Any temporary memory system ought to have some means by which information gets into such a memory system from the senses or from longer-term storage. Further, given that information is held on a temporary basis, the system must be subject to a process by which information may be lost over time, either through decay of the memory trace or by interference from new material displacing what is already in the system. Such a system should also have some means to extend the period of retention should this be necessary. Finally, given that it is a system that purports to store or manipulate visual and/or spatial material, the memory codes involved should have some relationship with the characteristics of the visual and spatial material with which the system has to deal.