Another way of understanding memory is to think about it in terms of stages that describe the length of time that information remains available to us. According to this approach (see Figure \(\PageIndex{1}\)), information begins in sensory memory, moves to short-term memory, and eventually moves to long- term memory. But not all information makes it through all three stages; most of it is forgotten. Whether the information moves from shorter-duration memory into longer-duration memory or whether it is lost from memory entirely depends on how the information is attended to and processed.
Figure \(\PageIndex{1}\): Memory duration. Memory can be characterized in terms of stages—the length of time that information remains available to us. [“Memory Duration” by University of Minnesota is licensed under CC BY-NC-SA 4.0. Adapted from Atkinson and Shiffrin (1968).]
Sensory Memory
Sensory memory refers to the brief storage of sensory information. Sensory memory is a memory buffer that lasts only very briefly and then, unless it is attended to and passed on for more processing, is forgotten. The purpose of sensory memory is to give the brain some time to process the incoming sensations and to allow us to see the world as an unbroken stream of events rather than as individual pieces.
Visual sensory memory is known as iconic memory. Iconic memory was first studied by the psychologist George Sperling (1960). In his research, Sperling showed participants a display of letters in rows, similar to that shown in Figure \(\PageIndex{2}\). However, the display lasted only about 50 milliseconds (one-twentieth of a second). Then, Sperling gave his participants a recall test in which they were asked to name all the letters they could remember. On average, the participants could remember only about one-quarter of the letters they had seen.
Figure \(\PageIndex{2}\): Measuring iconic memory. Sperling (1960) showed his participants displays such as this one for only one-twentieth of a second. He found that when he cued the participants to report one of the three rows of letters, they could do it, even if the cue was given shortly after the display had been removed. The research demonstrated the existence of iconic memory. [“Measuring Iconic Memory” by Judy Schmitt is licensed under CC BY-NC-SA 4.0. Adapted from Sperling (1960).]
Sperling reasoned that the participants had seen all the letters but could remember them only very briefly, making it impossible for them to report them all. To test this idea, in his next experiment he first showed the same letters, but then after the display had been removed, he signaled to the participants to report the letters from either the first, second, or third row. In this condition, the participants now reported almost all the letters in that row. This finding confirmed Sperling’s hunch: Participants had access to all of the letters in their iconic memories, and if the task was short enough, they were able to report on the part of the display he asked them to. The “short enough” is the length of iconic memory, which turns out to be about 250 milliseconds (1⁄4 of a second).
Auditory sensory memory is known as echoic memory. In contrast to iconic memories, which decay very rapidly, echoic memories can last as long as 4 seconds (Cowan et al., 1990). This is convenient as it allows you—among other things—to remember the words that you said at the beginning of a long sentence when you get to the end of it, and to take notes on your psychology professor’s most recent statement even after he or she has finished saying it.
In some people iconic memory seems to last longer, a phenomenon known as eidetic imagery (or “photographic memory”) in which people can report details of an image over long periods of time. These people, who often suffer from psychological disorders such as autism, claim that they can “see” an image long after it has been presented, and can often report accurately on that image. There is also some evidence for eidetic memories in hearing; some people report that their echoic memories persist for unusually long periods of time. The composer Wolfgang Amadeus Mozart may have possessed eidetic memory for music, because even when he was very young and had not yet had a great deal of musical training, he could listen to long compositions and then play them back almost perfectly (Solomon, 1995).
Short-Term Memory
Most of the information that gets into sensory memory is for- gotten, but information that we turn our attention to, with the goal of remembering it, may pass into short-term memory. Short-term memory (STM) is the place where small amounts of information can be temporarily kept for more than a few seconds but usually for less than one minute (Baddeley et al., 1990). Information in short-term memory is not stored permanently but rather becomes available for us to process, and the processes that we use to make sense of, modify, interpret, and store information in STM are known as working memory.
Although it is called “memory,” working memory is not a store of memory like STM but rather a set of memory procedures or operations. Imagine, for instance, that you are asked to participate in a task such as this one, which is a mea- sure of working memory (Unsworth & Engle, 2007). Each of the following questions appears individually on a computer screen and then disappears after you answer the question:
Is 10 × 2 − 5 = 15?
(Answer YES or NO)
Then remember “S”
Is 12 ÷ 6 − 2 = 1?
(Answer YES or NO)
Then remember “R”
Is 10 × 2 = 5?
(Answer YES or NO)
Then remember “P”
Is 8 ÷ 2 − 1 = 1?
(Answer YES or NO)
Then remember “I”
Is 6 × 2 − 1 = 8?
(Answer YES or NO)
Then remember “U”
Is 2 × 3 − 3 = 0?
(Answer YES or NO)
Then remember “Q”
To successfully accomplish the task, you have to answer each of the math problems correctly and at the same time remember the letter that follows the task. Then, after the six questions, you must list the letters that appeared in each of the trials in the correct order (in this case S, R, P, T, U, Q).
To accomplish this difficult task you need to use a variety of skills. You clearly need to use STM, as you must keep the letters in storage until you are asked to list them. But you also need a way to make the best use of your available attention and processing. For instance, you might decide to use a strategy of “repeat the letters twice, then quickly solve the next problem, and then repeat the letters twice again including the new one.” Keeping this strategy (or others like it) going is the role of working memory’s central executive—the part of working memory that directs attention and processing. The central executive will make use of whatever strategies seem to be best for the given task. For instance, the central executive will direct the rehearsal process, and at the same time direct the visual cortex to form an image of the list of letters in memory. You can see that although STM is involved, the processes that we use to operate on the material in memory are also critical.
Short-term memory is limited in both the length and the amount of information it can hold. Peterson and Peterson (1959) found that when people were asked to remember a list of three-letter strings and then were immediately asked to perform a distracting task (counting backward by threes), the material was quickly forgotten (see Figure \(\PageIndex{3}\)), such that by 18 seconds it was virtually gone.
Figure \(\PageIndex{3}\): Short-term memory decay. Peterson and Peterson (1959) found that information that was not rehearsed decayed quickly from memory. [“STM Decay” by Judy Schmitt is licensed under CC BY-NC-SA 4.0. Adapted from Peterson and Peterson (1959).]
One way to prevent the decay of information from short- term memory is to use working memory to rehearse it. Maintenance rehearsal is the process of repeating information mentally or out loud with the goal of keeping it in memory. We engage in maintenance rehearsal to keep something that we want to remember (e.g., a person’s name, e-mail address, or phone number) in mind long enough to write it down, use it, or potentially transfer it to long-term memory.
If we continue to rehearse information it will stay in STM until we stop rehearsing it, but there is also a capacity limit to STM. Try reading each of the following rows of numbers, one row at a time, at a rate of about one number each second. Then when you have finished each row, close your eyes and write down as many of the numbers as you can remember.
If you are like the average person, you will have found that on this test of working memory, known as a digit span test, you did pretty well up to about the fourth line, and then you started having trouble. I bet you missed some of the numbers in the last three rows, and did pretty poorly on the last one.
The digit span of most adults is between five and nine dig- its, with an average of about seven. The cognitive psychologist George Miller (1956) referred to “seven plus or minus two” pieces of information as the “magic number” in short-term memory. But if we can only hold a maximum of about nine digits in short-term memory, then how can we remember larger amounts of information than this? For instance, how can we ever remember a 10-digit phone number long enough to dial it?
One way we are able to expand our ability to remember things in STM is by using a memory technique called chunking. Chunking is the process of organizing information into smaller groupings (chunks), thereby increasing the number of items that can be held in STM. For instance, try to remember this string of 12 letters:
XOFCBANNCVTM
You probably won’t do that well because the number of letters is more than the magic number of seven. Now try again with this one:
MTVCNNABCFOX
Would it help you if I pointed out that the material in this string could be chunked into four sets of three letters each? I think it would, because then rather than remembering 12 letters, you would only have to remember the names of four television stations. In this case, chunking changes the number of items you have to remember from 12 to only four.
Experts rely on chunking to help them process complex information. Herbert Simon and William Chase (1973) showed chess masters and chess novices various positions of pieces on a chessboard for a few seconds each. The experts did a lot better than the novices in remembering the positions because they were able to see the “big picture.” They didn’t have to remember the position of each of the pieces individually, but chunked the pieces into several larger layouts. But when the researchers showed both groups random chess positions—positions that would be very unlikely to occur in real games—both groups did equally poorly, because in this situation the experts lost their ability to organize the layouts (see Figure \(\PageIndex{4}\)). The same occurs for basketball. Basketball players recall actual basketball positions much better than do nonplayers, but only when the positions make sense in terms of what is happening on the court, or what is likely to happen in the near future, and thus can be chunked into bigger units (Didierjean & Marmèche, 2005).
Figure \(\PageIndex{4}\): Impossible (left) and possible (right) chess positions. Experience matters: Experienced chess players are able to recall the positions of the game on the right much better than are those who are chess novices. But the experts do no better than the novices in remembering the positions on the left, which cannot occur in a real game. [“Impossible and Possible Chess Positions” by University of Minnesota is licensed under CC BY-NC-SA 4.0.]
Long-Term Memory
If information makes it past short term-memory it may enter long-term memory (LTM), memory storage that can hold information for days, months, and years. The capacity of long-term memory is large, and there is no known limit to what we can remember (Wang et al., 2003). Although we may forget at least some information after we learn it, other things will stay with us forever. In the next section we will discuss the principles of long-term memory.
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