Memory Articles

3/19/12 10:07 PMThe Human Memory - what it is, how it works and how it can go wrong

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

INTRODUCTION TYPES OF MEMORY MEMORY PROCESSES MEMORY DISORDERS MEMORY & THE BRAIN SOURCES & REFERENCES

Introduction

The Human Memory

What Is Memory?

The Study of Human Memory

THE HUMAN MEMORYSince time immemorial, humans have tried to understand whatmemory is, how it works and why it goes wrong. It is an important partof what makes us truly human, and yet it is one of the most elusive andmisunderstood of human attributes.

The popular image of memory is as a kind of tiny filing cabinet full ofindividual memory folders in which information is stored away, or perhaps asa neural super-computer of huge capacity and speed. However, in the lightof modern biological and psychological knowledge, these metaphors maynot be entirely useful and, today, experts believe that memory is in fact farmore complex and subtle than that

It seems that our memory is located not in one particular place in the brain,but is instead a brain-wide process. For example, the simple act of riding abike is actively and seamlessly reconstructed by the brain from manydifferent areas. The memory of how to operate the bike comes from onearea, the memory of how to get from here to the end of the block comesfrom another, the memory of biking safety rules from another, and thatnervous feeling when a car veers dangerously close comes from still another. Each element of a memory (sights, sounds, words,emotions) is encoded in the same 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 patterns generated during the original encoding. Thus, a better imagemight be that of a complex web, in which the threads symbolize the various elements of a memory, that join at nodes or intersectionpoints to form a whole rounded memory of a person, object or event. This kind of distributed memory ensures that even if part of thebrain is damaged, some parts of an experience may still remain. Neurologists are only beginning to understand how the parts arereassembled 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 areencoded and stored in different ways and in different parts of the brain, for reasons that we are only beginning to guess at. Years ofcase studies of patients suffering from accidents and brain-related diseases and other disorders have begun to indicate some of thecomplexities of the memory processes, and great strides have been made in neuroscience and cognitive psychology, but many ofthe exact mechanisms involved remain elusive.

This website, written by a layman for the layman, attempts to piece together some of what we DO know about the enigma that is...TheHuman Memory.

In this section:

What Is Memory?The Study of Human Memory

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memory, human memory, types of memory, memory processes, how memory works, how human memory works


3/19/12 10:07 PMWhat Is Memory? - The Human Memory

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??? 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 organtransplants where the recipientwas reported to havedeveloped new habits ormemories, but such theoriesare now consideredpseudoscientific and have notmade it into peer-reviewedscience journals.


Recent studies suggest thatrepeated bouts of jet lag maycause harm to the temporallobe, an area of the brainimportant to memory, causing itto 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 the

sleep disturbance, generalstress and fatigue caused bylong flights) are responsible forthis impairment of memory andother mental skills.


Introduction

The Human Memory

What Is Memory?


WHAT IS MEMORY?Memory is our ability to encode, store, retain and subsequently recall information and pastexperiences in the human brain.

It is the sum total of what we remember, and gives us the capability to learn and adapt fromprevious experiences as well as to build relationships. It is the ability to remember pastexperiences, and the power or process of recalling to mind previously learned facts, experiences,impressions, skills and habits. It is the store of things learned and retained from our activity orexperience, as evidenced by modification of structure or behaviour, or by recall and recognition.

Etymologically, the modern English word memory comes to us from the Middle English memorie,which in turn comes from the Anglo-French memoire or memorie, and ultimately from the Latinmemoria and memor, meaning "mindful" or "remembering".

In more physiological or neurological terms, memory is, at its simplest, a set of encoded neuralconnections in the brain. It is the re-creation or reconstruction of past experiences by thesynchronous firing of neurons that were involved in the original experience. As we will see, though,because of the way in which memory is encoded, it is perhaps better thought of as a kind ofcollage or jigsaw puzzle, rather than in the traditional manner as a collection of recordings orpictures or video clips, stored as discrete wholes. Our memories are not stored in our brains likebooks on library shelves, but are actually on-the-fly reconstructions from elements scatteredthroughout various areas of our brains.

Memory is related to but distinct from learning, which is the process by which we acquireknowledge of the world and modify our subsequent behaviour. During learning, neurons that firetogether to produce a particular experience are altered so that they have a tendency to firetogether again. For example, we learn a new language by studying it, but we then speak it by usingour memory to retrieve the words that we have learned. Thus, memory depends on learningbecause it lets us store and retrieve learned information. But learning also depends to some extenton memory, in that the knowledge stored in our memory provides the framework to which newknowledge is linked by association and inference. This ability of humans to call on past memoriesin order to imagine the future and to plan future courses of action is a hugely advantageousattribute in our survival and development as a species.

Since the development of the computer in the 1940s, memory is also used to describe thecapacity of a computer to store information subject to recall, as well as the physical components ofthe computer in which such information is stored. Although there are indeed some parallelsbetween the memory of a computer and the memory of a human being, there are also some

fundamental and crucial differences, principally that the human brain is organized as a distributednetwork in which each brain cell makes thousands of connections, rather than as an addressablecollection of discrete files.

The sociological concept of collective memory plays an essential role in the establishment ofhuman societies. Every social group perpetuates itself through the knowledge that it transmitsdown the generations, either through oral tradition or through writing. The invention of writing made it possible for the first time forhuman beings to preserve precise records of their knowledge outside of their brains. Writing, audiovisual media and computer recordscan be considered a kind of external memory for humans.

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what is memory, what is human memory


3/19/12 10:08 PMThe Study of Human Memory - The Human Memory

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Proponents of the tabularasa (blank slate) thesisfavour the nurture side of thenature versus nurture debate,when it comes to aspects ofpersonality, intelligence andsocial and emotional behaviour.The idea first surfaced in atreatise of Aristotle, but thenlay dormant for over a thousandyears until developed by the11th Century Persianphilosopher Avicenna, andthen John Lockes classicstatement of the theory in the17th Century.Sigmund Freud revived theidea in the 20th Century,depicting personality traits asbeing formed by familydynamics.


Flashbacks are involuntary(and often recurring) memories,in which an individual has asudden powerful re-experiencing of a past memory,sometimes so intense that theperson re-lives theexperience, unable to fullyrecognize it as a memory andnot something that is reallyhappening.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.


Introduction

The Human Memory

What Is Memory?


THE STUDY OF HUMAN MEMORYThe study of human memory stretches back at least 2,000 years to Aristotles early attempts tounderstand memory in his treatise On the Soul. In this, he compared the human mind to a blankslate and theorized that all humans are born free of any knowledge and are merely the sum oftheir experiences.

In antiquity, it was generally assumed that there were two sorts of memory: the natural memory(the inborn one that everyone uses every day) and the artificial memory (trained throughlearning and practice of a variety of mnemonic techniques, resulting in feats of memory that arequite extraordinary or impossible to carry out using the natural memory alone). Roman rhetoricianssuch as Cicero and Quintillian expanded on the art of memory or the method of loci (a methodoften first attributed to Simonides of Creos or the Pythagoreans), and their ideas were passeddown to the medieval Scholastics and later scholars of the Renaissance like Matteo Ricci andGiordano Bruno.

The 18th Century English philosopher David Hartley was the first to hypothesize that memorieswere encoded through hidden motions in the nervous system, although his physical theory for theprocess was rudimentary at best. William James in America and Wilhelm Wundt in Germany,both considered among the founding fathers of modern psychology, both carried out some earlybasic research into how the human memory functions in the 1870s and 1880s (Jameshypothesized the idea of neural plasticity many years before it was demonstrated). In 1881,Thodule-Armand Ribot proposed what became known as Ribot's Law, which states thatamnesia has a time-gradient in that recent memories are more likely to be lost than the moreremote memories (although in practice this is actually not always the case).

However, it was not until the mid-1880s that the young German philosopher Herman Ebbinghausdeveloped the first scientific approach to studying memory. He did experiments using lists of nonsense syllables, and then associatingthem with meaningful words, and some of his findings from this work (such as the concepts of the learning curve and forgettingcurve, and his classification of the three distinct types of memory: sensory, short-term and long-term) remain relevant to this day.

The German evolutionary biologist Richard Semon first proposed in 1904 the idea that experience leaves a physical trace, which hecalled an engram, on specific webs of neurons in the brain. The British psychologist Sir Frederick Bartlett is considered one of thefounding fathers of cognitive psychology, and his research in the 1930s into the recall of stories greatly influenced later ideas on howthe brain stores memories.

With advances in technology in the 1940s, the field of neuropsychology emerged and with it a biological basis for theories ofencoding. Karl Lashley devoted 25 years of his life 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 are not localized to one part of the brain at all, butare widely distributed throughout the cortex, and that, if certain parts of the brain are damaged, other parts of the brain may take onthe role of the damaged portion.The Canadian neurosurgeon Wilder Penfields work on the stimulation of the brain with electricalprobes in the 1940s and 1950s, initially in search of the causes of epilepsy, allowed him to createmaps of the sensory and motor cortices of the brain that are still used today, practically unaltered.He was also able to summon up memories or flashbacks (some of which the patients had noconscious recollection of) by probing parts of the temporal lobe of the brain.

As early as 1949, another Canadian, Donald Hebb, intuited that neurons that fire together, wiretogether, implying that the encoding of memories occurred as connections between neurons wereestablished through repeated use. This theoretical idea, sometimes referred to as Hebbs Rule,was supported by the discovery of the mechanics of memory consolidation, long-termpotentiation and neural plasticity in the 1970s, and remains the reigning theory today. EricKandels work on sea-slugs (whose brains are relatively simple and contain relatively large, andeasily-observed, individual neural cells) was particularly important in experimentally demonstratingHebbs Rule and identifying the molecular changes during learning, and the neurotransmittersinvolved.

As computer technology developed in the 1950s and 1960s, parallels between computer andbrain processes became apparent, leading to advances in the understanding of the encoding andstorage processes. The change in the overall study of memory during the 1950s and 1960s hascome to be known as the cognitive revolution, and led to several new theories on how to viewmemory, and yielded influential books by George Miller, Eugene Galanter, Karl Pribram,George Sperling and Ulric Neisser. In 1956, George Miller produced his influential paper onshort-term memory and his assessment that our short-term memory is limited to what he calledthe magical number seven, plus or minus two.

In 1968, Richard Atkinson and Richard Shiffrin created their modal, or multi-store, model of memory, which became the mostpopular model for studying memory for many years. Fergus Craik and Robert Lockhart offered an alternative model, known as thelevels-of-processing model, in 1972. In 1974, Alan Baddeley and Graham Hitch proposed their model of working memory, whichconsists 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 false memories. The pioneering research on human memory by Endel Tulving from the 1970sonwards has likewise been highly influential. He was the first to propose two distinct kinds of long-term memory, episodic andsemantic, in 1972 and he also devised the encoding specificity principle in 1983.


3/19/12 10:08 PMTypes of Memory - The Human Memory

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Types of Human Memory: Diagram by Luke Mastin


Types of Memory

Introduction

Sensory Memory

Short-Term (Working) Memory

Long-Term Memory

Explicit & Implicit

Declarative & Procedural

Episodic & Semantic

Retrospective & Prospective

TYPES OF MEMORYWhat we usually think of as memory in day-to-day usage isactually long-term memory, but there are also important short-term and sensory memory processes, which must be workedthrough before a long-term memory can be established. Thedifferent types of memory each have their own particularmode of operation, but they all cooperate in the process ofmemorization, and can be seen as three necessary steps informing a lasting memory.

This model of memory as a sequence of three stages, fromsensory to short-term to long-term memory, rather than as aunitary process, is known as the modal or multi-store orAtkinson-Shiffrin model, after Richard Atkinson andRichard Shiffrin who developed it in 1968, and it remains themost popular model for studying memory. It is often alsodescribed as the process of memory, but I have used thisdescription for the processes of encoding, consolidation,storage and recall in the separate Memory Processes section.

It should be noted that an alternative model, known as thelevels-of-processing model was proposed by Fergus Craikand Robert Lockhart in 1972, and posits that memory recall,and the extent to which something is memorized, is a functionof the depth of mental processing, on a continuous scalefrom shallow (perceptual) to deep (semantic). Under thismodel, there is no real structure to memory and no distinctionbetween short-term and long-term memory.

In this section:

Sensory MemoryShort-Term (Working) MemoryLong-Term Memory

Declarative (Explicit) and Procedural (Implicit) MemoryEpisodic and Semantic MemoryRetrospective and Prospective Memory

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3/19/12 10:09 PMSensory Memory - Types of Memory - The Human Memory

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Studies have shown thatattention significantly affectsmemory during the encodingphase, but hardly at all duringrecall.Thus, distractions or dividedattention during initial learningmay severely impairsubsequent retrieval success,whereas distractions at the timeof recall may slow down theprocess a little, but has little tono effect on its accuracy.


Types of Memory

Introduction

Sensory Memory


Long-Term Memory

Explicit & Implicit


Episodic & Semantic


SENSORY MEMORYSensory memory is the shortest-term element of memory. It is the ability to retain impressions ofsensory information after the original stimuli have ended. It acts as a kind of buffer for stimulireceived through the five senses of sight, hearing, smell, taste and touch, which are retainedaccurately, but very briefly. For example, the ability to look at something and remember what itlooked like with just a second of observation is an example of sensory memory.

The stimuli detected by our senses can be either deliberately ignored, in which case theydisappear almost instantaneously, or perceived, in which case they enter our sensory memory.This does not require any conscious attention and, indeed, is usually considered to be totallyoutside of conscious control. The brain is designed to only process information that will be useful ata later date, and to allow the rest to pass by unnoted. As information is perceived, it is thereforestored in sensory memory 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 inthe region of 200 - 500 milliseconds (1/5 - 1/2 second) after the perception of an item, and certainlyless than a second (although echoic memory is now thought to last a little longer, up to perhapsthree or four seconds). Indeed, it lasts for such a short time that it is often considered part of the process of perception, but itnevertheless represents an essential step for storing information in short-term memory.

The sensory memory for visual stimuli is sometimes known as the iconic memory, the memory for aural stimuli is known as theechoic memory, and that for touch as the haptic memory. Smell may actally be even more closely linked to memory than the othersenses, possibly because the olfactory bulb and olfactory cortex (where smell sensations are processed) are physically very close- separated by just 2 or 3 synapses - to the hippocampus and amygdala (which are involved in memory processes). Thus, smellsmay be more quickly and more strongly associated with memories and their associated emotions than the other senses, andmemories of a smell may persist for longer, even without constant re-consolidation.

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

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

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3/19/12 10:09 PMLong-Term Memory - Types of Memory - The Human Memory

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While older people have moredifficulty than the young withrote memorization, such asremembering lists of words ornumbers, 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 factsand experiences in a moreeasily accessible hierarchicalform.


Several studies have shownthat both episodic and semanticlong-term memories can bebetter recalled when the samelanguage is used for bothencoding and retrieval.For example, bilingual Russianimmigrants to the United Statescan recall moreautobiographical details of theirearly life when the questionsand cues are presented inRussian than when they arequestioned in English.


Types of Memory

Introduction

Sensory Memory


Long-Term Memory

Explicit & Implicit


Episodic & Semantic


LONG-TERM MEMORYLong-term memory is, obviously enough, intended for storage of information over a long period oftime. Despite our everyday impressions of forgetting, it seems likely that long-term memoryactually decays very little over time, and can store a seemingly unlimited amount of informationalmost indefinitely. Indeed, there is some debate as to whether we actually ever forget anythingat all, or whether it just becomes increasingly difficult to access or retrieve certain items frommemory.

Short-term memories can become long-term memory through the process of consolidation,involving rehearsal and meaningful association. Unlike short-term memory (which relies mostlyon an acoustic, and to a lesser extent a visual, code for storing information), long-term memoryencodes information for storage semantically (i.e. based on meaning and association). However,there is also some evidence that long-term memory does also encode to some extent by sound.For example, when we cannot quite remember a word but it is on the tip of the tongue, this isusually based on the sound of a word, not its meaning.

Physiologically, the establishment of long-term memory involves a process of physical changes inthe structure of neurons (or nerve cells) in the brain, a process known as long-term potentiation,although there is still much that is not completely understood about the process. At its simplest,whenever something is learned, circuits of neurons in the brain, known as neural networks, arecreated, altered or strengthened. These neural circuits are composed of a number of neurons thatcommunicate with one another through special junctions called synapses. Through a processinvolving the creation of new proteins within the body of neurons, and the electrochemical transferof neurotransmitters across synapse gaps to receptors, the communicative strength of certaincircuits of neurons in the brain is reinforced. With repeated use, the efficiency of these synapseconnections increases, facilitating the passage of nerve impulses along particular neural circuits, which may involve manyconnections to the visual cortex, the auditory cortex, the associative regions of the cortex, etc.

This process differs both structurally and functionally from the creation of working or short-termmemory. Although the short-term memory is supported by transient patterns of neuronalcommunication in the regions of the frontal, prefrontal and parietal lobes of the brain, long-termmemories are maintained by more stable and permanent changes in neural connections widelyspread throughout the brain. The hippocampus area of the brain essentially acts as a kind oftemporary transit point for long-term memories, and is not itself used to store information.However, it is essential to the consolidation of information from short-term to long-term memory,and is thought to be involved in changing neural connections for a period of three months or moreafter the initial learning.

Unlike with short-term memory, forgetting occurs in long-term memory when the formerlystrengthened synaptic connections among the neurons in a neural network become weakened, orwhen the activation of a new network is superimposed over an older one, thus causinginterference in the older memory.

Over the years, several different types of long-term memory have been distinguished, includingexplicit and implicit memory, declarative and procedural memory (with a further sub-division ofdeclarative memory into episodic and semantic memory) and retrospective and prospectivememory.

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3/19/12 10:09 PMDeclarative Memory (Explicit Memory) and Procedural Memory (Implicit Memory) - Types of Memory - The Human Memory

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Brain-scan studies have shownthat London taxi drivers, whospend years memorizing thecity's labyrinthine streets,develop physically largerhippocampi, much as amuscle is enlarged by weight-training.


Studies have show thatmusicians 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 more likelythan non-musicians to groupwords into similar semanticcategories, and less likely toverbalize pictures.


Children under the age ofabout seven pick up newlanguages easily without givingit much conscious thought,using procedural (or implicit)memory.Adults, on the other hand,actively learn the rules andvocabulary of a new languageusing declarative (or explicit)memory.


Types of Memory

Introduction

Sensory Memory


Long-Term Memory

Explicit & Implicit


Episodic & Semantic


DECLARATIVE (EXPLICIT) & PROCEDURAL (IMPLICIT) MEMORYLong-term memory is often divided into two further main types: explicit (or declarative) memoryand implicit (or procedural) memory.

Declarative memory (knowing what) is memory of facts and events, and refers to thosememories that can be consciously recalled. It is sometimes called explicit memory, since itconsists of information that is explicitly stored and retrieved, although it is more properly a subsetof explicit memory. Declarative memory can be further sub-divided into episodic memory andsemantic 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. Itis composed of automatic sensorimotor behaviours that are so deeply embedded that we are nolonger aware of them, and, once learned, these "body memories" allow us to carry out ordinarymotor actions automatically. Procedural memory is sometimes referred to as implicit memory,because previous experiences aid in the performance of a task without explicit and consciousawareness of these previous experiences, although it is more properly a subset of implicit memory.

These different types of long-term memory are stored in different regions of the brain and undergoquite different processes. Declarative memories are encoded by the hippocampus, entorhinalcortex and perirhinal cortex (all within the medial temporal lobe of the brain), but areconsolidated and stored in the temporal cortex and elsewhere. Procedural memories, on theother hand, do not appear to involve the hippocampus at all, and are encoded and stored by thecerebellum, 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; instinctive actions such asgrooming are stored in the caudate nucleus; and the cerebellum is involved with timing andcoordination of body skills. Thus, without the medial temporal lobe (the structure that includes thehippocampus), a person is still able to form new procedural memories (such as playing the piano,for example), but cannot remember the events during which they happened or were learned.

Perhaps the most famous study demonstrating the separation of the declarative and proceduralmemories is that of a patient known as H.M., who had parts of his medial temporal lobe,hippocampus and amygdala removed in 1953 in an attempt to cure his intractable epilepsy. Afterthe surgery, H.M. could still form new procedural memories and short-term memories, but long-lasting declarative memories could no longer be formed. The nature of the exact brain surgery heunderwent, and the types of amnesia he experienced, allowed a good understanding of howparticular areas of the brain are linked to specific processes in memory formation. In particular, hisability to 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 the hippocampus. The fact that hewas able to learn hand-eye coordination skills such as mirror drawing, despite having absolutelyno memory of having learned or practised the task before, also suggested the existence differenttypes of long-term memory, which are now known as declarative and procedural memories

There is strong evidence, notably by studying amnesic patients and the effect of priming, tosuggest that implicit memory is largely distinct from explicit memory, and operates through a different process in the brain. Studies ofthe effects of amnesia have shown that it is quite possible to have an intact implicit memory despite a severely impaired explicitmemory. Priming is the effect in which exposure to a stimulus influences response to a subsequent stimulus, so that, for instance, if aperson reads a list of words including the word concert, and is later asked to complete a word starting with con, there is a higherprobability 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 supports explicit memory.

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3/19/12 10:10 PMEpisodic Memory and Semantic Memory - Types of Memory - The Human Memory

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Females consistently performbetter 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 in women,which means that females ingeneral are more likely thanmales to recall their mistakes.


Experiments on rats in the1970s showed that there areover a million place cells in arats hippocampus, each ofwhich only becomes activewhen the rat is located in a veryspecific part of its environment.All together they can form avery precise cognitive mapthat 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.


Recent research into linksbetween memory andhandedness suggest that"mixed-handers" (whotypically 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, communicationbetween the brain'shemispheres than strong-handers, and possibly even athicker corpus callosum.


Types of Memory

Introduction

Sensory Memory


Long-Term Memory

Explicit & Implicit


Episodic & Semantic


EPISODIC & SEMANTIC MEMORYDeclarative memory can be further sub-divided into episodic memory and semantic memory.

Episodic memory represents our memory of experiences and specific events in time in a serialform, from which we can reconstruct the actual events that took place at any given point in ourlives. It is the memory of autobiographical events (times, places, associated emotions and othercontextual knowledge) that can be explicitly stated. Individuals tend to see themselves as actors inthese events, and the emotional charge and the entire context surrounding an event is usuallypart of the memory, not just the bare facts of the event itself.

The memories of the different elements of a particular event are distributed in the various visual,olfactory and auditory areas of the brain, but they are all connected together by the hippocampusto form an episode, rather than remaining a collection of separate memories. For example,memories of peoples faces, the taste of the wine, the music that was playing, etc, might all be partof the memory of a particular dinner with friends. By repeatedly reactivating or playing back thisparticular activity pattern in the various regions of the cortex, they become so strongly linked withone another that 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 be enough tobring back the entire scene of the dinner party.

Semantic memory, on the other hand, is a more structured record of facts, meanings, conceptsand knowledge about the external world that we have acquired. It refers to general factualknowledge, shared with others and independent of personal experience and of thespatial/temporal context in which it was acquired. Semantic memories may once have had apersonal context, but now stand alone as simple knowledge. It therefore includes such things astypes of food, capital cities, social customs, functions of objects, vocabulary, understanding ofmathematics, etc. Much of semantic memory is abstract and relational and is associated with themeaning of verbal symbols.

The semantic memory is generally derived from the episodic memory, in that we learn new factsor concepts from our experiences, and the episodic memory is considered to support andunderpin semantic memory. A gradual transition from episodic to semantic memory can takeplace, in which episodic memory reduces its sensitivity and association to particular events, so thatthe information can be generalized as semantic memory.

Both episodic memory and semantic memory require a similar encoding process. However,semantic memory mainly activates the frontal and temporal cortexes, whereas episodic memoryactivity is concentrated in the hippocampus, at least initially. Once processed in the hippocampus,episodic memories are then consolidated and stored in the neocortex. Our spatial memory inparticular appears to be much more confined to the hippocampus, particularly the righthippocampus, which seems to be able to create a mental 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 these personal memories are not mistaken for reallife. This difference in the neurological processing of episodic and semantic memory is illustrated by cases of anterograde amnesiacases (a good example being a case known as C.L.) in which episodic memory is almost completely lost while semantic memory isretained.

A further category of declarative memory, often referred to as autobiographical memory, issometimes distinguished (although episodic memory is also sometimes referred to asautobiographical memory). This is a memory system consisting of episodes recollected from anindividuals own life, based on a combination of episodic memory (personal experiences andspecific objects, people and events experienced at particular times and places) and semanticmemory (general knowledge and facts about the world).

One specific type of autobiographical memory is known as a "flashbulb memory", a highlydetailed, exceptionally vivid snapshot of a moment or circumstances in which surprising andconsequential (or emotionally arousing) news was heard, famous examples being theassassination of John Kennedy, the terrorist bombings on 9/11, etc. Such memories are believedby some to be highly resistant to forgetting, possibly due to the strong emotions that are typicallyassociated with them. However, a number of studies also suggest that flashbulb memories areactually not especially accurate, despite apparently being experienced with great vividness andconfidence.

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3/19/12 10:10 PMRetrospective and Prospective Memory - Types of Memory - The Human Memory

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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 when projectionsare made into the future.This is sometimes referred toas mental time travel as itallows us to project ourselvesat will either backwards orforwards in time within ourpersonal lives.


Types of Memory

Introduction

Sensory Memory


Long-Term Memory

Explicit & Implicit


Episodic & Semantic


RETROSPECTIVE & PROSPECTIVE MEMORYAn important alternative classification of long-term memory used by some researchers is basedon the temporal direction of the memories.

Retrospective memory is where the content to be remembered (people, words, events, etc) is inthe past, i.e. the recollection of past episodes. It includes semantic, episodic and autobiographicalmemory, and declarative memory in general, although it can be either explicit or implicit.

Prospective memory is where the content is to be remembered in the future, and may be definedas remembering to remember or remembering to perform an intended action. It may be eitherevent-based or time-based, often triggered by a cue, such as going to the doctor (action) at 4pm(cue), or remembering to post a letter (action) after seeing a mailbox (cue).

Clearly, though, retrospective and prospective memory are not entirely independent entities, andcertain aspects of retrospective memory are usually required for prospective memory. Thus, therehave been case studies where an impaired retrospective memory has caused a definite impact onprospective memory. However, there have also been studies where patients with an impairedprospective memory had an intact retrospective memory, suggesting that to some extent the twotypes of memory involve separate processes.

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3/19/12 10:11 PMMemory Processes - The Human Memory

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Memory Processes: Diagram by Luke Mastin


Memory Processes

Introduction

Memory Encoding

Memory Consolidation

Memory Storage

Memory Recall/Retrieval

MEMORY PROCESSESWe have already looked at the different stages of memory formation(from perception to sensory memory to short-term memory to long-termmemory) in the section on Types of Memory. This section, however,looks at the overall processes involved.

Memory is the ability to encode, store and recall information. The threemain processes involved in human memory are therefore encoding,storage and recall (retrieval). Additionally, the process of memoryconsolidation (which can be considered to be either part of theencoding process or the storage process) is treated here as a separateprocess in its own right.

Some of the physiology and neurology involved in these processes ishighly complex and technical (and some of it still not completelyunderstood), and lies largely outside the remit of this entry level guide,although at least a general introduction is given here. Moreinformation on the architecture of the human brain, and the neurologicalprocesses by which memory is encoded, stored and recalled can befound in the section on Memory and the Brain.

In this section:

Memory EncodingMemory ConsolidationMemory StorageMemory Recall/Retrieval

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Studies suggest thatcharacteristics of theenvironment are encoded aspart of the memory trace, andcan be used to enhanceretrieval of the other informationin 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 on land,and students perform better onexams by studying in silence,because exams are usuallydone in silence.


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


When presented with a visualstimulus, the part of the brain

which is activated the mostdepends on the nature of theimage.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 regionsmost, while the visual areas arescarcely stimulated at all.


Memory Processes

Introduction

Memory Encoding


Memory Storage


MEMORY ENCODINGEncoding is the crucial first step to creating a new memory. It allows the perceived item of interestto be converted into a construct that can be stored within the brain, and then recalled later fromshort-term or long-term memory.

Encoding is a biological event beginning with perception through the senses. The process oflaying down a memory begins with attention (regulated by the thalamus and the frontal lobe), inwhich a memorable event causes neurons to fire more frequently, making the experience moreintense and increasing the likelihood that the event is encoded as a memory. Emotion tends toincrease attention, and the emotional element of an event is processed on an unconsciouspathway in the brain leading to the amygdala. Only then are the actual sensations derived froman event processed.

The perceived sensations are decoded in the various sensory areas of the cortex, and thencombined in the brains hippocampus into one single experience. The hippocampus is thenresponsible for analyzing these inputs and ultimately deciding if they will be committed to long-termmemory. It acts as a kind of sorting centre where the new sensations are compared andassociated with previously recorded ones. The various threads of information are then stored invarious different parts of the brain, although the exact way in which these pieces are identified andrecalled later remains largely unknown.

Although the exact mechanism is not completely understood, encoding occurs on different 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 a process of memory consolidation. Theprocess begins with the creation of a memory trace or engram in response to the external stimuli. An engram is a hypotheticalbiophysical or biochemical change in the neurons of the brain, hypothetical in the respect that no-one has ever actually seen, or evenproved the existence of, such a construct.

An organ called the hippocampus, deep within the medial temporal lobe of the brain, receives connections from the primarysensory areas of the cortex, as well as from associative areas and the rhinal and entorhinal cortexes. While these 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 are linked to the individual memory.

There are three or four main types of encoding:

Acoustic encoding is the processing and encoding of sound, words and other auditoryinput for storage and later retrieval. This is aided by the concept of the phonological loop,which allows input within our echoic memory to be sub-vocally rehearsed in order tofacilitate remembering.Visual encoding is the process of encoding images and visual sensory information. Visualsensory information is temporarily stored within the iconic memory before being encodedinto long-term storage. The amygdala (within the medial temporal lobe of the brain whichhas a primary role in the processing of emotional reactions) fulfills an important role invisual encoding, as it accepts visual input 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 oftouch. Physiologically, neurons in the primary somatosensory cortex of the brain react tovibrotactile stimuli caused by the feel of an object.Semantic encoding is the process of encoding sensory input that has particular meaningor can be applied to a particular context, rather than deriving from a particular sense.

In general, encoding for short-term memory storage in the brain relies primarily on acousticencoding, while encoding for long-term storage is more reliant (although not exclusively) onsemantic encoding.

Human memory is fundamentally associative, meaning that a new piece of information is remembered better if it can be associatedwith previously acquired knowledge that is already firmly anchored in memory. The more personally meaningful the association, themore effective the encoding and consolidation. Memory encoding can therefore be improved by a strategy of organization of memorycalled elaboration, in which new pieces of information are associated with other information already recorded in long-term memory,thus incorporating them into a broader, coherent narrative which is already familiar.

An example of elaboration is the use of mnemonics, which are verbal, visual or auditoryassociations with other, easy-to-remember constructs, which can then be related back to the datathat is to be remembered. Common examples are Roy G. Biv to remember the order of thecolours of the rainbow, or Every Good Boy Deserves Favour for the musical notes on the lines ofthe treble clef, which most people find easier to remember than the original list of colours or letters.When we use mnemonic devices, we are effectively passing facts through the hippocampusseveral times, so that it can keep strengthening the associations, and therefore improve thelikelihood of subsequent memory recall. In the same way, associating words with images isanother commonly used mnemonic device, providing two alternative methods of remembering, andcreating additional associations in the mind.

Another method of improving memory encoding and consolidation is the use of a so-calledmemory palace (also known as the method of loci), a mnemonic techniques that relies onmemorized spatial relationships to establish, order and recollect other memories. The method is


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font considered easy to read.It is believed that presentinginformation in a way that is hardto digest means that a personhas to concentrate more,leading to deeper processingand therefore better retrievalafterwards.

memorized spatial relationships to establish, order and recollect other memories. The method isto assign objects or facts to different rooms in an imaginary house or palace, so that recall of thefacts can be cued by mentally walking though the palace until it is found. Many top memorizerstoday use the memory palace method to a greater or lesser degree.

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 largely discredited. Research, dating back to WilliamJames towards the end of the 19th Century, shows that long hours spent memorizing does notbuild up the powers of memory 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 James found, dailytraining in the memorization of a poetry of one author, for example, does not improves a persons ability to learn the poetry of anotherauthor, or poetry in general.

Many studies have shown that the most vivid autobiographical memories tend to be of emotional events, which are likely to berecalled more often and with more clarity and detail than neutral events. One theory suggests that high levels of emotional arousallead to attention narrowing, where the range of sensitive cues from the stimulus and its environment is decreased, so thatinformation central to the source of the emotional arousal is strongly encoded while peripheral details are not (e.g. the so-calledweapon focus effect, in which witnesses to a crime tend to remember the gun or knife in great detail, but not other more peripheraldetails such as the perpetrators clothing or vehicle).

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3/19/12 10:12 PMMemory Consolidation - Memory Processes - The Human Memory

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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 memory iswhy, for example, a policeofficer investigating a crimeshould not show a picture of asingle 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, there isno real way of knowing whetherthe victim is actuallyremembering the assailant orjust the picture.

??? Did You Know ???Studies have shown thatinformation is transferredbetween the hippocampus andthe cerebral cortex during deepsleep, and sleep appears to beessential for the properconsolidation of long-termmemories.However, even daytime napscan help improve memory tosome extent, and helps with thememorization of importantfacts.


Memory Processes

Introduction

Memory Encoding


Memory Storage


MEMORY CONSOLIDATIONConsolidation is the processes of stabilizing a memory trace after the initial acquisition. It mayperhaps be thought of part of the process of encoding or of storage, or it may be considered as amemory process in its own right. It is usually considered to consist of two specific processes,synaptic consolidation (which occurs within the first few hours after learning or encoding) andsystem consolidation (where hippocampus-dependent memories become independent of thehippocampus over a period of weeks to years).

Neurologically, the process of consolidation utilizes a phenomenon called long-term potentiation,which allows a synapse to increase in strength as increasing numbers of signals are transmittedbetween the two neurons. Potentiation is the process by which synchronous firing of neuronsmakes those neurons more inclined to fire together in the future. Long-term potentiation occurswhen the same group of neurons fire together so often that they become permanently sensitized toeach other. As new experiences accumulate, the brain creates more and more connections andpathways, and may re-wire itself by re-routing connections and re-arranging its organization.

As such a neuronal pathway, or neural network, is traversed over and over again, an enduringpattern is engraved and neural messages are more likely to flow along such familiar paths of leastresistance. For example, if a piece of music is played over and over, the repeated firing of certaincells in a certain order in your brain makes it easier to repeat this firing later on, with the result thatthe musician becomes better at playing the music, and can play it faster, with fewer mistakes.

In this way, the brain organizes and reorganizes itself in response to experiences, creating newmemories prompted by experience, education or training. The ability of the connection, or synapse,between two neurons to change in strength, and for lasting changes to occur in the efficiency ofsynaptic transmission, is known as synaptic plasticity or neural plasticity, and it is one of theimportant neurochemical foundations of memory and learning.

It should be remembered that each neuron makes thousands of connections with other neurons, and memories and neuralconnections are mutually interconnected in 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 encodedwithin a single neural network, by different patterns of synaptic connections. Conversely, a single memory may involve simultaneouslyactivating 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 take place, whereby the neural networks involved inerroneous movements are inhibited by the silencing of their synaptic connections. This can occur in the cerebellum, which is locatedtowards the back of the brain, in order to correct our motor procedures when learning how to perform a task (procedural memory), butalso in the synapses of the cortex, the hippocampus, the striatum and other memory-related structures.

Contrary to long-term potentiation, which is triggered by high-frequency stimulation of thesynapses, long-term depression is produced by nerve impulses reaching the synapses at very lowfrequencies, leading them to undergo the reverse transformation from long-term potentiation, and,instead of becoming more efficient, the synaptic connections are weakened. It is still not clearwhether long-term depression contributes directly to the storage of memories in some way, orwhether it simply makes us forget the traces of some things learned long ago so that new thingscan be learned.

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

Memory re-consolidation is the process of previously consolidated memories being recalled andthen actively consolidated all over again, in order to maintain, strengthen and modify memoriesthat are already stored in the long-term memory. Several retrievals of memory (either naturallythrough reflection, or through deliberate recall) may be needed for long-term memories to last formany years, depending on the depth of the initial processing. However, these individual retrievals can take place at increasingintervals, in accordance with the principle of spaced repetition (this is familiar to us in the way that cramming the night before anexam is not as effective as studying at intervals over a much longer span of time).

Research into a cognitive disorder known as Korsakoffs syndrome shows that the retrograde amnesia of sufferers follows a distincttemporal curve, in that the more remote the event in the past, the better it is preserved. This suggests that the more recentmemories are not fully consolidated and therefore more vulnerable to loss, indicating that the process of consolidation may continuefor much longer than initially thought, perhaps for many years.

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Genuine eidetic orphotographic memory (anunprocessed sensory memoryof sensory events that is asaccurate as if the person werestill experiencing the originalobjects or events) is extremelyrare, although not unheard of.Most extraordinary memoryskills which make claims ofphotographic memory,however, result from acombination of innate skills,learned tactics, mnemonicdevices and extraordinaryknowledge bases, rather thaneidetic memory as such.Young children, with their lackof verbal and conceptualsystems, are more likely tohave real eidetic memoriesthan adults.A classic case from the 1970swas a woman namedElizabeth, who was able towrite out poetry in a foreignlanguage (of which she had noprior knowledge) several yearsafter seeing the original text.


Research using functionalmagnetic resonance imaging(fMRI) suggests that verbs andnouns are stored in differentways in the brain.Concrete nouns are stored inareas of the brain used tosense or manipulate thereferent objects, leading to atheory of meaning basedlargely on function.


Memory Processes

Introduction

Memory Encoding


Memory Storage


MEMORY STORAGEStorage is the more or less passive process of retaining information in the brain, whether in thesensory memory, the short-term memory or the more permanent long-term memory. Each of thesedifferent stages of human memory function as a sort of filter that helps to protect us from the floodof information that confront us on a daily basis, avoiding an overload of information and helping tokeep us sane. The more the information is repeated or used, the more likely it is to be retained inlong-term memory (which is why, for example, studying helps people to perform better on tests).This process of consolidation, the stabilizing of a memory trace after its initial acquisition, is treatedin more detail in a separate section.

Since the early neurological work of Karl Lashley and Wilder Penfield in the 1950s and 1960s, ithas become clear that long-term memories are not stored in just one part of the brain, but arewidely distributed throughout the cortex. After consolidation, long-term memories are storedthroughout the brain as groups of neurons that are primed to fire together in the same pattern thatcreated the original experience, and each component of a memory is stored in the brain area thatinitiated it (e.g. groups of neurons in the visual cortex store a sight, neurons in the amygdala storethe associated emotion, etc). Indeed, it seems that they may even be encoded redundantly,several times, in various parts of the cortex, so that, if one engram (or memory trace) is wiped out,there are duplicates, or alternative pathways, elsewhere, through which the memory may still beretrieved.

Therefore, contrary to the popular notion, memories are not stored in our brains like books onlibrary shelves, but must be actively reconstructed from elements scattered throughout variousareas of the brain by the encoding process. Memory storage is therefore an ongoing process ofreclassification resulting from continuous changes in our neural pathways, and parallel processingof information in our brains.

The indications are that, in the absence of disorders due to trauma or neurological disease, thehuman brain has the capacity to store almost unlimited amounts of information indefinitely.Forgetting, therefore, is more likely to be result from incorrectly or incompletely encoded memories,and/or problems with the recall/retrieval process. It is a common experience that we may try toremember something one time and fail, but then remember that same item later. The information istherefore clearly still there in storage, but there may have been some kind of a mismatch betweenretrieval cues and the original encoding of the information. Lost memories recalled with the aid ofpsychotherapy or hypnosis are other examples supporting this idea, although it is difficult to be sure that such memories are realand not implanted by the treatment.

Having said that, though, it seems unlikely that, as Richard Schiffrin and others have claimed, ALL memories are stored somewherein the brain, and that it is only in the retrieval process that irrelevant details are fast-forwarded over or expurgated. It seems morelikely that the memories which are stored in some way edited and sorted, and that some of the more peripheral details are neverstored.

Forgetting, then, is perhaps better thought of as the temporary or permanent inability to retrieve a piece of information or a memorythat had previously been recorded in the brain. Unlike amnesia, forgetting is usually regarded as a normal phenomenon involvingspecific pieces of content, rather than relatively broad categories of memories or even entire segments of memory.

Theorists disagree over exactly what becomes of material that is forgotten. Some hold that long-term memories do actually decay and disappear completely over time; others hold that thememory trace remains intact as long as we live, but the bonds or cues that allow us to retrieve thetrace become broken, due to changes in the organization of the neural network, new experiences,etc, in the same way as a misplaced book in a library is lost even though it still exists somewherein the library.

Interestingly, it appears not to be possible to deliberately delete memories at will, which can havenegative consequences, for example if we experience traumatic events we would actually prefer toforget. In fact, such memories tend to be imprinted even more strongly than normal due to theiremotional content, although recent research involving the use of beta blockers (such aspropanonol) suggests that it may be possible to tone down the emotional aspects of suchmemories, even if the memories themselves cannot be erased. The way this works is that the actof recalling stored memories makes them "malleable" once more, as they were during the initialencoding phase, and their re-storage can then be blocked by drugs which inhibit the proteins thatenable the emotional memory to be re-saved.

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Memory recall appears to bestate-dependent, at least tosome extent.Studies have shown that, whenmaterial is learned under theinfluence of a drug or alcohol,for example, it is subsequentlyrecalled better when in thesame drug state than whensober.Similarly, individuals tend toretrieve information more easilywhen it has the sameemotional content as theircurrent emotional state, andwhen the emotional state at thetime of retrieval is similar to theemotional state at the time ofencoding.


Several studies have shownthat both episodic and semanticmemories can be betterrecalled when the samelanguage is used for bothencoding and retrieval.For example, bilingual Russianimmigrants to the United Statescan recall moreautobiographical details of theirearly life when the questionsand cues are presented inRussian than when they arequestioned in English.


Memory Processes

Introduction

Memory Encoding


Memory Storage


MEMORY RECALL/RETRIEVALRecall or retrieval of memory refers to the subsequent re-accessing of events or information fromthe past, which have been previously encoded and stored in the brain. In common parlance, it isknown as remembering. During recall, the brain "replays" a pattern of neural activity that wasoriginally generated in response to a particular event, echoing the brain's perception of the realevent. These replays are not quite identical to the original, though - otherwise we would not knowthe difference between the genuine 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-flyreconstruction of elements scattered throughout various areas of our brains. Memories are notstored in our brains like books on library shelves, or even as a collection of self-containedrecordings or pictures or video clips, but may be better thought of as a kind of collage or a jigsawpuzzle, involving different elements stored in disparate parts of the brain linked together byassociations and neural networks. Memory retrieval therefore requires re-visiting the nervepathways the brain formed when encoding the memory, and the strength of those pathwaysdetermines how quickly the memory can be recalled. Recall effectively returns a memory fromlong-term storage to short-term or working memory, where it can be accessed, in a kind of mirrorimage of the encoding process. It is then re-stored back in long-term memory, thusre-consolidating and strengthening it.

The efficiency of human memory recall is astounding. Most of what we remember is by directretrieval, where items of information are linked directly a question or cue, rather than by the kind ofsequential scan a computer might use (which would require a systematic search through theentire contents of memory until a match is found). Other memories are retrieved quickly and efficiently by hierarchical inference,where a specific question is linked to a class or subset of information about which certain facts are known. Also, the brain is usuallyable to determine in advance whether there is any point in searching memory for a particular fact (e.g. it instantly recognizes aquestion like What is Socrates telephone number? as absurd in that no search could ever produce an answer).

There are two main methods of accessing memory: recognition and recall. Recognition is the association of an event or physicalobject with one previously experienced or encountered, and involves a process of comparison of information with memory, e.g.recognizing a known face, true/false or multiple choice questions, etc. Recognition is a largely unconscious process, and the braineven has a dedicated face-recognition area, which passes information directly through the limbic areas to generate a sense offamiliarity, before linking up with the cortical path, where data about the person's movements and intentions are processed. Recallinvolves remembering a fact, event or object that is not currently physically present (in the sense of retrieving a representation, mentalimage or concept), and requires the direct uncovering 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 more effective), in that it requires just a singleprocess rather than two processes. Recognition requires only a simple familiarity decision, whereas a full recall of an item frommemory requires a two-stage process (indeed, this is often referred to as the two-stage theory of memory) in which the search andretrieval of candidate items from memory is followed by a familiarity decision where the correct information is chosen from thecandidates retrieved. Thus, recall involves actively reconstructing the information and requires the activation of all the neuronsinvolved in the memory in question, whereas recognition only requires a relatively simple decision as to whether one thing amongothers has been encountered before. Sometimes, however, even if a part of an object initially activates only a part of the neuralnetwork concerned, recognition may then suffice to activate the entire network.

In the 1980s, Endel Tulving proposed an alternative to the two-stage theory, which he called thetheory of encoding specificity. This theory states that memory utilizes information both from thespecific memory trace as well as from the environment in which it is retrieved. Because of itsfocus on the retrieval environment or state, encoding specificity takes into account context cues,and it also has some advantages over the two-stage theory as it accounts for the fact that, inpractice, recognition is not actually always superior to recall. Typically, recall is better when theenvironments are similar in both the learning (encoding) and recall phases, suggesting that contextcues are important. In the same way, emotional material is remembered more reliably in moodsthat match the emotional content of these memories (e.g. happy people will remember morehappy than sad information, whereas sad people will better remember sad than happyinformation).

According to the levels-of-processing effect theory, another alternative theory of memorysuggested by Fergus Craik and Robert Lockhart, memory recall of stimuli is also a function ofthe depth of mental processing, which is in turn determined by connections with pre-existingmemory, time spent processing the stimulus, cognitive effort and sensory input mode. Thus,shallow processing (such as, typically, that based on sound or writing) leads to a relatively fragilememory trace that is susceptible to rapid decay, whereas deep processing (such as that basedon semantics and meanings) results in a more durable memory trace. This theory suggests, then, that memory strength iscontinuously variable, as opposed to the earlier Atkinson-Shiffrin, or multi-store, memory model, which just involves a sequence ofthree 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 attentionat the time of recall tends to slow down the retrieval process to some extent, it typically has little to no effect on the accuracy ofretrieved memories. Distraction at the time of encoding, on the other hand, can severely impair subsequent retrieval success.

The efficiency of memory recall can be increased to some extent by making inferences from our personal stockpile of worldknowledge, and by our use of schemata. A schema (plural: schemata) is an organized mental structure or framework of pre-


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Several recent studies in thegrowing area of neuro-education have shown thevalue of the "testing effect" (or"retrieval effect"), wherequizzes a short time after initiallearning significantly improvessubsequent retrieval of factsand ideas, as well as overallunderstanding of topics and theability to solve relatedproblems.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.

knowledge, and by our use of schemata. A schema (plural: schemata) is an organized mental structure or framework of pre-conceived ideas about the world and how it works, which we can use to make realistic inferences and assumptions about how tointerpret and process information. Thus, our everyday communication consists not just of words and their meanings, but also of whatis left out and mutually understood (e.g. if someone says it is 3 oclock, our knowledge of the world usually allows us to knowautomatically whether it is 3am or 3pm). Such schemata are also applied to recalled memories, so that we can often flesh out detailsof a memory from just a skeleton memory of a central event or object.

There are three main types of recall:

Free recall is the process in which a person is given a list of items to remember and then is asked to recall them in any order(hence the name free). This type of recall often displays evidence of either the primacy effect (when the person recalls itemspresented at the beginning of the list earlier and more often) or the recency effect (when the person recalls items presented atthe end of the list earlier and more often), and also of the contiguity effect (the marked tendency for items from neighbouringpositions in the list to be recalled successively).Cued recall is the process in which a person is given a list of items to remember and isthen tested with the use of cues or guides. When cues are provided to a person, they tendto remember items on the list that they did not originally recall without a cue, and whichwere thought to be lost to memory. This can also take the form of stimulus-responserecall, as when words, pictures and numbers are presented together in a pair, and theresulting associations between the two items cues the recall of the second item in the pair.Serial recall refers to our ability to recall items or events in the order in which theyoccurred, whether chronological events in our autobiographical memories, or the order ofthe different parts of a sentence (or phonemes in a word) in order to make sense of them.

Serial recall in long-term memory appears to differ from serial recall in short-term memory,in that a sequence in long-term memory is represented in memory as a whole, rather thanas a series of discrete items. Testing of serial recall by psychologists have yielded severalgeneral rules:

more recent events are more easily remembered in order (especially with auditorystimuli);recall decreases as the length of the list or sequence increases;there is a tendency to remember the correct items, but in the wrong order;where errors are made, there is a tendency to respond with an item that resemblesthe original item in some way (e.g. dog instead of fog, or perhaps an itemphysically close to the original item);repetition errors do occur, but they are relatively rare;if an item is recalled earlier in the list than it should be, the missed item tends to beinserted immediately after it;if an item from a previous trial is recalled in a current trial, it is likely to be recalled atits position from the original trial.

If we assume that the "purpose" of human memory is to use past events to guide future actions, then keeping a perfect and completerecord of every past event is not necessarily a useful or efficient way of achieving this. So, in most people, some specific memoriesmay be given up or converted into general knowledge (i.e. converted from episodic to semantic memories) as part of theongoing recall/re-consolidation process, so that that we are able to generalize from experience. It is also possible that false orwrongly interpreted memories may be created during recall, and carried forward thereafter. One can also, up to a point, choose toforget, by blocking out unwanted memories during recall (a process achieved by frontal lobe activity, which inhibits the laying downor re-consolidation of a memory.

However, there is a rare condition called hyperthymesia (also known as hypermnesia or superior autobiographical memory) inwhich a few people show an extraordinary capacity to recall detailed specific events from a persons personal past, without relying onpractised mnemonic strategies. Although only a handful of cases of hyperthymesia have ever been definitively confirmed, some ofthese cases are quite startling, such as a California woman who could recall every day in complete detail from the age of 14onwards, a young English girl with an IQ of 191 who had a perfect photographic memory spanning almost 18 years, and a Russianman known simply as "S." who was only able to forget anything by a deliberate act of will. One of the most famous cases, known asA.J., described it as a burden rather than a gift, but others seem to be able to organize and compartmentalize their prodigiousmemories and do not appear to feel that their brains are "cluttered" with excess information. Interestingly, recent research has shownthat such individuals tend to have significantly larger than average temporal lobes and caudate nuclei, and many exhibit mildObsessive Compulsive Disorder-like behaviour (the caudate nucleus is also associated with OCD).

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3/19/12 10:12 PMMemory & the Brain - The Human Memory

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The live human brain is actually pinkish-beige in colour


Memory & the Brain

Introduction

Parts of the Brain

Neurons & Synapses

MEMORY & THE BRAINIn all vertebrates and most invertebrates, the brain is the centre of the nervous system.It allows them to collect information (sensory system), act on that information (motorsystem) and store the result for future reference (memory), thus effectively making lifepossible.

The human brain is the most complex of all, and indeed is perhaps the most complexliving structure known in the universe. Although it has the same general structure as thebrains of other mammals, is over three times as large as the brain of a typical mammalwith an equivalent body size, and much more complex.

The adult human brain weighs on average about 1.5 kg (3lbs), and is about the size of asmall head of cauliflower. It is very soft (having a consistency similar to soft gelatine orfirm tofu) and, despite being referred to as "grey matter", the live brain is actuallypinkish-beige in colour (although it may turn grey after death) and slightly off-white in theinterior. Almost 80% of the brain consists of water (mainly in the cytoplasm of its cells),with a further 10-12% being fatty lipids and 8% protein. Although it accounts for just 2%of body weight, it uses fully 20% of the body's glucose as fuel, supplied by constant bloodflow. It is protected by the thick bones of the skull, suspended in cerebrospinal fluid,and isolated from the bloodstream by the blood-brain barrier, but the delicate nature ofthe human brain nevertheless makes it susceptible to many types of damage anddisease.

It is a hugely complex organ, with an estimated 100 billion neurons passing signals toeach other via as many as 1,000 trillion synaptic connections. It continuously receivesand analyzes sensory information, responding by controlling all bodily actions andfunctions. It is also the centre of higher-order thinking, learning and memory, and givesus the power to think, plan, speak, imagine, dream, reason and experience emotions.

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Parts of the BrainNeurons & Synapses

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memory, human memory, types of memory, memory processes, how memory works, how human memory works


3/19/12 10:13 PMParts of the Brain - Memory & the Brain - The Human Memory

Page 1 of 2http://www.human-memory.net/brain_parts.html

Important structures in the human brainPicture from Wikipedia

(http://en.wikipedia.org/wiki/Human_brain)

Lobes of the cerebral cortexPicture from Wikipedia

(http://en.wikipedia.org/wiki/Human_brain)

The Limbic System and Basal GangliaPicture from How Stuff Works


Memory & the Brain

Introduction

Parts of the Brain

Neurons & Synapses

PARTS OF THE BRAINThe human brain is hugely interconnected but three major components can beidentified: the cerebrum, the cerebellum and the brain stem.

The brainstem which includes the medulla, the pons and the midbrain,controls breathing, digestion, heart rate and other autonomic processes, as wellas connecting the brain with the spinal cord and the rest of the body.

The cerebellum plays an important role in balance, motor control, but is alsoinvolved in some cognitive functions such as attention, language, emotionalfunctions (such as regulating fear and pleasure responses) and in theprocessing of procedural memories.

The cerebrum (or forebrain), which makes up 75% of the brain by volume and85% by weight, is divided by a large groove, known as the longitudinal fissure,into two distinct hemispheres. The left and right hemispheres ("left" and "right"refer to the owner's point of view, not an outside viewer's) are linked by a largebundle of nerve fibres called the corpus callosum, and also by other smallerconnections called commissures.

Most of the important elements of the cerebrum, are split into symmetrical pairsin the left and right hemispheres. Thus, we often speak of the temporal lobes,hippocampi, etc (in the plural), although this website generally follows theconvention of speaking of the temporal lobe, hippocampus, etc (in the singular),which should therefore be taken to mean both sides, within both hemispheres.The two hemispheres look similar, but are slightly different in structure andperform differen

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