least likely to be remembered, which accounts for this middle part of the serial position curve. Did you remember the words table and lemon from the beginning of the lesson? If not, your forgetfulness of the two words in the middle of the list would be evidence in support of the serial position curve.
In summary, researchers believe that short-term memory, sometimes called working memory, is an auditory process. The number of items you can keep in short-term memory at any given time is seven plus or minus two, meaning between five and nine items. When you try to remember a list of items, you’re most likely to remember the items at the beginning of the list (a tendency called primacy) and the items at the end of the list (a tendency called recency). Put together, primacy and recency create what’s called the serial position curve, a graphed image of how likely words are to be remembered based on their position in a list. Together, these concepts are everything you need to know about short-term memory. If you need to go over this lesson again, be sure to pay special attention to the information in the middle of the lesson!
After watching this lesson, you should be able to:
- Define short-term memory and explain why it is considered an auditory process
- Identify the number of items that can be stored in short-term memory
- Define primacy and recency and understand how they relate to the serial position curve