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For computer memory, see computer storage.

Memory is one of the activities of the human mind, much studied by cognitive psychology. It is the capacity to retain an impression of past experiences. There are multiple types of classifications for memory based on duration, nature and retrieval of perceived items.

The main stages in the formation and retrieval of memory, from an information processing perspective, are:

Table of contents
1 Classification by duration
2 Classification by information type
3 Memory disorders
4 The physiology of memory
5 Related topics

Classification by duration

A basic and generally accepted classification of memory is based on the duration of memory retention, and identifies three distinct types of memory: sensory memory, short-term memory, and long-term memory.

The sensory memory corresponds approximately to the initial moment that an item is perceived. Some of this information in the sensory area proceeds to the sensory store, which is referred to as short-term memory. Sensory memory is characterized by the duration of memory retention from miliseconds to seconds and short-term memory from seconds to minutes. These stores are generally characterised as of strictly limited capacity and duration, whereas in general stored information can be retrieved in a period of time which ranges from days to years; this type of memory is called long-term memory.

It may be that short-term memory is supported by transient changes in neuronal communication, whereas long-term memories are maintained by more stable and permanent changes in neural structure that are dependent on protein synthesis. Some psychologists, however, argue that the distinction between long- and short-term memories is arbitrary, and is merely a reflection of differing levels of activation within a single store.

If we are given a random seven-digit number, we may remember it only for a few seconds and then forget (short-term memory). On the other hand, we can remember telephone numbers for many years, since we have stored them in our brain after long periods of consolidation and rehearsal (long-term memory).

Additionally, the term working memory is used to refer to the short-term store needed for certain mental tasks - it is not a synonym for short-term memory, since it is defined not in terms of duration, but rather in terms of purpose. Some theories consider working memory to be the combination of short-term memory and some attentional control. For instance, when we are asked to mentally multiply 45 by 4, we have to perform a series of simple calculations (additions and multiplications) to arrive at the final answer. The ability to store the information regarding the instructions and intermediate results is what is referred to as working memory.

Classification by information type

Long-term memory, the largest part of any model, can be divided into declarative (explicit) and procedural (implicit) memories.

Declarative memory requires conscious recall, in that some conscious process must call back the information. It is sometimes called explicit memory, since it consists of information that is explicitly stored and retrieved.

Declarative memory can be further sub-divided into semantic memory, which concerns facts taken independent of context; and episodic memory, which concerns information specific to a particular context, such as a time and place. Semantic memory allows the encoding of abstract knowledge about the world, such as "Paris is the capital of France". Episodic memory, on the other hand, is used for more personal memories, such as the sensations, emotions, and personal associations of a particular place or time. Autobiographical memory - memory for particular events within one's own life - is generally viewed as either equivalent to, or a subset of, episodic memory.

In contrast, procedural memory (or implicit memory) is not based on the conscious recall of information, but on an implicit learning of certain patterns about the world. It is revealed when we do better in a given task due only to repetition - no new explicit memories have been formed, but we are unconsciously accessing aspects of those previous experiences. Classical conditioning can be seen as a form of implicit memory, as can memory resulting from motor learning, which depends upon the cerebellum and basal ganglia.

Memory disorders

Much of the current knowledge of memory has come from studying memory disorders, which are known collectively as amnesia. There are many sorts of amnesia, and by studying their different forms, it has become possible to observe apparent defects in individual sub-systems of the brain's memory systems, and thus hypothesize their function in the normally working brain.

The physiology of memory

Brain areas such as the mammillary bodies and hippocampus are thought to be involved in memory. It has been demonstrated that damage to these structures can result in impaired performance on certain memory tasks.

Related topics