Learn more about Memory
In psychology, 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 a 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 200 - 500 ms after an item is perceived. Your ablitity to look at an item, and remember what it looked like with just a second of observation, or memorization, is an example of sensory memory. With very short presentations, participants often report that they seem to "see" more than they can actually report. The first experiments exploring this form of sensory memory were conducted by George Sperling using the "partial report paradigm." Subjects were presented with a grid of 12 letters, arranged into three rows of 4. After a brief presentation, subjects were then played either a high, medium or low tone, cuing them which of the rows to report. Based on these partial report experiments, Sperling was able to show that the capacity of sensory memory was approximately 12 items, but that it degraded very quickly (within a few hundred milliseconds). Because this form of memory degrades so quickly, participants would see the display, but be unable to report all of the items (12 in the "whole report" procedure) before they decayed. However, by using a smaller number of items, the items in sensory memory could be transferred to short-term memory. This type of memory cannot be prolonged via rehearsal.
Some of the information in sensory memory is then transferred to short-term memory. Short-term memory allows one to recall something from several seconds to as long as a minute without rehearsal. If people are allowed to rehearse (repeat the information in their head) they can remember for even longer periods of time. George Miller, when working at Bell Laboratories, conducted experiments showing that the store of short term memory was 7 + or - 2 items (the title of his famous paper, "The magic number 7 +/- 2"). Modern estimates of the capacity of short-term memory are lower, typically on the order of 4-5 items, and we know that memory capacity can be increased through a process called chunking. For example, if presented with the string:
FB IPH DTW AIB M
people are able to remember only a few items. However, if the same information is presented in this following way:
FBI PHD TWA IBM
people can remember a great deal more letters. This is because they are able to chunk the information into meaningful groups of letters.
The storage in the sensory memory and short term memory generally have a strictly limited capacity and duration, which means that information is available for a certain period of time, but is not retained indefinitely. 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.
Short-term memory is supported by transient patterns of neuronal communication, dependent on regions of the frontal lobe (especially dorsolateral prefrontal cortex) and the parietal lobe. Long-term memories, on the other hand, are maintained by more stable and permanent changes in neural connections widely spread throughtout the brain. The hippocampus is essential to the consolidation of information from short-term to long-term memory, although it does not seem to store information itself. Rather, it may be involved in changing neural connections for a period of three months, or more, after the initial learning. One of the main functions of sleep is thought to be to improve consolidation of information, as it can be shown that memory depends on getting sufficient sleep between training and test, and that the hippocampus replays activity from the current day while sleeping.
For example, if we are given a random seven-digit number, we may remember it only for a few seconds and then forget,which means it was stored into our short-term memory. On the other hand, we can remember telephone numbers for many years through repitition;those long-lasting memories are said to be stored in our long term memory.
Early theorists argued that the distinction between long and short term memories was arbitrary, reflecting nothing more than differing the levels of activity. However, since the 1960s, and the study of patients who have had lesions to the hippocampus and other brain stuctures, it is now clear that there are multiple memory systems in the brain, and that information must be transferred between these systems.
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. The most well-known, and well-developed theory of working memory was developed by Alan Baddeley who argued that it was composed of three parts: a "verbal store" (akin to rehearsing a phone number) a "visuo-spatial sketchpad" (for visual memory) and a "central executive" involved in control and rehearsal. It is possible to show that the verbal store and visuo-spatial sketchpad are independent systems by asking people to remember two sets of information, one verbal the other visual. Although there is a slight decrease in memory performance (thought to be due to increased load on the central executive) this decrease is much less than when people are asked to remember two sets of verbal or two sets of visual information. The neural bases of the verbal store and visuospatial sketchpad are thought to lie in the relevant sensory areas of the brain (auditory and visual cortex, respectively) while the central executive is thought to depend on frontal lobe structures.
 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 directionretrospective 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 specific types 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, namely 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.
Impaired Memory can be a symptom of Hypothyroidism.
Memorization, which includes (but is not limited to) rote learning, is a method of learning that allows an individual to recall important information verbatim. Rote rehearsal involves repetition, that an individual can learn a necessary process or amount of information through repetitive action or study, so that it gets to the point that it becomes almost automatic for them to recite it with ease. This technique is the most often one used. In memorizing items such as words or facts, it can often increase efficiency to be used during more involved techniques, such as story telling. The spacing effect 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.
 Quotes about memory
- "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
- "Memories are sometimes based in fact" - John Leax professor of writing at Houghton College.
 Popular Culture
- The experimental film Memento emulates the experience of not being able to form long term memories.
- Memory plays a central role in the video game series, 'Kingdom Hearts.'
- The short stories of Philip K. Dick and the movies based on those works deal extensively with the nature of memory and the consequences to society if memories can be artificially generated.
- Strange Days (film) is a film about memory. New technology allows people to record their all the sensory data associated with their experiences. Playing back one of these recordings is like exactly reliving moments. The character played by Ralph Fiennes is still in love with his ex-girlfriend, played by Juliette Lewis. He keeps replaying recorded memories of their time together. She tells him to stop, saying, "Memories fade. They're made that way for a reason."
- Eternal Sunshine of the Spotless Mind is a film about removing a person from one's memory; all events shared with this person are erased through a medical procedure.
Anderson, J.R. (1976) Language, Memory and Thought. Mahwah, NJ: Erlbaum
 See also
- Aging and memory
- Attention versus memory in prefrontal cortex
- Cellular memory
- Emotion and memory
- False Memory
- Forgetting curve
- Giordano Bruno
- 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
 External links
- Stanford Encyclopedia of Philosophy entry
- Memory-related resources from the National Institutes of Health.
- Scientific American Magazine (February 2005 Issue) Making Memories Stick
- Aristotle's On Memory and Reminiscence
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