Memory in health and disease has been the focus of medical studies throughout history. In the first century, Pliny the Elder described a man who fell off a roof and afterward could not remember his mother, neighbors, and friends. Galen (130-200) placed emphasis on the ventricles as the anatomical key to mental processing, a view advocated by Nemesius of Syria, Posidonius of Byzantium, and several other early religious leaders whose views influenced medical thought. The perceived role of the ventricles declined during the Renaissance, and Thomas Willis entertained the idea that the cerebellum and brain stem controlled memory. Later, such figures as David Harley (1Z05-1Z5Z) and Charles Bonnet viewed memory in terms of vibration waves that oscillated in the brain. Scientific measurement of memory began in the late 1800s with Ebbinghaus' monograph and turned the study of memory away from philosophers and toward experimentalists. Studies of dementia, brain atrophy and memory decline, and trauma in relation to amnesia were conducted by several neurological luminaries including Samuel Wilks, Jean Esquirol, and Alois Alzheimer. Linkage of memory with emotional elements was presented by James Papez, whose anatomical studies form much of the basis of modern knowledge of memory circuitry.
Memory is the recording, retention, and retrieval of knowledge. It accounts for all knowledge gained from experience-facts that are known, events that are remembered, and skills that are gained and applied. Memory is not a unitary faculty but rather an ensemble of various forms of learning that differ in their uses, their operating characteristics, and the neural networks that mediate their processing. A memory system may be defined as a particular neural network that mediates a specific form of mnemonic processing. Neurological or psychiatric diseases result in characteristic mnemonic deficits that reflect which memory systems are injured by a particular disease.
In regard to memory, several distinctions should be made between aspects of memory that are useful from the clinical perspective and those that are useful from the neuroscience perspective. One such distinction is between declarative and nondeclarative forms of memory.^ Declarative memory corresponds to the everyday sense of memory and is responsible for the learning and remembrance of new events, facts, and materials. Thus, it encompasses both episodic memories (remembrance of personal experiences that took place at a particular place and time) and semantic memories (knowledge of generic information, such as the meaning of a word). y It is the form of memory people use to recollect facts and events consciously and intentionally, and is therefore also referred to as explicit memory.
Nondeclarative memory refers to the many forms of memory that are not retrieved explicitly or intentionally but reflexively or incidentally. Remembering how to swim or ride a bicycle belong in this category. Nondeclarative forms of memory do not depend on the psychological processes or brain regions that are vital for declarative memory. These forms of memory guide current behavior on the basis of past experiences unrelated to any conscious awareness of those experiences, and therefore are referred to as implicit memory. Implicit forms of memory include perceptual, motor, and cognitive skill learning (sometimes referred to as procedural memory), which is the increased
accuracy, speed, or skill acquired for a given task during multiple training sessions in the absence of conscious awareness; classic and other sorts of conditioning, in which repeated pairing of an unconditioned stimulus such as a tone with an unconditioned response such as salivation at the sight of food, leads to a conditioned response (salivation) when the tone is presented in the absence of the evoking stimulus (food); and repetition priming, which is the facilitated processing of a stimulus, such as a word or picture, due to prior exposure to that stimulus.
Temporal properties also distinguish one form of memory from another. Immediate memory refers to the recall of information without delay, either immediately after presentation or after uninterrupted rehearsal. Immediate memory is characterized by sharply limited capacities for how much and how long information can be remembered. For example, most people can remember no more than seven random digits and only as long as they rehearse the digits. Immediate memory often has perceptual characteristics. For example, errors in immediate memory for words are more likely to reflect word sounds than word meanings. Working memory is a multicomponent psychological system that mediates the temporary processing and storage of internal representations that guide and control action. Information is held in working memory only as long as it is useful for solving a problem at hand.
Long-term memory refers to permanent and large stores of episodic and semantic memories. Long-term memories are not, however, passively stored records of experience. Rather, they are constantly used to interpret new experiences. That ongoing use can alter the original memory (retrograde interference) and yield inaccurate memories (false or distorted memories). Long-term memory is often organized by meaning (semantics) or gist rather than by perceptual characteristics of experience. For example, people remember the content of a sentence they have read far better than the specific order of words or the font in which the sentence was seen. Similarly, people remember a set of related words (e.g., fruits) better than a set of unrelated words. It is common, however, to believe falsely that a particular fruit was in the set of words when it was not actually presented.
The relation between short-term and long-term memory is often misunderstood, in part because of various meanings of the term short-term memory. Sometimes the term short-term memory is used to refer to attention. An individual who cannot repeat back even a single digit or word is better described as having a deficit in attention than a deficit in any sort of memory. A second use of short-term memory is to refer to a memory mechanism that supports remembrance of information after a delay of seconds or minutes and without rehearsal. There is, however, no evidence of any anatomical memory system that has a temporal span between immediate and long-term. There is, instead, constant and rapid loss of long-term memory for most events and facts. Therefore, memory of information after a brief delay of seconds or minutes will be superior to memory after a longer delay, but this difference reflects the dynamics of long-term memory and not the existence of any particular short-term memory store. It is often thought that information must go through short-term memory in order to reach long-term memory. This is correct if short-term memory refers to attention because people do not remember that which they do not notice. Surprisingly, this notion is incorrect if short-term memory refers to immediate memory. Patients with severe immediate memory deficits show minimal long-term memory deficits. This finding indicates that information from the environment enters immediate-memory and long-term memory stores in parallel rather than serially.
There is also a relation between working and long-term memory that is apparent on strategic memory tasks. Strategic memory tasks require manipulation of long-term memories rather than the mere retrieval of those memories. The mere retrieval of memories, for example, is sufficient to support performance on a recognition test in which an individual is asked to differentiate items that were seen earlier from those that were not. In contrast, an individual must initiate and use a strategy when asked to recall what items were seen or in what order the items were seen. For these strategic memory tasks, individuals must use a plan to "work with" long-term memories in order to solve a memory problem.
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