Three faces of memory

There are, in fact, no fewer than three kinds of memory: a sensory store, a short-term memory and a long-term memory.

- A sensory store is sometimes referred to as a visual image. Here the raw material of perception is briefly retained while information-processing operations begin. Perception is not instantaneous. The decision-making process of integrating incoming information with what is already known and expected takes a significant amount of time, approximately a quarter of a second or more. Information in the sensory store decays very rapidly, in less than a second, and is erased by a new intake of information through a receptor.

- A short-term memory. Here raw information is held temporarily while it is processed, i.e. identified, categorised. E.g.: the configuration of black marks on a white page that forms the letter ‘A’ has not been fully processed until the letter is, in fact, identified, or categorised, as an ‘A’.

There’s much less information in short-term memory than in sensory store. In fact, it can hold only about 4 or 5 separate items. This information is lost unless some form of internal rehearsal constantly renews it. Because its capacity is very limited, short-term memory is decrypted (erased) if new information comes in before the information it contains has been disposed of. Unlike sensory store, there’s no absolute limit to how long information can be kept in short-term memory. But because we constantly want to pass new information through the bottleneck of its limited capacity, its effective duration is only a few seconds at the most. However, re-encoding visual information into its sound correlation usually makes it possible to hold this information from 15 up to 30 seconds.

- A long-term memory. It appears to have no storage limitations at all. Anything that ever gets into long-term memory stays there permanently. However, two limitations on long-term memory prevent us from using it as effectively as we might. 1) In the first place, it takes time to get information into it (aggravating the short-term memory bottleneck); 2) secondly, we need special retrieval procedures (or rules) for getting information out. The reason we so often ‘forget’ knowledge we once had is not that it has left our long-term memory, but that we have lost the means of access to it.

In reading, all three aspects of memory must be involved. Visual information is picked up from the printed page and held for less than a second in a sensory store. Much of the information in the sensory store must necessarily be lost, but some is transferred to short-term memory. There it can be held for a few seconds while further information from the sensory store is acquired. New visual information effectively wipes out the content of the sensory store immediately.

How much information gets into short-term memory depends on its form. Short-term memory may contain only 4 or 5 elements at any one time. But each of these elements may be a single letter, or a single word, or a meaning extracted from several words. Since sentence meaning cannot be determined on a sequential word-by-word basis, it is obvious that information from several printed words has to be held in short-term memory at any one time. The load on short-term memory can be reduced by ‘ chunking' information into larger units. But this involves making use of syntactic and semantic information that must already be stored in long-term memory. In fact, nothing could be identified (i.e. nothing could be perceived), if a contribution were not made by long-term memory. This is so, because it is there that is lodged the knowledge of the world to which all incoming information must be referred (i.e. to the data of a situational-empirical channel).

The skilled reader keeps his eyes 4 or 5 words ahead of his voice while reading. In other words, a skilled reader keeps his short-term memory fully loaded while he is reading. The short-term memory bottleneck also suggests why even the slightest distraction is enough to make us lose the thread of what we are reading.

The processed elements in short-term memory must be disposed of: either lost altogether or transferred to long-term memory. Getting something into a long-term memory is not an easy task. We can only get items into long-term memory at the rate of one every five seconds. By chunking, we can make sure that each of these items is larger than a single word. Many experiments have shown that we remember meanings much better than actual words. Even so, a good deal of information must be lost between short-term memory and long-term memory. A good reader is the individual who can ensure that the information, lost in the perceptual process, is that, which is least important. He knows in advance what information is the least important, because he already has acquired, as part of his knowledge of the world, the competence to predict just what the nature and the relevance of incoming information is likely to be.