Human memory consists of information patterns being stored temporarily or permanently in the interconnection patterns and synaptic weightings among neurons in the brain. Although specific brain regions such as the hippocampus, amygdala, cerebellum, and basal ganglia have been implicated as being highly involved in specific aspects of memory, many researchers believe that memory may be a "field phenomenon" of the brain — not localized strongly in any one point, but in the entirety of the interconnective map that makes up the brain. This would be consistent with the observation that evolution prefers redundancy and animals with critical functions localized in any particular brain structure would be more subject to the degenerative threats of malnutrition or injury than those with distributed functions.
There are three ways of classifying memory. They include duration of retention, information type, and temporal direction. Duration of retention is seen as the most universal and useful.
From the perspective of duration of retention, there are three memory types: sensory, short term memory (STM) and long term memory (LTM). Sensory memory operates 200-500 ms immediately after a perceptual event and can hold approximately 12 items for a negligible quantity of time. Occasionally, experiences that begin as sensory memories transfer to short term memory, which can hold 5, plus or minus 2 items without rehearsal for somewhere between a minute to an hour. This type is responsible for the "phonological loop" — our internal monologue reciting something to remember it.
The type that is most pervasive, and with the largest capacity, is long-term memory. Long-term memories are built especially well through repetition and training and the complex web of memories that associate freely with other memories. Sometimes this web of long-term memories is called knowledge.
Within long-term memory, there are declarative (explicit) and procedural (implicit) memories. Procedural memories are motor-based and controlled by older sections of the brain. They include things like learning how to ride a bike. Declarative memory, which further breaks down into semantic and episodic/autobiographical memories, are at the core of what we consider the human experience. Semantic memories are abstract knowledge and the recital of facts, and episodic memories contain stories. The two types of declarative memory are intimately interrelated. If anything in this article was new to a reader, he or she has just added some substantial information to his or her semantic memory database.