WHEN YOU BUY something in a shop and hand it to the shop assistant you remember the price of the object long enough to see that you get the right change. She in turn remembers what money you give her long enough to work out the right change. A few minutes later she will have forgotten this, and hours later you will have forgotten the price. If the cat has been sitting on your lap for some time and then, as all cats do, jumps down for no apparent reason, you may still feel the cat to be sitting there for some time after it has gone.
Short-term memory is the impression left by an event for a short while after it has gone. It is a sort of afterglow or shadow that lingers a bit longer than its cause. The short-term memory effect may become converted into a long-term memory effect, or it may disappear. Unless there is an actual conversion to a long-term memory effect the nature of the short-term memory effect is to disappear – but by then it may have served some purpose.
With the jelly model, the sculpting of the surface by the hot water was a long-term, or permanent, memory effect. But if you actually tried out the jelly model instead of just reading about it you would notice a curious effect. If you place successive spoonfuls of hot water on the surface in such a way that each spoonful spreads into an area that is already wet, then a single network of channels forms on the surface. As a result, a spoonful of water placed anywhere on the surface will always find its way to the same spot. This always happens if you carry out the experiment in one session. If, however, you take several days over it, adding only a few spoonfuls each day, then the result is quite different. Although the spoonfuls may be placed in exactly the same positions and in exactly the same sequence as before, a single network does not appear. Instead several separate networks appear.
The explanation for this curious difference in behaviour of the jelly surface if the experiment is spread over several days lies in the way it dries out between sessions. No matter where the water actually goes, a spoonful of water always leaves a wet spot where it is first placed. Another spoonful placed alongside and spreading into this wet area will tend to flow into the wet area even if there is no significant depression. This is a surface-tension effect. The wetness of the surface constitutes a short-term memory of the placement of a spoonful of water. It makes it easier for water to flow into that area than into surrounding areas. If the wetness has dried out, then the second spoonful forms its own depression without flowing into and becoming part of the first network. This effect is shown here.
The wetness of the jelly surface constitutes a short-term memory, and its effect is to unite things together into a single pattern, whereas without it separate patterns tend to form.
As we have seen, the major tendency of the special memory-surface is to break things up into separate fragments and to make these fragments as firm and distinct as possible. Fragmentation, separation, selection follow from the way the surface is organized. A large picture presented to the memory-surface is broken down into separate attention areas, each of which provides a specific pattern. Each of these patterns will leave its permanent effect on the surface. But as far as the surface is concerned there is nothing to tell whether the parts occurred together or as separate patterns presented on different days. This effect is shown here. With a short-term memory effect each pattern leaves an afterglow which makes that part of the memory-surface easy to activate. From the combination of these afterglow areas a new pattern emerges as a synthesis of all the separate patterns.
A better example is provided by a huge picture filled with figures doing different things. One pays attention to the figures individually and sees what they are up to. As one’s attention moves on to another part of the picture there remains a short-term memory of each figure, and in the end these are all synthesized into a complete pattern which comes to exist on its own. It may be a battle scene or a wedding feast.
The great usefulness of the recent memory effect is to put together again what has been fragmented by the limited attention span of the system. Things which existed as a whole can be recreated as a whole.
The limited attention span of the special memory-surface divides a large picture up into attention areas that are attended to one after another, so that what is spread out in space becomes spread out in time. As far as the memory-surface is concerned the picture might just as well have been spread out in time. A glass falling to shatter on the floor will be recorded as a sequence of patterns (glass in hand, glass falling, glass shattered, fragments). Whether the sequence of attention areas come from something that has been spread out in space or in time the effect is the same, and short-term memory acts to put the pieces together to enable a new composite pattern to emerge.
So far the short-term memory effect has only been considered as compensating for the limited attention span that breaks things up. But short-term memory can have a positive synthesizing effect that can create new patterns. If the attention area flits from part of one object to part of another, then those two parts may be combined to give a new pattern which only exists on the memory-surface. The pattern has been invented by the memory-surface. The same thing can happen with time. If two totally unconnected events follow upon each other, then the short-term memory effect can synthesize them into a pattern which again only exists on the memory-surface.
This synthesizing effect of short-term memory is the basis of association and learning. Teaching involves arranging an artificial but useful sequence so that short-term memory can create a pattern which links the things together permanently. A conditioned reflex is established in the same way. Two unconnected events are put together time and again until, through short-term memory, a linking pattern develops to convert them into a single pattern.
If the long-term memory effect is relatively slow, then short-term memory can serve to hold things around long enough for them to leave a long-term impression. This probably happens in the brain where long-term memory effects may involve chemical changes which take some time. Until this chemical change can be effected, the actual picture is preserved as electrical activity in the form of short-term memory. This system has obvious advantages in that short-term memory acts as a moving average which sorts things out before the long-term record is made. This is much more convenient than making the long-term record at once and then sorting things out.
Breaking things down into units, establishing these units and then putting them together again in different combinations is what the special memory-surface does. The ability to do these things makes the memory-surface a powerful computing system. The combining property is common to most computing systems, but the unusual feature is the extracting and selecting property which allows the memory surface to organize its own input.
The separating behaviour of the memory-surface leads to discrimination and selection.
The combining behaviour of the memory-surface leads to association, learning and creation.
All this behaviour is really due to the self-organization of information in the ideal surroundings offered by the special memory-surface.
