Short-Term Memory

Of the thousands of visual and auditory sensations, only a small percentage remains beyond that brief length of sensory memory. Once you attend to information in sensory memory Opens in new window, however, that information is automatically transferred into short-term memory.

What Is Short-Term Memory?

Short-Term Memory (STM) refers to a process that can hold a limited amount of information—an average of 7 times—for a short period of time, often 2 to 30 seconds. However, the relatively short duration can be lengthened if you repeatedly repeat or rehearse Opens in new window the information.

Some of the information that enters STM goes on to be processed in long-term memory (LTM) Opens in new window where memories for facts, images, thoughts, feelings, skills, and experiences may reside for as long as a lifetime.

Using the computer model for a moment, you might think of the STM as the material that can be retrieved onto the computer’s display screen at a given time. Information held in STM is active information, information to which you are paying attention—hence, it is sometimes called active memory.

As its name suggests, this memory is limited. If the information is new, it can be held in STM for only a brief time whether the information is new (i.e., from the sensory memory Opens in new window) or old (that is, retrieved from long-term memory Opens in new window for examination); only a certain amount of it can be held at any given time.

Most people’s STM can hold about seven “items,” give or take two. However, people can expand their short-term memory capacity by increasing the size of each unit of information. This involves an encoding strategy called chunking Opens in new window.

Consider what happens when you look up a new number in the telephone book. As you close the book, you must remember all seven digits in order to dial the telephone. There is one way you can keep the information in STM for longer than approximately 30 seconds. This is by using rehearsal—by saying the number over and over again to yourself, constantly restoring it in your STM (Dworetzky, 1988).

Has the following ever happened to you? You repeat a telephone number to yourself again and again until you dial it. Once you’ve dialed the number, you no longer rehearse it. But the line is busy. Guess what? Back to the telephone book to look it up again!

The telephone number, however, will disappear after this relatively short time unless it is transferred to permanent storage, called long-term memory Opens in new window. Typically, the new information in STM was lost because it is no longer being rehearsed Opens in new window or was not stored.

How does STM handle overloading?

What happens if you overload your short-term memory by trying to place 20 items into it? Find out here Opens in new window.

Features of Short-Term Memory

Two known features of short-term memory are as follows:

1. Limited duration

The telephone number you looked up will remain in short-term memory for a brief time, usually from 2 to 30 seconds, and then disappear. However, you can keep information longer in STM by using a mental practice technique called rehearsal Opens in new window. Rehearsal is the practice of deliberately repeating or rehearsing information so that it remains longer in STM. Mental practice or rehearsal can extend the time that information remains in short-term memory.

2. Limited Capacity

Not only does short-term memory have a limited duration, it also has a limited capacity. In an often cited paper, George Miller (1956) reported that our short-term memory can hold only about seven items or bits, plus or minus two. Although this seems like an unreasonably limited amounts, researchers have repeatedly confirmed Miller’s original finding (Baddeley, 1994; Shiffrin and Nosofsky, 1994). Thus, one reason why telephone numbers world-wide are generally limited to seven digits is because they match the capacity of STM.

We can demonstrate the capacity of STM with a memory span test, which measures the total number of digits that we repeat back in the correct order after a single hearing. Students make few errors when asked to repeat seven or eight digits, make some errors with a list of eight or nine digits, and make many errors when repeating a list that is longer than nine digits. One of the main reasons information disappears from short-term memory is interference (M.C. Anderson, 2009a).