Memory systems and levels

The stage model (Atkinson and Shiffrin, 1968) was the first major model of memory. It describes three memory systems, each with its own capacity and duration:

• Sensory memory: every piece of information from the senses enters here first. Its capacity is very large but it lasts only about a second. Visual sensory memory is called iconic memory; auditory sensory memory is echoic memory. On a long road journey your eyes and ears take in trees, vehicles and horns, and most of it vanishes at once.
• Short-term memory (STM): only the information we attend to moves here. Its capacity is small, about 7 plus or minus 2 items, and information fades within about 30 seconds unless rehearsed. It is encoded mainly by sound.
• Long-term memory (LTM): information that passes beyond STM. It has a vast capacity and can last a lifetime. It is encoded mainly by meaning.

Control processes manage the flow of information between the systems:

• Selective attention decides what passes from sensory memory into STM.
• Maintenance rehearsal is simple repetition. Saying a delivery code over and over until you reach the door keeps it in STM, but once you stop, it is lost. This is what cramming does.
• Chunking groups items into larger meaningful units so more fit into STM. A ten-digit mobile number is easier to hold as three groups.
• Elaborative rehearsal links new information to what you already know, through meaning, logic or a mental image. This is what moves information from STM into LTM. Asking yourself why a rule works, and connecting it to an example you know, is elaborative rehearsal.

Working memory (Alan Baddeley). Short-term memory is not just a passive box; it is a working space where we hold and use information, as when you do a sum in your head. It has three parts:

• Phonological loop: holds a limited number of sounds for a short time (repeating a word in your head).
• Visuospatial sketchpad: holds visual and spatial information (picturing the route to a friend's house).
• Central executive: allocates attention and coordinates the other two, deciding what to focus on.

The idea that STM and LTM are completely separate was questioned by the case of a patient known as KF, whose brain injury badly damaged his short-term memory for spoken material while his long-term memory remained largely normal. If information had to pass through STM to reach LTM, that should not have been possible.

Levels of processing (Craik and Lockhart, 1972) says how long something is remembered depends on how deeply it is processed, not on which 'box' it is in:

• Structural (shallowest): noticing only the physical features, such as the shape of the letters or whether a word is in capitals.
• Phonetic (intermediate): noticing how it sounds, such as what it rhymes with.
• Semantic (deepest): understanding its meaning and connecting it to other knowledge.
• Deeper processing leads to longer-lasting memory. Understanding a topic beats repeating its words.