Sensory Memory and Incoming Stimulation
The memory's huge storage system is divided into three storage subsystems, the first of which is sensory memory. This is the entranceway through which all incoming stimulation must pass. This subsystem is divided even further into storage areas for each of the senses: hearing, vision, smell, touch, and taste. Information relating to everything you sense is sent directly to one of these memory areas.
The memory of these impressions is stored briefly. Have you ever noticed that after you shut off the radio, you can still hear the sound for a split second? Or perhaps you've looked at the sun (though you know you're not supposed to!) and then shut your eyes: Do you recall the image still being there? These lingering impressions are possible because they are temporarily stored in your sensory memory.
Because you are constantly receiving new information, the sensory memory cannot store impressions for long. For instance, the vision area stores impressions for only a fraction of a second. The hearing area stores impressions for a bit longer, but no more than about two seconds. Amazingly, this is enough time for your brain to decide what to do with this information.
Sensory memory also explains the bird-in-a-cage trick. If you aren't familiar with this “trick,” it is where the image of a bird is placed on one side of a card and the image of a cage is placed on the other. When the card is spun rapidly, it appears as if the image you are seeing is the bird inside the cage.
During that short amount of time that the impression is stored, the brain decides whether it is important or insignificant information. If it is deemed important, that impression moves on to the short-term memory. If it is deemed insignificant, the brain discards it and it is lost forever.