Duration of Short-Term Memory

To study how long short-term memory lasts, rehearsal must not be allowed to refresh information in short-term memory or to transfer it to long-term memory. The Peterson and Peterson procedure (also known as the Brown-Peterson procedure) minimizes rehearsal and recoding by tying up active processes in short-term memory during the retention interval (time between getting items and recalling them). They are tied up by having participants count backwards by 3's. Counting backwards requires mental computation and speaking, so it leaves little chance for rehearsal and recoding (Peterson,1966)

A typical experiment goes like this. The participant receives three items to remember, say three separate words, like DOCTOR, COOK, PLUMBER. Immediately afterward the participant receives a 3- digit number and starts counting backwards by 3's (378, 375, 372, 369, . . . ) until the experimenter asks the participant to report the three words. Practiced subjects get about 50% of the 3 words after a 15- second retention interval or about 30% correct after a 30-second retention interval (the interval between getting the words to remember and reporting them).

Figure 1. Decrease in percent correct over the first few trials of the Peterson and Peterson procedure to measure the duration of short-term memory.

The number of items to be remembered affects how rapidly information is lost from STM. Figure 1 shows the rate of loss when participants get one, three, or seven items to retain. If only one item is to be recalled, then the decline in recall as the retention interval increases is quite small, as the figure shows. If seven items (about the maximum STM can hold) are to be recalled, decline in recall is very rapid. Correct recall is almost completely gone in a few seconds.

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