Human memory and the storage of information

The amount of selective information in a message can be increased either by increasing the variety of the symbols from which it is composed or by increasing the length of the message. Psychological experiments indicate that the variety of the symbols is far less important than the length of the message in controlling what human subjects are able to remember. Two messages equal in length but differing in the amount of information per symbol are equally easy to memorize. This fact provides an opportunity for the effective use of recoding procedures and reveals the mental economy involved in organizing the materials we want to remember. An apparent exception to the rule that length, not variety, is the limiting factor in human memory occurs in the case of redundant messages. If two messages of the same length differ because one contains redundancy familiar to the learner and the other does not, the redundant message will usually be easier to learn and remember. In terms of the theory of information, redundancy can be viewed equally well as a reduction in the information per symbol or as a reduction in the effective length of the message. Psychologically, however, these two alternatives are not equivalent; redundancy permits a reorganization into familiar sequences in a way that effectively shortens the length of the message and so makes it easier to memorize, but this is not psychologically equivalent to reducing the amount of information per symbol. It is as if each storage register could accept any one of a tremendous variety of alternative symbols, but the number of registers available was quite limited. If we use these registers to store binary symbols, the storage is inefficient. If we group the binary symbols into sequences, give each sequence a different name, and store the recoded names, we can make much more efficient use of the registers. Familiar redundancy is helpful because it enables us to recode more efficiently. These results for human memory are all the more striking in view of the fact that the amount of information per symbol is a critically important variable controlling the accuracy of our perceptions.