Story of Psychology - Morton Hunt [345]
To explain this disparity and determine how memory works, cognitive psychologists conducted a great many experiments during the 1960s, 1970s, and 1980s; the findings, pieced together, gave shape to an information-processing picture of human memory. In it, memory consists of three forms of storage, ranging from a fraction of a second to a lifetime. Experiences or items of information needed only for an instant fade away as soon as used, but those needed longer are transformed and held for longer, or even worked into the semipermanent or permanent register of long-term memory. Researchers and theorists portrayed the three types and the transfer of information among them in flow charts something like the one on p. 608.
The briefest form of memory consists of sensory “buffers” in which incoming sensations are first received and held. By means of the tachistoscope, researchers verified that buffers exist and also measured how long memories endure in them before disappearing. In a classic experiment in 1960, the psychologist George Sperling flashed on a screen, before attentively watching volunteers, patterns of letters like this:
The letters appeared for a twentieth of a second, too brief a time for the volunteers to have seen all of them, although immediately afterward they could write down the letters of any one line. (A tone, right after the flash, told them which line to record.) They could still “see” all three lines when they heard the tone, but by the time they had written down one line, they could no longer remember the others; the memory had vanished in less than a second. (Experiments by others yielded comparable results with sounds.) Evidently, incoming perceptions are stored in buffers, from which they vanish almost at once—fortunately, for if they lasted longer, we would see the world as a continuous blur.35
FIGURE 40
An information-processing model of human memory
Since, however, we need to retain somewhat longer the things we are currently concerned with, there must be another and longer-lasting form of temporary storage. When we pay attention to material in a sensory buffer, we process it in any of several ways. A digit becomes not just a perceived shape but a symbol—a 4 gets a name (four) and a meaning (the quantity it stands for); similarly, words we read or hear get meanings. This processing transfers whatever we are attending to from the buffers to the immediate or short-term memory that Miller was talking about.
In lay usage, short-term memory refers to the retention of events of recent hours or days, but in technical usage it denotes whatever is part of current mental activity but is not retained after use. This form of memory is brief. We have all looked up a phone number, dialed it, gotten a busy signal, and had to look up the number again to redial it. Yet we can retain it for many seconds or even minutes by continuously repeating it to ourselves—psychologists call this activity “rehearsal”—until we have used it.
To measure the normal duration of short-term memory, therefore, researchers had to prevent rehearsal. A team at Indiana University did so by telling their subjects that they were to try to remember a set of three consonants, a very easy task, but that as soon as they had seen them, they were to count backward by threes in time with a metronome; this preempted their attention and made rehearsal impossible. The researchers cut the volunteers’ backward counting short at different times to see how long they would retain the three consonants; none did so longer than eighteen seconds. Many later experiments confirmed that the decay rate of short-term memory is between fifteen and thirty seconds.36
Later, other studies distinguished between two kinds of short-term memory (not shown in the above diagram). One is verbal: the immediate memory for numbers, words,