Memory is the ability of an organism to store, retain, and subsequently recall information. Although traditional studies of memory began in the realms of philosophy, the late nineteenth and early twentieth century put memory within the paradigms of cognitive psychology. In the recent decades, it has become one of the principal pillars of a new branch of science that represents a marriage between cognitive psychology and neuroscience, called cognitive neuroscience.
There are several ways of classifying memories, based on duration, nature and retrieval of information. From an information processing perspective there are three main stages in the formation and retrieval of memory:
- Encoding (processing and combining of received information)
- Storage (creation of a permanent record of the encoded information)
- Retrieval/Recall (calling back the stored information in response to some cue for use in some process or activity)
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 milliseconds 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 (assuming we use them often enough). Those long-lasting memories are said to be stored in 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
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. Visual memory is part of memory preserving some characteristics of our senses pertaining to visual experience. We are able to place in memory information that resembles objects, places, animals or people in sort of a mental image. Visual memory can result in priming and it is assumed some kind of perceptual representational system or PRS underlies this phenomenon. 
In contrast, procedural memory (or implicit memory) is not based on the conscious recall of information, but on implicit learning. Procedural memory is primarily employed in learning motor skills and should be considered a subset of implicit memory. 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. Procedural memory involved in motor learning depends on the cerebellum and basal ganglia.
So far, nobody has successfully been able to isolate the time dependence of these suggested memory structures.
Classification by temporal direction
A further major way to distinguish different memory functions is whether the content to be remembered is in the past, retrospective memory, or whether the content is to be remembered in the future, prospective memory. Thus, retrospective memory as a category includes semantic memory and episodic/autobiographical memory. In contrast, prospective memory is memory for future intentions, or remembering to remember (Winograd, 1988). Prospective memory can be further broken down into event- and time-based prospective remembering. Time-based prospective memories are triggered by a time-cue, such as going to the doctor (action) at 4pm (cue). Event-based prospective memories are intentions triggered by cues, such as remembering to post a letter (action) after seeing a mailbox (cue). Cues do not need to be related to the action (as the mailbox example is), and lists, sticky-notes, knotted hankerchiefs, or string around the finger (see box) are all examples of cues that are produced by people as a strategy to enhance prospective memory.
Overall, the mechanisms of memory are not well understood. Brain areas such as the hippocampus, the amygdala, or the mammillary bodies are thought to be involved in certain kinds of memory. For example, the hippocampus is believed to be involved in spatial learning and declarative learning. Damage to certain areas in patients and animal models and subsequent memory deficits is a primary source of information. However, rather than implicating a specific area, it could be that damage to adjacent areas, or to a pathway traveling through the area is actually responsible for the observed deficit. Further, it is not sufficient to describe memory, and its counterpart, learning, as solely dependent on specific brain regions. Learning and memory are attributed to changes in neuronal synapses, thought to be mediated by long-term potentiation and long-term depression. Other scientists who have investigated the nature of memory, namley neurologists John Carew Eccles and Wilder Penfield and biologist Rupert Sheldrake, have suggested that memories are a field phenomenon and are not stored in the brain at all, but rather accessed through neurological structures.
Much of the current knowledge of memory has come from studying memory disorders. Loss of memory is known 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. Other neurological disorders such as Alzheimer's disease can also affect memory and cognition.
Memorization, which often includes (but is not limited to) rote learning, is a method of learning that conditions an individual to recall important information verbatim. Rote rehearsal involves repetition, with the assumption that an individual can learn a necessary process or an amount of information through repetitive action or study, such to the point that it becomes near-automatic. This technique is the most often one used, though there are alternatives. In memorizing items such as words, it can often increase efficiency to be used during more involved techniques, such as story-telling. Related subjects include the spacing-effect, which shows that an individual is more likely to remember a list of items when rehearsal is spaced over an interval of time. (For example, in memorizing words such as dog, fish, house, book, it is more effective to rehearse dog-fish-house-book-dog-fish-house-book, etc., than dog-dog-fish-fish-house-house-book-book.)
Artworks often explore the nature of memory. The film Memento, about a man afflicted with anterograde amnesia, reflects on the nature and meaning of memory, and implications of its loss. The film Eternal Sunshine of the Spotless Mind deals with the consequences of forcefully forgetting a loved one, and the regret that may bring. The paintings of Howard Hodgkin, while apparently abstract, are said by the artist to be representations of his memories and their emotional associations. The late works of the 20th-century composer Morton Feldman explore the nature of memory and methods through which it can be disorientated. Several works of the Czech author Milan Kundera explore the nature of personal memory in relation to social or historical memory, especially the novels Ignorance, The Book of Laughter and Forgetting, and Immortality. Perhaps the most renowned work of literature dealing largely with the concept of memory is Marcel Proust's monumental In Search of Lost Time. Many of Jorge Luis Borges's works explored memory, often as a symbol of the infinite. In Funes the Memorious, for example, a young man loses the ability to forget, and consequently the ability to generalize and reduce his experiences to abstract concepts. Several works by British playwright Harold Pinter also dramatize aspects of memory and are often referred to as his "memory plays," including his stage play and tv film Old Times, his stage play and tv film No Man's Land, his stage play and film Betrayal, his as-yet unfilmed The Proust Screenplay, based on Remembrance of Things Past, by Marcel Proust, and his collaborative stage adaptation featuring the latter title.
Looking at wider cultural influences, Sigmund Freud's concept of memory has been extremely important to twentieth century art, especially as stated in his 1899 essay Screen Memories. In this essay he discussed the way in which mild and inoffensive childhood memories might "screen" other more frightening or dramatic ones. The masking of one memory by another, and its gradual revelation, has featured as a plot device in film, particularly, from Hitchcock's Spellbound, to Total Recall, to Ridley Scott's Blade Runner.
- Aging and memory
- Attention versus memory in prefrontal cortex
- Cellular memory
- Educational psychology
- Emotion and memory
- False Memory
- Forgetting curve
- Hebbian learning
- Involuntary Memory
- Kinesthetic learning
- List of memory biases
- Long-term potentiation
- Memory bias
- Memory inhibition
- The Memory-Prediction Framework - an acclaimed "unifying theory of memory"
- MemoryArchive, a wiki collecting memories
- Misinformation effect
- Muscle memory or proprioception - the sense and memory of how parts of the body are trained to move
- Neural adaptation
- Source amnesia
- Synaptic plasticity
- Draaisma D (2005) Why Life Speeds Up As You Get Older: how memory shapes our past (CUP, Cambridge).
- Tulving E and Craik F (eds). (2000). The Oxford Handbook of Memory. Oxford University Press, New York.
- Jung, Carl Gustave. Synchronicity -- An Acausal Connecting Principle, 1952, Princeton University Press.
- Wieman C and Perkins K, Transforming Physics Education, Physics Today, November 2005
- "Memory is but the storage of fragmentary but 'relevant' features" - Walter J. Ong
- "It's a poor sort of memory that only works backwards" - Through the Looking-Glass by Lewis Carroll, Ch. 5, Wool and Water.
- "The existence of writing changed the nature of memory" - Jennifer Wise, in Dionysus Writes: The Invention of Theatre in Ancient Greece, 1998, p.25
- "The language of the Homeric epic exhibits a 'formulaic' linguistic style [to aid the memory]" - Jennifer Wise, in Dionysus Writes: The Invention of Theatre in Ancient Greece, 1998, p.27
- "I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skilful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it - there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones." A Study In Scarlet by Sir Arthur Conan Doyle, Ch. 2, The science of Deduction
- "Great is this power of memory, exceedingly great, O my God -- a large and boundless inner hall! Who has plumbed the depths of it? Yet it is a power of my mind, and it belongs to my nature." Confessions (St. Augustine) Book 10, Chapter 8
Anderson, J.R. (1976) Language, Memory and Thought. Mahwah, NJ: Erlbaum