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Short-Term Memory: How STM Works

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  • 0:12 Introduction
  • 1:20 An Auditory Process
  • 4:05 Storage Capacity of…
  • 5:52 The Serial Position Curve
  • 8:48 Lesson Summary
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Lesson Transcript
Instructor: Wind Goodfriend
All memories must first pass through short-term memory (STM) before becoming permanent. But how does STM really work? This lesson covers several aspects of STM, including why it seems to be an auditory-based system, how much information can be stored and the serial position curve.

Introduction

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.

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