the human memory - luke mastin [2010]

8/17/2019 The Human Memory - Luke Mastin [2010]

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© 2010 Luke Mastin

http://www.human-memory.net/index.html

INTRODUCTION

THE HUMAN MEMORY

Since time immemorial, humans havetried to understand what memory is,

how it works and why it goes wrong. Itis an important part of what makes ustruly human, and yet it is one of themost elusive and misunderstood ofhuman attributes.

The popular image of memory is as akind of tiny filing cabinet full ofindividual memory folders in whichinformation is stored away, or perhaps asa neural super-computer of huge

capacity and speed. However, in the lightof modern biological and psychologicalknowledge, these metaphors may not be entirely useful and, today, experts believe thatmemory is in fact far more complex and subtle than that

It seems that our memory is located not in one particular place in the brain, but isinstead a brain-wide process. For example, the simple act of riding a bike is activelyand seamlessly reconstructed by the brain from many different areas. The memory ofhow to operate the bike comes from one area, the memory of how to get from here tothe end of the block comes from another, the memory of biking safety rules fromanother, and that nervous feeling when a car veers dangerously close comes from still

another. Each element of a memory (sights, sounds, words, emotions) is encoded in thesame part of the brain that originally created that fragment (visual cortex, motor cortex,language area, etc), and recall of a memory effectively reactivates the neural patternsgenerated during the original encoding. Thus, a better image might be that ofa complex web, in which the threads symbolize the various elements of a memory, that

join at nodes or intersection points to form a whole rounded memory of a person, objector event. This kind of distributed memory ensures that even if part of the brain is

The human brain, one of the most complex livingstructures in the universe, is the seat of memory

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damaged, some parts of an experience may still remain. Neurologists are onlybeginning to understand how the parts are reassembled into a coherent whole.

Neither is memory a single unitary process but there are different types of memory.Our short term and long-term memories are encoded and stored in different ways and

in different parts of the brain, for reasons that we are only beginning to guess at.Years of case studies of patients suffering from accidents and brain-related diseasesand other disorders have begun to indicate some of the complexities of the memoryprocesses, and great strides have been made in neuroscience and cognitivepsychology, but many of the exact mechanisms involved remain elusive.

This website, written by a layman for the layman, attempts to piece together some ofwhat we DO know about the enigma that is...The Human Memory.

WHAT IS MEMORY?

Memory is our ability to encode, store, retain andsubsequently recall information and past experiences inthe human brain.

It is the sum total of what we remember , and gives us thecapability to learn and adapt from previous experiences aswell as to build relationships. It is the ability toremember past experiences, and the power or process ofrecalling to mind previously learned facts, experiences,impressions, skills and habits. It is the store of thingslearned and retained from our activity or experience, as

evidenced by modification of structure or behaviour, or byrecall and recognition.

Etymologically, the modern English word “memory” comesto us from the Middle English memorie, which in turn comesfrom the Anglo-French memoire or memorie, and ultimatelyfrom the Latin memoria and memor , meaning "mindful" or"remembering".

In more physiological or neurological terms, memory is, at its simplest, a set ofencoded neural connections in the brain. It is the re-creation or reconstruction of past

experiences by the synchronous firing of neurons that were involved in the originalexperience. As we will see, though, because of the way in which memory is encoded, itis perhaps better thought of as a kind of collage or jigsaw puzzle, rather than in thetraditional manner as a collection of recordings or pictures or video clips, stored asdiscrete wholes. Our memories are not stored in our brains like books on libraryshelves, but are actually on-the-fly reconstructions from elements scatteredthroughout various areas of our brains.

??? Did You Know ???

For a time during the 1960s, itwas hypothesized that all thecells of the human body werecapable of storing memories,not only those in the brain, anidea known as cell memory orcellular memory.This was based on memorytransfer research usingcannibal flatworms, and onanecdotal evidence of organ

transplants where the recipientwas reported to havedeveloped new habits ormemories, but such theoriesare nowconsidered pseudoscientificand have not made it intopeer-reviewed science

journals.

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Memory is related to but distinct from learning, which is theprocess by which we acquire knowledge of the world andmodify our subsequent behaviour. During learning, neuronsthat fire together to produce a particular experience arealtered so that they have a tendency to fire together again.

For example, we learn a new language by studying it, butwe then speak it by using our memory to retrieve the wordsthat we have learned. Thus, memory depends on learningbecause it lets us store and retrieve learned information. Butlearning also depends to some extent on memory, in thatthe knowledge stored in our memory provides theframework to which new knowledge is linked by associationand inference. This ability of humans to call on pastmemories in order to imagine the future and to plan futurecourses of action is a hugely advantageous attribute in oursurvival and development as a species.

Since the development of the computer in the 1940s, memory is also used to describethe capacity of a computer to store information subject to recall, as well as the physicalcomponents of the computer in which such information is stored. Although there areindeed some parallels between the memory of a computer and the memory of a humanbeing, there are also some fundamental and crucial differences, principally that thehuman brain is organized as a distributed network in which each brain cell makesthousands of connections, rather than as an addressable collection of discrete files.

The sociological concept of collective memory plays an essential role in theestablishment of human societies. Every social group perpetuates itself through the

knowledge that it transmits down the generations, either through oral tradition orthrough writing. The invention of writing made it possible for the first time for humanbeings to preserve precise records of their knowledge outside of their brains. Writing,audiovisual media and computer records can be considered a kind of externalmemory for humans.


Recent studies suggest thatrepeated bouts of jet lag maycause harm to the temporallobe, an area of the brain

important to memory, causingit to shrink in size, andcompromising performance onspatial memory tests.It is thought that stresshormones, such as cortisol,released by the body duringtimes of stress (such as thesleep disturbance, generalstress and fatigue caused bylong flights) are responsible forthis impairment of memory andother mental skills.


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THE STUDY OF HUMAN MEMORY

The study of human memory stretches back at least 2,000years to Aristotle’s early attempts to understand memory inhis treatise “On the Soul” . In this, he compared the human

mind to a blank slate and theorized that all humans areborn free of any knowledge and are merely the sum of theirexperiences. Aristotle compared memory to makingimpressions in wax, sometimes referred to asthe "storehouse metaphor", a theory of memory whichheld sway for many centuries.

In antiquity, it was generally assumed that there were twosorts of memory: the “natural memory” (the inborn onethat everyone uses every day) and the “artificialmemory” (trained through learning and practice of a variety

of mnemonic techniques, resulting in feats of memory thatare quite extraordinary or impossible to carry out using thenatural memory alone). Roman rhetoricians suchas Cicero and Quintillian expanded on the art ofmemory or the method of loci (a method often firstattributed to Simonides of Creos or the Pythagoreans),and their ideas were passed down to the medievalScholastics and later scholars of the Renaissancelike Matteo Ricci and Giordano Bruno.

The 18th Century English philosopher David Hartley was the first to hypothesize that

memories were encoded through hidden motions in the nervous system, although hisphysical theory for the process was rudimentary at best. William James in Americaand Wilhelm Wundt in Germany, both considered among the founding fathers ofmodern psychology, both carried out some early basic research into how the humanmemory functions in the 1870s and 1880s (James hypothesized the idea of neuralplasticity many years before it was demonstrated). In 1881,Théodule-ArmandRibot proposed what became known as Ribot's Law, which states that amnesia has atime-gradient in that recent memories are more likely to be lost than the more remotememories (although in practice this is actually not always the case).

However, it was not until the mid-1880s that the young German philosopher Herman

Ebbinghaus developed the first scientific approach to studying memory. He didexperiments using lists of nonsense syllables, and then associating them withmeaningful words, and some of his findings from this work (such as the concepts ofthe learning curve and forgetting curve, and his classification of the threedistinct types of memory: sensory, short-term and long-term) remain relevant to thisday.


Proponents of the “tabula

rasa” (blank slate) thesisfavour the nurture side of thenature versusnurture debate, when it comesto aspects of personality,intelligence and social andemotional behaviour.The idea first surfaced in atreatise of Aristotle, but thenlay dormant for over athousand years untildeveloped by the 11th CenturyPersianphilosopher Avicenna, andthen John Locke’s classicstatement of the theory in the17th Century.Sigmund Freud revived theidea in the 20th Century,depicting personality traits asbeing formed by familydynamics.


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The German evolutionary biologist Richard Semon first proposed in 1904 the idea thatexperience leaves a physical trace, which he called an engram, on specific websof neurons in the brain. The British psychologist Sir Frederick Bartlett is consideredone of the founding fathers of cognitive psychology, and his research in the 1930s intothe recall of stories greatly influenced later ideas on how the brain stores memories.

With advances in technology in the 1940s, the field of neuropsychology emerged andwith it a biological basis for theories of encoding. Karl Lashley devoted 25 years of hislife to research on rats in mazes, in a systematic attempt to pinpoint where memorytraces or engrams are formed in the brain, only to conclude in 1950 that memories arenot localized to one part of the brain at all, but are widely distributed throughout thecortex, and that, if certain parts of the brain are damaged, other parts of the brain maytake on the role of the damaged portion.

The Canadian neurosurgeon Wilder Penfield’s work on thestimulation of the brain with electrical probes in the 1940s

and 1950s, initially in search of the causes of epilepsy,allowed him to create maps of the sensory and motorcortices of the brain that are still used today, practicallyunaltered. He was also able to summon up memoriesor flashbacks (some of which the patients had noconscious recollection of) by probing parts of the temporallobe of the brain.

As early as 1949, another Canadian, Donald Hebb, intuitedthat “neurons that fire together, wire together”, implying thatthe encoding of memories occurred as connections

between neurons were established through repeated use.This theoretical idea, sometimes referred to as Hebb’sRule, was supported by the discovery of the mechanics ofmemory consolidation, long-term potentiation and neuralplasticity in the 1970s, and remains the reigning theorytoday. Eric Kandel’s work on sea-slugs (whose brains arerelatively simple and contain relatively large, and easily-observed, individual neuralcells) was particularly important in experimentally demonstrating Hebb’s Rule andidentifying the molecular changes during learning, and the neurotransmittersinvolved.

As computer technology developed in the 1950s and 1960s, parallels betweencomputer and brain processes became apparent, leading to advances in theunderstanding of the encoding, storage and retrieval processes of memory. Thecomputer metaphor is, however, essentially just a more sophisticated version of theearlier storehouse view of memory, based on the rather simplistic and misleadingassumption that a memory is just a simple copy of the original experience.


Flashbacks are involuntary(and often recurring)memories, in which anindividual has a suddenpowerful re-experiencing of apast memory, sometimes sointense that the person “re-lives” the experience, unableto fully recognize it as amemory and not somethingthat is really happening.Such involuntary memories areoften of traumatic events orhighly-charged emotionalhappenings, and often occur attimes of high stress or fooddeprivation, although the exactcauses and mechanisms arenot clear.

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The change in the overall study of memory during the 1950s and 1960s has come to beknown as the “cognitive revolution”, and led to several new theories on how to viewmemory, and yielded influential books by George Miller , Eugene Galanter , KarlPribram, George Sperling and Ulric Neisser . In 1956, George Miller produced hisinfluential paper on short-term memory and his assessment that our short-term

memory is limited to what he called “the magical number seven, plus or minus two”.

In 1968, Richard Atkinson and Richard Shiffrin first described their modal, or multi-store, model of memory - consisting of a sensory memory, a short-term memory anda long-term memory - which became the most popular model for studying memory formany years. Fergus Craik and Robert Lockhart offered an alternative model, knownas the levels-of-processing model, in 1972. In 1974, Alan Baddeley and GrahamHitch proposed their model of working memory, which consists of the central executive,visuo-spatial sketchpad and phonological loop as a method of encoding.

The 1970s also saw the early work of Elizabeth Loftus, who carried out her influential

research on the misinformation effect, memory biases and the nature of falsememories. The pioneering research on human memory by Endel Tulving from the1970s onwards has likewise been highly influential. He was the first to propose twodistinct kinds of long-term memory, episodic and semantic, in 1972 and he also devisedthe encoding specificity principle in 1983.

During the 1980s and 1990s, several formal models of memory were developed thatcan be run as computer simulations, including theSearch of Associative Memory(SAM) model proposed by Jerome Raaijmaker and Richard Shiffrin in 1981,the Parallel Distributed Processing (PDP) model of James McClelland, DavidRumelhart and Geoffrey Hinton's in 1986, and various versions of the Adaptive Control

of Thought (ACT) model developed by John Anderson in 1993.

Nowadays, the study of human memory is considered part of the disciplinesof cognitive psychology and neuroscience, and the interdisciplinary link between thetwo which is known as cognitive neuroscience.


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TYPES OF MEMORY

What we usually think of as“memory” in day-to-day usageis actually long-term memory,

but there are alsoimportant short-term and sensory memoryprocesses, which must beworked through before a long-term memory can beestablished. The differenttypes of memory each havetheir own particular mode ofoperation, but they allcooperate in the process of

memorization, and can beseen as three necessarysteps in forming a lastingmemory.

This model of memory as asequence of three stages,from sensory to short-term to long-term memory, rather than as a unitary process, is known asthe modal or multi-store or Atkinson-Shiffrin model, after Richard Atkinson andRichard Shiffrin who developed it in 1968, and it remains the most popular model for

studying memory. It is often also described as the process of memory, but I have usedthis description for the processes of encoding, consolidation, storage and recall in theseparate Memory Processes section.

It should be noted that an alternative model, known as the levels-of-processingmodel was proposed by Fergus Craik and Robert Lockhart in 1972, and posits thatmemory recall, and the extent to which something is memorized, is a function of thedepth of mental processing, on a continuous scale from shallow (perceptual)to deep (semantic). Under this model, there is no real structure to memory and nodistinction between short-term and long-term memory.

Types of Human Memory: Diagram by Luke Mastin


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SENSORY MEMORY

Sensory memory is the shortest-term element of memory.It is the ability to retain impressions of sensory informationafter the original stimuli have ended. It acts as a kind

of buffer for stimuli received through the five senses ofsight, hearing, smell, taste and touch, which are retainedaccurately, but very briefly. For example, the ability to lookat something and remember what it looked like with just asecond of observation is an example of sensory memory.

The stimuli detected by our senses can be eitherdeliberately ignored, in which case they disappear almostinstantaneously, or perceived, in which case they enter oursensory memory. This does not require any consciousattention and, indeed, is usually considered to be totally

outside of conscious control. The brain is designed to onlyprocess information that will be useful at a later date, and to allow the rest to pass byunnoted. As information is perceived, it is therefore stored in sensorymemory automatically and unbidden. Unlike other types of memory, the sensorymemory cannot be prolonged via rehearsal.

Sensory memory is an ultra-short-term memory and decays or degrades very quickly,typically in the region of 200 - 500 milliseconds (1/5 - 1/2 second) after the perception ofan item, and certainly less than a second (although echoic memory is now thought tolast a little longer, up to perhaps three or four seconds). Indeed, it lasts for such a shorttime that it is often considered part of the process of perception, but it nevertheless

represents an essential step for storing information in short-term memory.

The sensory memory for visual stimuli is sometimes known as the iconic memory, thememory for aural stimuli is known as the echoic memory, and that for touch asthe haptic memory. Smell may actually be even more closely linked to memory thanthe other senses, possibly because the olfactory bulb and olfactory cortex (wheresmell sensations are processed) are physically very close - separated by just 2 or 3synapses - to the hippocampus and amygdala (which are involved in memoryprocesses). Thus, smells may be more quickly and more strongly associated withmemories and their associated emotions than the other senses, and memories of asmell may persist for longer, even without constant re-consolidation.

Experiments by George Sperling in the early 1960s involving the flashing of a grid ofletters for a very short period of time (50 milliseconds) suggest that the upper limit ofsensory memory (as distinct from short-term memory) is approximately 12 items,although participants often reported that they seemed to "see" more than they couldactually report.


Studies have shown that

attention significantly affectsmemory during the encodingphase, but hardly at all duringrecall.Thus, distractions or dividedattention during initial learningmay severely impairsubsequent retrieval success,whereas distractions at thetime of recall may slow downthe process a little, but haslittle to no effect on itsaccuracy.


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Information is passed from the sensory memory into short-term memory via the processof attention (the cognitive process of selectively concentrating on one aspect of theenvironment while ignoring other things), which effectively filters the stimuli to only thosewhich are of interest at any given time.

SHORT-TERM (WORKING) MEMORY

Short-term memory acts as a kind of “scratch-pad” fortemporary recall of the information which is being processedat any point in time, and has been referred to as "the brain'sPost-it note". It can be thought of as the abilityto remember and process information at the same time. Itholds a small amount of information (typically around 7items or even less) in mind in an active, readily-available state for a short period of time (typically from 10 to15 seconds, or sometimes up to a minute).

For example, in order to understand this sentence, thebeginning of the sentence needs to be held in mind whilethe rest is read, a task which is carried out by the short-termmemory. Other common examples of short-term memory inaction are the holding on to a piece of informationtemporarily in order to complete a task (e.g. “carrying over”a number in a subtraction sum, or remembering apersuasive argument until another person finishes talking), and simultaneous translation(where the interpreter must store information in one language while orally translating itinto another).

However, this information will quickly disappear forever unless we make a consciouseffort to retain it, and short-term memory is a necessary step toward the next stage ofretention, long-term memory. The transfer of information to long-term memory for morepermanent storage can be facilitated or improved by mental repetition of theinformation or, even more effectively, by giving it a meaning and associating it withother previously acquired knowledge. Motivation is also a consideration, in thatinformation relating to a subject of strong interest to a person, is more likely to beretained in long-term memory.


A 2010 University of Stirlingstudy has suggested apossible link between poorshort-term or working memoryand depression.The 10 to 15% with thepoorest working memory in thestudy tended to mull thingsover and brood too much,leading to a risk of depression.People with a good workingmemory, on the other hand,are more likely tobe optimistic and self-assured, and more likely tolead a happy and successfullife.


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The term working memory is often used interchangeablywith short-term memory, although technically workingmemory refers more to the whole theoretical framework ofstructures and processes used for the temporary storageand manipulation of information, of which short-term

memory is just one component.

The central executive part of the prefrontal cortex at thefront of the brain appears to play a fundamental role inshort-term and working memory. It both serves as atemporary store for short-term memory, where information iskept available while it is needed for current reasoningprocesses, but it also "calls up" information from elsewherein the brain. The central executive controls two neuralloops, one for visual data (which activates areas nearthe visual cortex of the brain and acts as a visual scratch

pad), and one for language (the "phonological loop",which uses Broca's area as a kind of "inner voice" that repeats word sounds to keepthem in mind). These two scratch pads temporarily hold data until it is erased by thenext job.

Although the prefrontal cortex is not the only part of the brain involved - it must alsocooperate with other parts of the cortex from which it extracts information for briefperiods - it is the most important, and Carlyle Jacobsen reported, as early as 1935,that damage to the prefrontal cortex in primates caused short-term memory deficits.

The short-term memory has a limited capacity, which can be readily illustrated by the

simple expedient of trying to remember a list of random items (without allowingrepetition or reinforcement) and seeing when errors begin to creep in. The often-citedexperiments by George Miller in 1956 suggest that the number of objects an averagehuman can hold in working memory (known as memory span) is between 5 and 9 (7 ±2, which Miller described as the “magical number”, and which is sometimes referred toas Miller's Law). However, although this may be approximately true for a population ofcollege students, for example, memory span varies widely with populations tested, andmodern estimates are typically lower, of the order of just 4 or 5 items.


A recent study at theUniversity of Michigansuggests that attention andshort-term memory processing

are directly affected by aperson's surroundings andenvironment.Two groups of individuals weretested on their attention andworking memory performance,one group after a relaxedwalk in a quiet park and theother group afternavigating busy city streets.Those who had been walkingthe city streets scored farlower on the tests.


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The type or characteristics of the information also affects thenumber of items which can be retained in short-termmemory. For instance, more words can be recalled if theyare shorter or more commonly used words, or if they arephonologically similar in sound, or if they are taken from a

single semantic category (such as sports, for example)rather than from different categories, etc. There is alsosome evidence that short-term memory capacity andduration is increased if the words or digits are articulatedaloud instead of being read sub-vocally (in the head).

The relatively small capacity of the short-term memory,compared to the huge capacity of long-term memory, hasbeen attributed by some to the evolutionary survivaladvantage in paying attention to a relatively small number of important things (e.g. theapproach of a dangerous predator, the proximity of a nearby safe haven, etc) and not to

a plethora of other peripheral details which would only interfere with rapid decision-making.

"Chunking" of information can lead to an increase in the short-term memory capacity.Chunking is the organization of material into shorter meaningful groups to make themmore manageable. For example, a hyphenated phone number, split into groups of 3 or4 digits, tends to be easier to remember than a single long number. Experimentsby Herbert Simon have shown that the ideal size for chunking of letters and numbers,whether meaningful or not, is three.

It is usually assumed that the short-term

memory spontaneously decays over time, typically in theregion of 10 - 15 seconds, but items may be retained for upto a minute, depending on the content. However, it can beextended by repetition or rehearsal (either by reading itemsout loud, or by mental simulation), so that the information re-enters the short-term store and is retained for a furtherperiod. When several elements (such as digits, words orpictures) are held in short-term memory simultaneously,they effectively compete with each other for recall. Newcontent, therefore, gradually pushes out older content(known as displacement), unless the older content isactively protected against interference by rehearsal or bydirecting attention to it. Any outside interference tends tocause disturbances in short-term memory retention, and forthis reason people often feel a distinct desire to completethe tasks held in short-term memory as soon as possible.

The forgetting of short-term memories involves a differentprocess to the forgetting of long-term memories. When something in short-term memory


Short-term working memoryappears to operatephonologically.For instance, whereas English

speakers can typically holdseven digits in short-termmemory, Chinese speakerscan typically remember tendigits.This is because Chinesenumber words are all singlesyllables, whereas Englishare not.


The use of mnemonicdevices can significantlyincrease memory, particularlythe recall of long lists ofnames, numbers, etc.One case, known as “S.F.”,was able to increase his digitspan (the longest list ofnumber that a person canrepeat back in correct order)from 7 to 79 with the use ofmnemonic strategies.

Akira Haraguchi and LuChao’s record-breakingrecitations of the digits of thenumber Pi (100,000 and67,890 digits respectively) alsomake use of mnemonicsystems.


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is forgotten, it means that a nerve impulse has merely ceased being transmitted througha particular neural network. In general, unless an impulse is reactivated, it stopsflowing through a network after just a few seconds.

Typically, information is transferred from the short-term or working memory to the long-

term memory within just a few seconds, although the exact mechanisms by which thistransfer takes place, and whether all or only some memories are retained permanently,remain controversial topics among experts. Richard Schiffrin, in particular, is wellknown for his work in the 1960s suggesting that ALL memories automatically pass froma short-term to a long-term store after a short time (known as the modal or multi-store or Anderson-Schiffrin model).

However, this is disputed, and it now seems increasingly likely that some kindof vetting or editing procedure takes place. Some researchers (e.g. Eugen Tarnow)have proposed that there is no real distinction between short-term and long-termmemory at all, and certainly it is difficult to demarcate a clear boundary between them.

However, the evidence of patients with some kinds of anterograde amnesia, andexperiments on the way distraction affect the short-term recall of lists, suggest that thereare in fact two more or less separate systems.

LONG-TERM MEMORY

Long-term memory is, obviously enough, intended forstorage of information over a long period of time. Despiteour everyday impressions of forgetting, it seems likely thatlong-term memory actually decays very little over time, andcan store a seemingly unlimited amount of information

almost indefinitely. Indeed, there is some debate as towhether we actually ever “forget” anything at all, or whetherit just becomes increasingly difficult to access or retrievecertain items from memory.

Short-term memories can become long-term memorythrough the process of consolidation,involving rehearsal and meaningful association.Unlike short-term memory (which relies mostly onan acoustic, and to a lesser extent a visual, code forstoring information), long-term memory encodes information

for storage semantically (i.e. based on meaning andassociation). However, there is also some evidence thatlong-term memory does also encode to some extentby sound. For example, when we cannot quite remember aword but it is “on the tip of the tongue”, this is usually based on the sound of a word,not its meaning.


While older people have moredifficulty than the young withrote memorization, such asremembering lists of words or

numbers, they actually tend toperform better than youngpeople in the recognition andrecall of facts and tasks.This is partly because olderpeople, having accumulatedmore real-life experience andinformation, have a densernetwork of linkages andassociations in their long-termmemory, and partly becausethey have had time to moreefficiently organize their facts

and experiences in a moreeasily accessible hierarchicalform.


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Physiologically, the establishment of long-term memory involves a process of physicalchanges in the structure of neurons (or nerve cells) in the brain, a process knownas long-term potentiation, although there is still much that is not completelyunderstood about the process. At its simplest, whenever something is learned, circuitsof neurons in the brain, known as neural networks, are created, altered or

strengthened. These neural circuits are composed of a number of neurons thatcommunicate with one another through special junctions called synapses. Through aprocess involving the creation of new proteins within the body of neurons, and theelectrochemical transfer of neurotransmitters across synapse gaps to receptors, thecommunicative strength of certain circuits of neurons in the brain is reinforced. Withrepeated use, the efficiency of these synapse connections increases, facilitating thepassage of nerve impulses along particular neural circuits, which may involve manyconnections to the visual cortex, the auditory cortex, the associative regions of thecortex, etc.

This process differs both structurally and functionally from

the creation of working or short-term memory. Althoughthe short-term memory is supported by transient patterns ofneuronal communication in the regions ofthe frontal, prefrontal and parietal lobes of the brain, long-term memories are maintained by more stable andpermanent changes in neural connections widely spreadthroughout the brain. The hippocampus area of the brainessentially acts as a kind of temporary transit point for long-term memories, and is not itself used to store information.However, it is essential to the consolidation of informationfrom short-term to long-term memory, and is thought to be

involved in changing neural connections for a period of threemonths or more after the initial learning.

Unlike with short-term memory, forgetting occurs in long-term memory when the formerly strengthened synaptic connections among the neuronsin a neural network become weakened, or when the activation of a new network issuperimposed over an older one, thus causing interference in the older memory.

Over the years, several different types of long-term memory have been distinguished,including explicit and implicit memory, declarative and procedural memory (with afurther sub-division of declarative memory into episodic and semantic memory)and retrospective and prospective memory.


Several studies have shownthat both episodic andsemantic long-term memoriescan be better recalled whenthe same language is used forboth encoding and retrieval.For example, bilingualRussian immigrants to theUnited States can recall moreautobiographical details oftheir early life when thequestions and cues are

presented in Russian thanwhen they are questioned inEnglish.

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DECLARATIVE (EXPLICIT) & PROCEDURAL (IMPLICIT) MEMORY

Long-term memory is often divided into two further maintypes: explicit (or declarative) memoryand implicit (or procedural) memory.

Declarative memory (“knowing what”) is memory of factsand events, and refers to those memories that canbe consciously recalled. It is sometimes called explicitmemory, since it consists of information that is explicitlystored and retrieved, although it is more properly a subset ofexplicit memory. Declarative memory can be further sub-divided into episodic memory and semantic memory.

Procedural memory (“knowing how”) is the unconscious memory of skills and how to do things, particularly the use of

objects or movements of the body, such as playing a guitar or riding a bike. It iscomposed of automatic sensorimotor behaviours that are so deeply embedded that weare no longer aware of them, and, once learned, these "body memories" allow us tocarry out ordinary motor actions automatically. Procedural memory is sometimesreferred to as implicit memory, because previous experiences aid in the performanceof a task without explicit and conscious awareness of these previous experiences,although it is more properly a subset of implicit memory.

These different types of long-term memory are stored indifferent regions of the brain and undergo quite different

processes. Declarative memories are encoded by thehippocampus, entorhinal cortex and perirhinal cortex (allwithin the medial temporal lobe of the brain), but areconsolidated and stored in the temporal cortex andelsewhere. Procedural memories, on the other hand, do notappear to involve the hippocampus at all, and are encodedand stored by the cerebellum, putamen, caudate nucleus and the motor cortex, all of which are involved in motorcontrol. Learned skills such as riding a bike are stored in the putamen; instinctiveactions such as grooming are stored in the caudate nucleus; and the cerebellum isinvolved with timing and coordination of body skills. Thus, without the medial temporal

lobe (the structure that includes the hippocampus), a person is still able to form newprocedural memories (such as playing the piano, for example), but cannot rememberthe events during which they happened or were learned.


Studies have shown that

musicians tend to have abetter memory than non-musicians, not just for music,but for words and pictures too.Interestingly, they also tend touse different strategies formemorization, being morelikely than non-musicians togroup words intosimilar semantic categories,and less likely toverbalize pictures.


Brain-scan studies haveshown that London taxidrivers, who spend yearsmemorizing the city'slabyrinthine streets, developphysically larger hippocampi,much as a muscle is enlargedby weight-training.


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Perhaps the most famous study demonstrating theseparation of the declarative and procedural memories isthat of a patient known as “H.M.”, who had parts of his

medial temporal lobe, hippocampus and amygdala removedin 1953 in an attempt to cure his intractable epilepsy. Afterthe surgery, H.M. could still form new procedural memoriesand short-term memories, but long-lasting declarativememories could no longer be formed. The nature of theexact brain surgery he underwent, and the typesof amnesia he experienced, allowed a good understandingof how particular areas of the brain are linked to specificprocesses in memory formation. In particular, his abilityto recall memories from well before his surgery, but his inability to create new long-termmemories, suggests that encoding and retrieval of long-term memory information is

mediated by distinct systems within the medial temporal lobe, particularly thehippocampus. The fact that he was able to learn hand-eye coordination skills suchas mirror drawing, despite having absolutely no memory of having learned or practisedthe task before, also suggested the existence different types of long-term memory,which are now known as declarative and procedural memories

There is strong evidence, notably by studying amnesic patients and the effectof priming, to suggest that implicit memory is largely distinct from explicit memory, andoperates through a different process in the brain. Studies of the effects of amnesia haveshown that it is quite possible to have an intact implicit memory despite a severelyimpaired explicit memory. Priming is the effect in which exposure to a stimulus

influences response to a subsequent stimulus, so that, for instance, if a person reads alist of words including the word “concert”, and is later asked to complete a word startingwith “con”, there is a higher probability that they will answer “concert” than, say,“contact”, “connect”, etc. Studies from amnesic patients indicate that priming iscontrolled by a brain system separate from the medial temporal system that supportsexplicit memory.


Children under the age of

about seven pick up newlanguages easily withoutgiving it much consciousthought, using procedural (orimplicit) memory.Adults, on the other hand,actively learn the rules andvocabulary of a new languageusing declarative (or explicit)memory.


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EPISODIC & SEMANTIC MEMORY

Declarative memory can be further sub-dividedinto episodic memory and semantic memory.

Episodic memory represents our memoryof experiences and specific events in time in a serial form,from which we can reconstruct the actual events that tookplace at any given point in our lives. It is the memoryof autobiographical events (times, places, associatedemotions and other contextual knowledge) that can beexplicitly stated. Individuals tend to see themselves asactors in these events, and the emotional charge and theentire context surrounding an event is usually part of thememory, not just the bare facts of the event itself.

Semantic memory, on the other hand, is a more structuredrecord of facts, meanings, concepts and knowledgeabout the external world that we have acquired. It refers to general factual knowledge,shared with others and independent of personal experience and of thespatial/temporal context in which it was acquired. Semantic memories may once havehad a personal context, but now stand alone as simple knowledge. It therefore includessuch things as types of food, capital cities, social customs, functions of objects,vocabulary, understanding of mathematics, etc. Much of semantic memory is abstractand relational and is associated with the meaning of verbal symbols.

The semantic memory is generally derived from the episodic memory, in that we learn

new facts or concepts from our experiences, and the episodic memory is consideredto support and underpin semantic memory. A gradual transition from episodic tosemantic memory can take place, in which episodic memory reduces its sensitivity andassociation to particular events, so that the information can be generalized as semanticmemory.


Females consistently perform

better than males on episodiclong-term memory tasks,especially those involvingdelayed recall and recognition.However, males and femalesdo not differ significantly onworking memory and semanticmemory tasks.There is also evidence for anegative recall bias inwomen, which means thatfemales in general are morelikely than males to recall theirmistakes.


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Both episodic memory and semantic memory require asimilar encoding process. However, semantic memorymainly activates the frontal and temporal cortexes,whereas episodic memory activity is concentrated inthe hippocampus, at least initially. Once processed in the

hippocampus, episodic memories arethen consolidated and stored in the neocortex. Thememories of the different elements of a particular event aredistributed in the various visual, olfactory and auditory areasof the brain, but they are all connected together bythe hippocampus to form an episode, rather thanremaining a collection of separate memories.

For example, memories of people’s faces, the taste of thewine, the music that was playing, etc, might all be part of thememory of a particular dinner with friends. By repeatedly

reactivating or “playing back” this particular activity patternin the various regions of the cortex, they become so strongly linked with one anotherthat they no longer need the hippocampus to act as their link, and the memory of themusic that was playing that night, for example, can act as an index entry, and may beenough to bring back the entire scene of the dinner party.

Our spatial memory in particular appears to be much more confined to thehippocampus, particularly the right hippocampus, which seems to be able to create amental map of space, thanks to certain cells called "place cells". Episodic memorydoes also trigger activity in the temporal lobe, but mainly in order to ensure that thesepersonal memories are not mistaken for real life. This difference in the neurological

processing of episodic and semantic memory is illustrated by cases of anterogradeamnesia cases (a good example being a case known as “C.L.”) in which episodicmemory is almost completely lost while semantic memory is retained.


Experiments on rats in the1970s showed that there areover a million “place cells” ina rat’s hippocampus, each of

which only becomes activewhen the rat is located in avery specific part of itsenvironment.

All together they can form avery precise cognitive map

that tells the animal where it isat any given time.When the rat explores a newenvironment, it forms a newcognitive map of place cellsthat can be very stable, lastingweeks or months.


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A further category of declarative memory, referred toas autobiographical memory, is sometimes distinguished,although really it is just one area of episodic memory.

Autobiographical memory refers to a memory systemconsisting of episodes recollected from an individual’s own

life, often based on a combination of episodic memory(personal experiences and specific objects, people andevents experienced at particular times and places) andsemantic memory (general knowledge and facts about theworld).

One specific type of autobiographical memory is known asa "flashbulb memory", a highly detailed, exceptionallyvivid “snapshot” of a moment or circumstances in whichsurprising and consequential (or emotionally arousing) newswas heard, famous examples being the assassination of

John Kennedy, the terrorist bombings on 9/11, etc. Suchmemories are believed by some to be highly resistant toforgetting, possibly due to the strong emotions that are typically associated with them.However, a number of studies also suggest that flashbulb memories are actually notespecially accurate, despite apparently being experienced with great vividness andconfidence.

RETROSPECTIVE & PROSPECTIVE MEMORY

An important alternative classification of long-termmemory used by some researchers is based on

the temporal direction of the memories.

Retrospective memory is where the content to beremembered (people, words, events, etc) is in the past, i.e.the recollection of past episodes. Itincludes semantic, episodic and autobiographical memory,and declarative memory in general, although it can beeither explicit or implicit.

Prospective memory is where the content is to beremembered in the future, and may be defined

as “remembering to remember” or remembering toperform an intended action. It may be either event-based ortime-based, often triggered by a cue, such as going to thedoctor (action) at 4pm (cue), or remembering to post a letter (action) after seeing amailbox (cue).

Clearly, though, retrospective and prospective memory are notentirely independent entities, and certain aspects of retrospective memory are usually


Recent research into linksbetween memory andhandedness suggest that"mixed-handers" (who

typically perform some taskswith one hand and some withthe other) tend to show betterautobiographcial memory than"strong-handers" (whoperform almost all tasks witheither one hand or the other).It is hypothesized that mixed-handers may have more, orbetter, communication

between the brain'shemispheres than strong-handers, and possibly even a

thicker corpus callosum.


MRI studies have shown thatthe same parts of the brain areused when remembering thepast as when imagining asimilar event in the future,which shows that pastmemories are also accessedand drawn on whenprojections are made into thefuture.This is sometimes referred toas “mental time travel” as itallows us to project ourselvesat will either backwards orforwards in time within ourpersonal lives.


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required for prospective memory. Thus, there have been case studies where animpaired retrospective memory has caused a definite impact on prospective memory.However, there have also been studies where patients with an impaired prospectivememory had an intact retrospective memory, suggesting that to some extent the twotypes of memory involve separate processes.

MEMORY PROCESSES

We have already looked at thedifferent stages of memoryformation (from perceptionto sensory memory to short-termmemory to long-term memory) in thesection on Types of Memory. Thissection, however, looks at the overallprocesses involved.

Memory is the ability to encode, storeand recall information. The threemain processes involved in humanmemory are therefore encoding,storage and recall (retrieval).

Additionally, the process of memoryconsolidation (which can beconsidered to be either part ofthe encoding process orthe storage process) is treated here

as a separate process in its own right.

Some of the physiology and neurology involved in these processes is highly complexand technical (and some of it still not completely understood), and lies largely outsidethe remit of this entry level guide, although at least a general introduction is givenhere. More information on the architecture of the human brain, and the neurologicalprocesses by which memory is encoded, stored and recalled can be found in thesection on Memory and the Brain.

Memory Processes: Diagram by Luke Mastin


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MEMORY ENCODING

Encoding is the crucial first step to creating a new memory.It allows the perceived item of interest to be converted intoa construct that can be stored within the brain, and then

recalled later from short-term or long-term memory.

Encoding is a biological event beginningwith perception through the senses. The process of layingdown a memory begins with attention (regulated bythe thalamus and the frontal lobe), in which a memorableevent causes neurons to fire more frequently, making theexperience more intense and increasing the likelihood thatthe event is encoded as a memory. Emotion tends toincrease attention, and the emotional element of an event isprocessed on an unconscious pathway in the brain leading

to the amygdala. Only then are theactual sensations derived from an event processed.

The perceived sensations are decoded in thevarious sensory areas of the cortex, and then combined inthe brain’s hippocampus into one single experience. The hippocampus is thenresponsible for analyzing these inputs and ultimately deciding if they will be committedto long-term memory. It acts as a kind of sorting centre where the new sensationsare compared and associated with previously recorded ones. The various threads ofinformation are then stored in various different parts of the brain, although the exact wayin which these pieces are identified and recalled later remains largely unknown.

Although the exact mechanism is not completely understood, encoding occurs ondifferent levels, the first step being the formation of short-term memory from the ultra-short term sensory memory, followed by the conversion to a long-term memory by aprocess of memory consolidation. The process begins with the creation of a memorytrace or engram in response to the external stimuli. An engram is a hypotheticalbiophysical or biochemical change in the neurons of the brain, hypothetical in therespect that no-one has ever actually seen, or even proved the existence of, such aconstruct.

An organ called the hippocampus, deep within the medial temporal lobe of the brain,

receives connections from the primary sensory areas of the cortex, as well asfrom associative areas and the rhinal and entorhinal cortexes. Whilethese anterogradeconnections converge at the hippocampus,other retrograde pathways emerge from it, returning to the primary cortexes. A neuralnetwork of cortical synapses effectively records the various associations which arelinked to the individual memory.

There are three or four main types of encoding:


Studies suggest that

characteristics of theenvironment are encoded aspart of the memory trace, andcan be used to enhanceretrieval of the otherinformation in the trace.In other words, you can recallmore when the environmentsare similar in both the learning(encoding) and recall phases.Thus, deep-sea divers tend toremember their training moreeffectively when trainedunderwater rather than onland, and students performbetter on exams by studying insilence, because exams areusually done in silence.


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Acoustic encoding is the processing and encodingof sound, words and other auditory inputfor storage and later retrieval. This is aided by theconcept of the phonological loop, which allowsinput within our echoic memory to be sub-vocally

rehearsed in order to facilitate remembering. Visual encoding is the process of encoding images

and visual sensory information. Visual sensoryinformation is temporarily stored within the iconicmemory before being encoded into long-termstorage. The amygdala (within the medial temporallobe of the brain which has a primary role in theprocessing of emotional reactions) fulfills animportant role in visual encoding, as it accepts visualinput in addition to input from other systems andencodes the positive or negative values

of conditioned stimuli. Tactile encoding is the encoding of how something feels, normally through the

sense of touch. Physiologically, neurons in the primary somatosensorycortex of the brain react to vibrotactile stimuli caused by the feel of an object.

Semantic encoding is the process of encoding sensory input that hasparticular meaning or can be applied to a particular context, rather than derivingfrom a particular sense.

It is believed that, in general, encoding for short-term memory storage in the brain reliesprimarily on acoustic encoding, while encoding for long-term storage is more reliant(although not exclusively) on semantic encoding.

Human memory is fundamentally associative, meaning thata new piece of information is remembered better if it can beassociated with previously acquired knowledge that isalready firmly anchored in memory. The more personallymeaningful the association, the more effective theencoding and consolidation. Elaborate processing thatemphasizes meaning and associations that are familiartends to leads to improved recall. On the other hand,information that a person finds difficult to understand cannotbe readily associated with already acquired knowledge, andso will usually be poorly remembered, and may even beremembered in a distorted form due to the effort tocomprehend its meaning and associations. For example,given a list of words like "thread", "sewing", "haystack","sharp", "point", "syringe", "pin", "pierce", "injection" and"knitting", people often also (incorrectly) remember the word"needle" through a process of association.


When presented with a visualstimulus, the part of the brainwhich is activated the mostdepends on the nature of the

image. A blurred image, for example,activates the visual cortex atthe back of the brain most.

An image of an unknown faceactivates the associative andfrontal regions most.

An image of a face which isalready in working memoryactivates the frontal regions

most, while the visual areasare scarcely stimulated at all.


In a positive example ofdisfluency (the subjectivefeeling of difficulty associatedwith any mental task), a recentstudy at Princeton Universityhas shown that studentslearning new material printedin a difficult-to-read font ortypeface were able to recallsignificantly more than those

learning the same material in afont considered easy to read.It is believed that presentinginformation in a way that ishard to digest means that aperson has to concentratemore, leading to deeperprocessing and thereforebetter retrieval afterwards.

http://www.human-memory.net/brain.htmlhttp://www.human-memory.net/brain.html


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Because of the associative nature of memory, encoding can be improved by a strategyof organization of memory called elaboration, in which new pieces of information areassociated with other information already recorded in long-term memory, thusincorporating them into a broader, coherent narrative which is already familiar. Anexample of this kind of elaboration is the use of mnemonics, which are verbal, visual or

auditory associations with other, easy-to-remember constructs, which can then berelated back to the data that is to be remembered. Common examples are “Roy G. Biv”to remember the order of the colours of the rainbow, or “Every Good Boy DeservesFavour” for the musical notes on the lines of the treble clef, which most people findeasier to remember than the original list of colours or letters. When we use mnemonicdevices, we are effectively passing facts through the hippocampus several times, sothat it can keep strengthening the associations, and therefore improve the likelihood ofsubsequent memory recall.

In the same way, associating words with images is anothercommonly used mnemonic device, providing two alternative

methods of remembering, and creating additionalassociations in the mind. Taking this to a higher level,another method of improving memory encodingand consolidation is the use of a so-called memorypalace (also known as the method of loci), a mnemonictechniques that relies on memorized spatialrelationships to establish, order and recollect othermemories. The method is to assign objects or facts todifferent rooms in an imaginary house or palace, sothat recall of the facts can be cued by mentally “walkingthough” the palace until it is found. Many top memorizers

today use the memory palace method to a greater or lesserdegree.

The old and popular notion of the brain as a kind of “muscle”which strengthens with repeated use (also known as faculty theory) is now largelydiscredited. Research, dating back to William James towards the end of the 19thCentury, shows that long hours spent memorizing does not build up the powers ofmemory at all, and, on the contrary, may even diminish it. This is not to saythat individual memories cannot be strengthened by repetition, but that, as Jamesfound, daily training in the memorization of a poetry of one author, for example, doesnot improves a person’s ability to learn the poetry of another author, or poetry ingeneral.

Many studies have shown that the most vivid autobiographical memories tend to beof emotional events, which are likely to be recalled more often and with more clarityand detail than neutral events. One theory suggests that high levels of emotionalarousal lead to attention narrowing, where the range of sensitive cues from thestimulus and its environment is decreased, so that information central to the source ofthe emotional arousal is strongly encoded while peripheral details are not (e.g. the so-


It has been shown that usingtwo separate studysessions, with time betweenthe sessions, can result intwice the learning as a singlestudy session of the sametotal time length.This is known as spacedlearning (the opposite ofcramming), and is designed toavoid the situation where thesynapses become "maxed out"or lose their ability to learn newinformation (also known as thelong-term depression orweakening of a synapseconnection).


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called “weapon focus effect”, in which witnesses to a crime tend to remember the gunor knife in great detail, but not other more peripheral details such as the perpetrator’sclothing or vehicle).

MEMORY CONSOLIDATION

Consolidation is the processes of stabilizing a memorytrace after the initial acquisition. It may perhaps be thoughtof part of the process of encoding or of storage, or it may beconsidered as a memory process in its own right. It isusually considered to consist of two specific processes,synaptic consolidation (which occurs within the first fewhours after learning or encoding) and systemconsolidation (where hippocampus-dependent memoriesbecome independent of the hippocampus over a period ofweeks to years).

Neurologically, the process of consolidation utilizes aphenomenon called long-term potentiation, which allowsa synapse to increase in strength as increasing numbers ofsignals are transmitted between thetwo neurons. Potentiation is the process by whichsynchronous firing of neurons makes those neurons moreinclined to fire together in the future. Long-term potentiationoccurs when the same group of neurons fire together sooften that they become permanently sensitized to eachother. As new experiences accumulate, the brain creates

more and more connections and pathways, and may “re-wire” itself by re-routingconnections and re-arranging its organization.

As such a neuronal pathway, or neural network, is traversed over and over again, anenduring pattern is engraved and neural messages are more likely to flow along suchfamiliar paths of least resistance. For example, if a piece of music is played over andover, the repeated firing of certain cells in a certain order in your brain makes it easier torepeat this firing later on, with the result that the musician becomes better at playing themusic, and can play it faster, with fewer mistakes.

In this way, the brain organizes and reorganizes itself in response to experiences,

creating new memories prompted by experience, education or training. The ability of theconnection, or synapse, between two neurons to change in strength, and for lastingchanges to occur in the efficiency of synaptic transmission, is known as synapticplasticity or neural plasticity, and it is one of the important neurochemical foundationsof memory and learning.

It should be remembered that each neuron makes thousands of connections withother neurons, and memories and neural connections are mutually interconnected in


Studies have shown that weoften construct our memoriesafter the fact, and that we aresusceptible to suggestionsfrom others that help us fill inthe gaps in our memories.This malleability of memoryis why, for example, a policeofficer investigating a crimeshould not show a picture of a

single individual to a victim andask if the victim recognizes theassailant.If the victim is then presentedwith a line-up and picks out theindividual whose picture thevictim had been shown, thereis no real way of knowingwhether the victim is actuallyremembering the assailant or

just the picture.

http://www.human-memory.net/brain_neurons.htmlhttp://www.human-memory.net/brain_neurons.html


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extremely complex ways. Unlike the functioning of a computer, each memory isembedded in many connections, and each connection is involved in several memories.Thus, multiple memories may be encoded within a single neural network, by differentpatterns of synaptic connections. Conversely, a single memory may involvesimultaneously activating several different groups of neurons in completely different

parts of the brain.

The inverse of long-term potentiation, known as long-term depression, can also takeplace, whereby the neural networks involved in erroneous movements are inhibited bythe silencing of their synaptic connections. This can occur in the cerebellum, which islocated towards the back of the brain, in order to correct our motor procedures whenlearning how to perform a task (procedural memory), but also in the synapses ofthe cortex, the hippocampus, the striatum and other memory-related structures.

Contrary to long-term potentiation, which is triggeredby high-frequency stimulation of the synapses, long-termdepression is produced by nerve impulses reachingthe synapses at very low frequencies, leading them toundergo the reverse transformation from long-termpotentiation, and, instead of becoming more efficient, thesynaptic connections are weakened. It is still not clearwhether long-term depression contributes directly tothe storage of memories in some way, or whether it simplymakes us forget the traces of some things learned long agoso that new things can be learned.

Sleep (particularly slow-wave, or deep, sleep, during thefirst few hours) is also thought to be important in improvingthe consolidation of information in memory, and activation patterns in the sleeping brain,which mirror those recorded during the learning of tasks from the previous day, suggestthat new memories may be solidified through such reactivation and rehearsal.

Memory re-consolidation is the process of previously consolidated memoriesbeing recalled and then actively consolidated all over again, in order to maintain,strengthen and modify memories that are already stored in the long-term memory.Several retrievals of memory (either naturally through reflection, or throughdeliberate recall) may be needed for long-term memories to last for many years,depending on the depth of the initial processing. However, these individual retrievalscan take place at increasing intervals, in accordance with the principle of spacedrepetition (this is familiar to us in the way that “cramming” the night before an exam isnot as effective as studying at intervals over a much longer span of time).

The very act of re-consolidation, though, may change the initial memory. As aparticular memory trace is reactivated, the strengths of the neural connections may


Studies have shown thatinformation is transferredbetween the hippocampus andthe cerebral cortexduring deep sleep, and sleepappears to be essential for theproper consolidation of long-term memories.However, even daytime napscan help improve memory tosome extent, and helps with

the memorization of importantfacts.


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throughout various areas of the brain by the encoding process. Memory storage istherefore an ongoing process of reclassification resulting from continuous changes inour neural pathways, and parallel processing of information in our brains.

The indications are that, in the absence of disorders due to trauma or neurological

disease, the human brain has the capacity to store almost unlimited amounts ofinformation indefinitely. Forgetting, therefore, is more likely to be result from incorrectlyor incompletely encoded memories, and/or problems with the recall/retrieval process. Itis a common experience that we may try to remember something one time and fail, butthen remember that same item later. The information is therefore clearly still there instorage, but there may have been some kind of a mismatch between retrieval cues andthe original encoding of the information. “Lost” memories recalled with the aidof psychotherapy or hypnosis are other examples supporting this idea, although it isdifficult to be sure that such memories are real and not implanted by the treatment.

Having said that, though, it seems unlikely that, as Richard Schiffrin and others have

claimed, ALL memories are stored somewhere in the brain, and that it is only inthe retrieval process that irrelevant details are “fast-forwarded” over or expurgated. Itseems more likely that the memories which are stored are in someway edited and sorted, and that some of the more peripheral details are never stored.

Forgetting, then, is perhaps better thought of as the temporary or permanent inabilityto retrieve a piece of information or a memory that had previously been recorded inthe brain. Forgetting typically follows a logarithmic curve, so that information loss isquite rapid at the start, but becomes slower as time goes on. In particular, informationthat has been learned very well (e.g. names, facts, foreign-language vocabulary, etc),will usually be very resistant to forgetting, especially after the first three years.

Unlike amnesia, forgetting is usually regarded as a normal phenomenoninvolving specific pieces of content, rather than relatively broad categories of memoriesor even entire segments of memory.

Theorists disagree over exactly what becomes of materialthat is forgotten. Some hold that long-term memories doactually decay and disappear completely over time; othershold that the memory trace remains intact as long as welive, but the bonds or cues that allow us to retrieve the tracebecome broken, due to changes in the organization ofthe neural network, new experiences, etc, in the same wayas a misplaced book in a library is “lost” even though it stillexists somewhere in the library.

Interestingly, it appears not to be possible todeliberately delete memories at will, which can havenegative consequences, for example if we experience traumatic events we wouldactually prefer to forget. In fact, such memories tend to be imprinted even more stronglythan normal due to their emotional content, although recent research involving the use


Research using functionalmagnetic resonanceimaging (fMRI) suggests thatverbs and nouns are stored indifferent ways in the brain.Concrete nouns are stored in

areas of the brain used tosense or manipulate thereferent objects, leading to atheory of meaning basedlargely on function.

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of beta blockers (such as propanolol) suggests that it may be possible to tone downthe emotional aspects of such memories, even if the memories themselves cannot beerased. The way this works is that the act of recalling stored memories makes them"malleable" once more, as they were during the initial encoding phase, and their re-storage can then be blocked by drugs which inhibit the proteins that enable the

emotional memory to be re-saved.

MEMORY RECALL/RETRIEVAL

Recall or retrieval of memory refers to the subsequent re-accessing of events or information from the past, whichhave been previously encoded and stored in the brain. Incommon parlance, it is known as remembering. Duringrecall, the brain "replays" a pattern of neural activity thatwas originally generated in response to a particular event,echoing the brain's perception of the real event. These

replays are not quite identical to the original, though -otherwise we would not know the difference between thegenuine experience and the memory - but are mixed with anawareness of the current situation.

Because of the way memories are encoded and stored,memory recall is effectively an on-the-fly reconstruction ofelements scattered throughout various areas of our brains.Memories are not stored in our brains like books on libraryshelves, or even as a collection of self-contained recordingsor pictures or video clips, but may be better thought of as a

kind of collage or a jigsaw puzzle, involving differentelements stored in disparate parts of the brain linkedtogether by associations and neural networks. Memory retrieval therefore requires re-visiting the nerve pathways the brain formed when encoding the memory, and thestrength of those pathways determines how quickly the memory can be recalled. Recalleffectively returns a memory from long-term storage to short-term or working memory,where it can be accessed, in a kind of mirror image of the encoding process. It is thenre-stored back in long-term memory, thus re-consolidating and strengthening it.

The efficiency of human memory recall is astounding. Most of what we remember isby direct retrieval, where items of information are linked directly a question or cue,

rather than by the kind of sequential scan a computer might use (which would requirea systematic search through the entire contents of memory until a match is found).Other memories are retrieved quickly and efficiently by hierarchical inference, where aspecific question is linked to a class or subset of information about which certain factsare known. Also, the brain is usually able to determine in advance whether there is anypoint in searching memory for a particular fact (e.g. it instantly recognizes a questionlike “What is Socrates’ telephone number?” as absurd in that no search could everproduce an answer).


Memory recall appears to bestate-dependent, at least tosome extent.Studies have shown that,when material is learned underthe influence of

a drug or alcohol, forexample, it is subsequentlyrecalled better when in thesame drug state than whensober.Similarly, individuals tend toretrieve information moreeasily when it has the sameemotional content as theircurrent emotional state, andwhen the emotional state atthe time of retrieval is similar tothe emotional state at the time

of encoding.

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There are two main methods of accessing memory: recognition andrecall. Recognition is the association of an event or physical object with one previouslyexperienced or encountered, and involves a process of comparison of information withmemory, e.g. recognizing a known face, true/false or multiple choice questions, etc.Recognition is a largely unconscious process, and the brain even has a dedicated face-

recognition area, which passes information directly through the limbic areas togenerate a sense of familiarity, before linking up with the cortical path, where dataabout the person's movements and intentions are processed. Recall involvesremembering a fact, event or object that is not currently physically present (in the senseof retrieving a representation, mental image or concept), and requires the directuncovering of information from memory, e.g. remembering the name of a recognizedperson, fill-in the blank questions, etc.

Recognition is usually considered to be “superior” to recall (in the sense of being moreeffective), in that it requires just a single process rather than two processes. Recognitionrequires only a simple familiarity decision, whereas a full recall of an item from

memory requires a two-stage process (indeed, this is often referred to as the two-stagetheory of memory) in which the search and retrieval of candidate items from memoryis followed by a familiarity decision where the correct information is chosen from thecandidates retrieved. Thus, recall involves actively reconstructing the information andrequires the activation of all the neurons involved in the memory in question, whereasrecognition only requires a relatively simple decision as to whether one thing amongothers has been encountered before. Sometimes, however, even if a part of an objectinitially activates only a part of the neural network concerned, recognition may thensuffice to activate the entire network.

In the 1980s, Endel Tulving proposed an alternative to the

two-stage theory, which he called the theory of encodingspecificity. This theory states that memory utilizesinformation both from the specific memory trace as well asfrom the environment in which it is retrieved. Because of itsfocus on the retrieval environment or state, encodingspecificity takes into account context cues, and it also hassome advantages over the two-stage theory as it accountsfor the fact that, in practice, recognition is not actuallyalways superior to recall. Typically, recall is better when theenvironments are similar in both the learning (encoding) andrecall phases, suggesting that context cues are important. Inthe same way, emotional material is remembered morereliably in moods that match the emotional content of thesememories (e.g. happy people will remember more happythan sad information, whereas sad people will betterremember sad than happy information).

According to the levels-of-processing effect theory, another alternative theory ofmemory suggested by Fergus Craik and Robert Lockhart, memory recall of stimuli is


Several studies have shownthat both episodic andsemantic memories can bebetter recalled when the samelanguage is used for bothencoding and retrieval.For example, bilingualRussian immigrants to theUnited States can recall moreautobiographical details oftheir early life when thequestions and cues are

presented in Russian thanwhen they are questioned inEnglish.


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also a function of the depth of mental processing, which is in turn determined byconnections with pre-existing memory, time spent processing the stimulus, cognitiveeffort and sensory input mode. Thus, shallow processing (such as, typically, thatbased on sound or writing) leads to a relatively fragile memory trace that is susceptibleto rapid decay, whereas deep processing (such as that based on semantics and

meanings) results in a more durable memory trace. This theory suggests, then, thatmemory strength is continuously variable, as opposed to the earlier Atkinson-Shiffrin,or multi-store, memory model, which just involves a sequence of three discrete stages,from sensory to short-term to long-term memory.

The evidence suggests that memory retrieval is a more or less automatic process.Thus, although distraction or divided attention at the time of recall tends to slowdown the retrieval process to some extent, it typically has little to no effect onthe accuracy of retrieved memories. Distraction at the time of encoding, on the otherhand, can severely impair subsequent retrieval success.

The efficiency of memory recall can be increased to some extent bymaking inferences from our personal stockpile of world knowledge, and by our useof schema (plural: schemata). A schema is an organized mental structure orframework of pre-conceived ideas about the world and how it works, which we can useto make realistic inferences and assumptions about how to interpret and processinformation. Thus, our everyday communication consists not just of words and theirmeanings, but also of what is left out and mutually understood (e.g. if someone says “itis 3 o’clock”, our knowledge of the world usually allows us to know automaticallywhether it is 3am or 3pm). Such schemata are also applied to recalled memories, sothat we can often flesh out details of a memory from just a skeleton memory of a centralevent or object. However, the use of schemata may also lead to memory errors as

assumed or expected associated events are added that did not actually occur.

There are three main types of recall:

Free recall is the process in which a person is given a list of items to rememberand then is asked to recall them in any order (hence the name “free”). This typeof recall often displays evidence of either the primacy effect (when the personrecalls items presented at the beginning of the list earlier and more often) orthe recency effect (when the person recalls items presented at the end of the listearlier and more often), and also of the contiguity effect (the marked tendencyfor items from neighbouring positions in the listto be recalled successively).


Several recent studies in thegrowing area of neuro-education have shown thevalue of the "testingeffect" (or "retrieval effect"),where quizzes a short time


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Cued recall is the process in which a personis given a list of items to remember and is thentested with the use of cues or guides. Whencues are provided to a person, they tend toremember items on the list that they did not

originally recall without a cue, and which werethought to be lost to memory. This can alsotake the form of stimulus-response recall, aswhen words, pictures and numbers arepresented together in a pair, and the resultingassociations between the two items cues therecall of the second item in the pair.

Serial recall refers to our ability to recall itemsor events in the order in which they occurred,whether chronological events in ourautobiographical memories, or the order of the different parts of a sentence (or

phonemes in a word) in order to make sense of them. Serial recall in long-termmemory appears to differ from serial recall in short-term memory, in that asequence in long-term memory is represented in memory as a whole, rather thanas a series of discrete items. Testing of serial recall by psychologists haveyielded several general rules:

o more recent events are more easily remembered in order (especially withauditory stimuli);

o recall decreases as the length of the list or sequence increases;o there is a tendency to remember the correct items, but in the wrong order;o where errors are made, there is a tendency to respond with an item that

resembles the original item in some way (e.g. “dog” instead of “fog”, orperhaps an item physically close to the original item);

o repetition errors do occur, but they are relatively rare;o if an item is recalled earlier in the list than it should be, the missed item

tends to be inserted immediately after it;o if an item from a previous trial is recalled in a current trial, it is likely to be

recalled at its position from the original trial.

If we assume that the "purpose" of human memory is to use past events to guide futureactions, then keeping a perfect and complete record of every past event is notnecessarily a useful or efficient way of achieving this. So, in most people, some specificmemories may be given up or converted into general knowledge (i.e. convertedfrom episodic to semantic memories) as part of the ongoing recall/re-consolidation process, so that that we are able to generalize from experience. It is alsopossible that false or wrongly interpreted memories may be created during recall, andcarried forward thereafter. One can also, up to a point, choose to forget, by blocking outunwanted memories during recall (a process achieved by frontal lobe activity, whichinhibits the laying down or re-consolidation of a memory.

after initial learningsignificantly improvessubsequent retrieval of factsand ideas, as well as overallunderstanding of topics andthe ability to solve related

problems.Testing helps protect against"proactive interference" (thefamiliar feeling of beingoverwhelmed by too muchinformation), and the studiessuggest that a quick test ismuch more effective than enextra hour of study or re-reading.


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also aided in the analysis of how memory disorders affect the brain physiologically andneurologically.

AGE ASSOCIATED MEMORY IMPAIRMENT

Age associated memory impairment is a label for thegeneral degradation of memory which results from ageing.It is a natural process, seen in many animals as well ashumans, which often begins in our 20s and tends to getnoticeably worse as we reach our 50s. While some specificabilities do decline with age, though, overall memorygenerally remains strong for most people through their 70s.

Episodic memory (our memory of experiences and specificevents in time) in particular is impaired in normal ageing. Onthe other hand, in the absence of specific neurological

disorders, implicit or procedural memory typically showslittle or no decline with age, short-term memory shows onlya little decline, and semantic knowledge, such asvocabulary, actually tends to improve somewhat with age.

Normal ageing is not responsible for causing memorydisorders as such, but it is associated with a generaldecline in cognitive and neural systems, including memory.

As people age, the likelihood of cholinergicdysfunction, beta-amyloid deposits, hippocampalneurofibrillary tangles or neuritic plaques in the cortex of

the brain increases, so that memory connections canbecome blocked, memory functions decrease and the likelihood of memorydisorders like dementia and Alzheimer’s disease increases. Ageing is the singlegreatest risk factor for neurodegenerative diseases in general.

Recent research has identified a transitional state between the cognitive changes ofnormal ageing and Alzheimer's disease, known as mild cognitive impairment, wheresome memory loss occurs, but is not so severe that it interferes with normal dailyfunctioning. More severe memory loss is defined as dementia, of which Alzheimer's is

just one common variant. Those who experience mild cognitive impairment are at asignificantly higher risk of developing Alzheimer’s disease or other types of dementia,

especially after events like strokes.

Although the brain does not change its overall structure or grow whole new batchesof neurons over time, the connections between them change during the normal processof learning, as synapses are reinforced and neural cells make more and strongerconnections with each other. As we begin to age, however, these connections begin tofalter and weaken, in the same way as other biological processes deteriorate and


Normal human memorypowers peak at the age of 25,after which they start todecline.

At this time, the brain iscapable of remembering over200 bits of information persecond, as well as controllingbody movements at the sameti

the human memory - luke mastin [2010]

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