Types of Memory

Sensory, working and long-term memory; episodic, semantic and procedural; explicit and implicit — a map of memory systems and the brain regions behind each.

Psychologists and neuroscientists divide memory in several ways. The most useful maps combine how long information is kept with what kind of information it is.

By duration

Sensory memory holds a brief, high-capacity trace of what you just saw or heard — a fraction of a second for vision (iconic memory), a few seconds for sound (echoic memory). Short-term or working memory holds a small amount of information in mind for seconds while you use it. George Miller's famous 1956 estimate was "seven, plus or minus two" items; later research suggests about four meaningful chunks. Alan Baddeley and Graham Hitch's 1974 model splits working memory into a verbal loop, a visuospatial sketchpad and a controlling central executive; it relies heavily on the prefrontal and parietal cortex. Long-term memory can last a lifetime and has no known capacity limit.

Explicit (declarative) memory

Memory you can consciously recall and describe. In 1972 the Estonian-Canadian psychologist Endel Tulving split it into episodic memory — personal experiences tied to a time and place ("my tenth birthday party") — and semantic memory — general knowledge ("Paris is the capital of France"). Both depend on the hippocampus and medial temporal lobe for forming new memories, with long-term storage spread across the cortex.

Implicit (non-declarative) memory

Memory expressed through performance rather than recollection. Procedural memory for skills and habits (riding a bike, typing) relies on the basal ganglia, cerebellum and motor cortex. Priming — faster processing of something encountered before — involves sensory cortex. Classical conditioning of emotional responses involves the amygdala, and of simple reflexes the cerebellum.

Evidence for separate systems

The strongest evidence comes from patients. Henry Molaison lost new declarative memory but learned skills normally; people with Parkinson's or Huntington's disease can show the reverse pattern. Such double dissociations suggest the systems are genuinely distinct.