Stages of Memory

Classically, memory can be thought of as a series of time based stages. Each succeeding stage contains fewer items than the preceding one. Also, movement of items from one stage to another is accomplished by separate memory processes.

The sensory store is the first stage. Basically, everything you perceive enters sensory memory but very little remains. Partial report experiments for visual stimuli have shown that items are retained for two seconds or less. Here, subjects can remember any of 16 letters that were flashed in front of them for a fraction of a second if they are asked to report within two seconds. After two seconds they cannot remember any of the letters.

Short term memory (STM) is the next stage. Pure STM lasts around 20 seconds. Pure STM occurs when subjects are prevented from rehearsing. One way to prevent rehearsing is to have subjects count backwards from 100 by threes (e.g,. 100, 97, 94, ...) after you show them a word. In real contexts, rehearsal makes STM last longer. Think of the following example. Your car breaks down in an unfamiliar place. You walk to a phone booth, look up Al's Garage, dial the number, it's busy. A minute later you want to call again, but you have to look up the number again. Why? Because you did not rehearse it. If you had said "555-4231" over and over to yourself for that minute, you would not have had to look it up. That is a form of rehearsal.

STM is limited in size. George Miller described it as 7 items + or - 2 items. That finding is now referred to as the "magic number 7" for STM. Most people can easily hold 7 items in STM. Notice that telephone numbers have seven places. Do you remember ZIP + 4, the new ZIP code? It is nine numbers long. Most people refuse to use 9 digit ZIP codes. Do you know yours? Mine is 71752-6231, SAU's is 71753-5000. What do you think is going to happen when telephone numbers get longer? Have you heard of (500) telephone numbers? You get to keep those no matter where you move to.

Long-term memory (LTM) lasts from minutes to years. Consolidation is the name of the process that puts items into LTM. Little is known of the details of how consolidation works. LTM can also be broken down into parts. Procedural memories are memories about how to perform activities, for example, riding a bicycle. Semantic memories are basically the same as knowledge. For example, 3 X 7 = 21, or knowing the capital of France. Semantic memories are not time tagged, meaning you cannot usually remember when you learned those items. Finally, episodic memories are personal and are time tagged: the day a relative died, the time the dog bit you, where you met your spouse, for example. Think of them as the episodes of your life.

Working memory is a newer concept proposed by Baddeley to better describe the workings of short term memory. According to Baddeley, working memory includes: the rehearsal loop, the visuospatial sketchpad, and the executive control system. The rehearsal loop is essentially the same as the traditional view of STM. The visuospatial sketchpad component allows for the handling of images. The executive control system is in charge of making decisions based on the limited amounts of information that exist in STM.

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