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For the component in computers and electronics, see memory (computers)

In neuroscience, a memory is the trace of some experienced past event by which that experienced past event can subsequently be re-experienced.

In normal use, we think of memory as information that is encoded in our brains (or minds) in a way that enables it to be stored quietly away, and then subsequently retrieved when needed. That notion has been generalised to the idea of digital electronic information stores as memory, following the perhaps unsafe analogy between animal brains and digital computers. Metaphorically, we sometimes talk of memory in physical objects, such as when a deformable object holds an imprint of a previous form, such as paper in-formed by an ink-press, creating a book.

This article is about memory as information stored in our brains for subsequent deliberate retrieval. As we experience new things every day of our lives, our brains change in small but sometimes important ways. Nothing in our brains is fixed, neurons are dying steadily, but some new ones are being born throughout our lives. Each of the nerve cells neurons)in our brain makes many thousands of connections, called synapses, with other nerve cells - and these are constantly in flux, the number and strength of these connections changes according to our experience.

Thus experience changes our brains in those and many other ways, and in a sense, all of those changes are part of learning - in that they are the mechanisms by which past experience influences future behaviour. In a sense all learning involves 'memory', but memory is not just about learning. In natural usage, a memory is a detailed reconstruction in the mind of some past event. We believe that ultimately memories must be encoded in the brain by changes in the patterns or strengths of connectivity between neurons. We also know that some parts of the brain like the hippocampus are especially important for remembering certain types of things.

However, to pretend that neuroscientists have an adequate understanding of how even the simplest memory of an event is encoded in the brain would be quite wrong. There is an enormous "gap" in our understanding; we know a great deal about the fundamental mechanisms by which nerve cells operate, but we don't know how these allow us to store and retrieve memories as we understand them.

The typical person can store approximately seven items in short term memory.[1]

Memory and mind

In relation to the processes constituting the human mind, the term memory serves the generic function of interrelating numerous species of physicochemical processes targeting storage and retrieval of past experiencing, defined as the receiving, processing, and responding to information about events of reality internal or external to the organism. Those interrelated physicochemical processes enable what we call ‘remembering’, the foundation of the human mind and its manifestation as intelligence, the ability to adapt, through learning, to impinging events of reality.

Receiving information about fire burning your hand—an event of reality—processing that information to generate a response to it in accord with the biological imperative for self-preservation, experiencing the event, in other words, initiates the interrelated processes targeting storage of that experiencing for remembering it the next time you have the opportunity to decide what to do with your hand near a fire. Memory enables the learning, learning enhances the ability for intelligent behavior.

Neurological Basis of Memory

The neuropsychologist and theoretical neuroscientist Donald Hebb (1904 - 1985) was the first to distinguish between short-term memory and long-term memory. When the brain receives a sensory input, for example, visual and auditory stimuli, a sensory memory retains an exact copy of what is seen or heard, but this memory lasts for at most only a few seconds. What is retained longer than this depends on selective attention - things that we "notice" may be stored in short-term memory for up to a few minutes. This memory is thought to depend on electrical activity in neuronal circuits, and is very easily destroyed by interruption or interference. Short term memory includes iconic memory, to hold visual images; acoustic memory, to hold sounds; and working memory, an active process to keep a memory until it is put to use.

Memories stored for longer than this are stored in long-term memory. Hebb's main contribution to neuroscience was his theory that the basis of long-term memory was a form of synaptic plasticity - a long term alteration in the strength of connections between neurons now thought to involve a phenomenon called "long-term potentiation" (LTP). This is relatively permanent storage, and it requires the synthesis of new proteins.

Whether information is stored in long-term memory depends on its 'importance'; for any animal, memories associated with stress or trauma are potentially important for the adaptive value that such memories have for future avoidance behaviour, and the hormones that are released during stress are thought to have an important role in determining what memories are preserved. In humans, acute traumatic stress is associated with acute secretion of epinephrine and norepinephrine (adrenaline and noradrenaline) from the adrenal medulla and more prolonged secretion of cortisol from the adrenal cortex. Acute increases in these hormones are thought to facilitate memory while chronic stress associated with prolonged hypersecretion of cortisol may have the opposite effect. The limbic system, including the hippocampus and amygdala in particular, is critically involved in memory storage and retrieval as well as giving emotional significance to sensory inputs.The hippocampus is important for explicit memory, and for memory consolidation; it is also very sensitive to stress and has a role in recording the emotions of a stressful event. The hippocampus receives input from many different parts of the neocortex and sends its output out to different parts of the brain. The amygdala is thought to assign emotional values to sensory inputs which are then elaborated upon by the neocortex and imbued with personal meaning; thus patients with amygdalar damage are no more likely to remember emotionally charged words than nonemotionally charged ones. The amygdala may also integrate internal representations of the external world in memory image form associating emotional experiences with these memories. The septo-hippocampal system is thought to record memory in temporal and spatial dimensions, and plays an important role in storing and categorizing incoming stimuli in memory.

Types of Memory

Transactive memory [2] and extrasomatic knowledge[3] are similar concepts.

Memory can also be described by described by the degree of memorization into the categories:[4]

  • Remembered. The fact is remembered as well as details of how the fact was learned
  • Know
  • Familiar. The specific fact is not remembered, but felt familiar, perhaps like recognizing a telephone number that has been stored in speed dial.

"Familiar" knowledge should be subject to successful information recovery, especially is the user knows how to use tools for information recovery such as Google.[5]

The benefits of enhancing personal knowledge with information retrieval (more specifically, information recovery) of extrasomatic knowledge has been shown in comparisons with rote memory.[6][5]


  1. Miller GA (1956) The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychol Rev 63:81–97
  2. Sparrow, Betsy; Jenny Liu, Daniel M. Wegner (2011-07-14). "Google Effects on Memory: Cognitive Consequences of Having Information at Our Fingertips". Science. DOI:10.1126/science.1207745. Retrieved on 2011-07-16. Research Blogging.
  3. Sagan, Carl (1993). The Dragons of Eden: Speculations on the Evolution of Human Intelligence. New York: Ballantine Books. ISBN 0-345-34629-7. 
  4. Barber SJ, Rajaram S, Marsh EJ (2008). "Fact learning: how information accuracy, delay, and repeated testing change retention and retrieval experience.". Memory 16 (8): 934-46. DOI:10.1080/09658210802360603. PMID 18949663. Research Blogging.
  5. 5.0 5.1 Sparrow B, Liu J, Wegner DM (2011). "Google effects on memory: cognitive consequences of having information at our fingertips.". Science 333 (6043): 776-8. DOI:10.1126/science.1207745. PMID 21764755. Research Blogging.
  6. de Bliek R, Friedman CP, Wildemuth BM, Martz JM, Twarog RG, File D (1994). "Information retrieved from a database and the augmentation of personal knowledge". J Am Med Inform Assoc 1 (4): 328–38. PMID 7719819[e]