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Memory is "the capacity of the nervous system to acquire and retain usable skills and knowledge, which allows living organisms to benefit from experience." (source:

Today's memory is stuffed with faces, birthdays, telephone numbers, meetings, mathematical functions and the like - we may call these things informations. But how is this information stored, accessed and manipulated? Or, on the other side, why or how do we forget? Can we influence this process to save information or can we store more information in less time? It is indisputable that the process of saving and retrieving informations has affected human life until now. If, for example, our ancestors had not been able to memorize the location and the edibility of certain roots and berries, the development of crafts, house building, in short, of civilization, would surely have been a rather complicated and slow process.

First we are going to give an basic overview on how memory works. First of all we will approach the question "What is Memory?". We will discuss three types of memory - the Sensory Memory, the Short Term Memory and the Long Term Memory.

We will also mention memory models, like the "Atkinson's & Shiffrin's Memory Model", are going to be introduced and advance with Baddeley‘s "Working Memory Model". Further we will compare declarative and implicit memory (different types of Long Term Memory, LTM).

Finally, we will turn to another interesting phenomenon related to our topic, errors. By examining the occurrence of errors we will be able to introduce the designated functions of the memory parts in the brain.

What is Memory

What ability enables us to learn new things and thereby helps us survive? The answer to this question is: memory. Without memory we would not have been able to develop into what we now are, without memory we would probably have died out in the early years of human life. We would not have known which berries to collect, which ones were good and healthy and which ones were poisonous. Or how to find our way home without getting lost, or how to start a fire. We most probably would not be what we are now: a technologically advanced society that rules the planet and explores the universe. So how does this amazing thing called memory work? What is it? Where is it?

What is memory?

Memory is the ability of an organism to store, retain, and subsequently recall information. Although traditional studies of memory began in the realms of philosophy, the late nineteenth and early twentieth century put memory within the paradigms of cognitive psychology. In the recent decades, it
has become one of the principal pillars of a new branch of science that represents a marriage between cognitive psychology and neuroscience, called cognitive neuroscience. There are several ways of classifying memories, based on duration, nature and retrieval of information. From an information processing perspective there are three main stages in the formation and retrieval of memory: encoding (processing and combining of received information), storage (creation of a permanent record of the encoded information), retrieval/recall (calling back the stored information in response to some cue for use in some process or activity). We can classify memory also by: duration, information type, temporal direction

Classification by duration

A basic and generally accepted classification of memory is based on the duration of memory retention, and identifies three distinct types of memory: sensory memory, short term memory, and long term memory. The sensory memory corresponds approximately to the initial moment that an item is perceived. Some of this information in the sensory area proceeds to the sensory store, which is referred to as short-term memory. Sensory memory is characterized by the duration of memory retention from milliseconds to seconds and short-term memory from seconds to minutes. These stores are generally characterized as of strictly limited capacity and duration, whereas in general stored information can be retrieved in a period of time which ranges from days to years; this type of memory is called long-term memory. It may be that short-term memory is supported by transient changes in neuronal communication, whereas long-term memories are maintained by more stable and permanent changes in neural structure that are dependent on protein synthesis. Some psychologists, however, argue that the distinction between long- and short-term memories is arbitrary, and is merely a reflection of differing levels of activation within a single store. If we are given a random seven-digit number, we may remember it only for a few seconds and then forget (short-term memory). On the other hand, we can remember telephone numbers for many years (assuming we use them often enough). Those long-lasting memories are said to be stored in long term memory. Additionally, the term working memory is used to refer to the short-term store needed for certain mental tasks - it is not a synonym for short-term memory, since it is defined not in terms of duration, but rather in terms of purpose. Some theories consider working memory to be the combination of short-term memory and some attentional control. For instance, when we are asked to mentally multiply 45 by 4, we have to perform a series of simple calculations (additions and multiplications) to arrive at the final answer. The ability to store the information regarding the instructions and intermediate results is what is referred to as working memory.

Classification by information type

Long-term memory can be divided into declarative (explicit) and procedural (implicit) memories. Declarative memory requires conscious recall, in that some conscious process must call back the information. It is sometimes called explicit memory, since it consists of information that is explicitly stored and retrieved. Declarative memory can be further sub-divided into semantic memory, which concerns facts taken independent of context; and episodic memory, which concerns information specific to a particular context, such as a time and place. Semantic memory allows the encoding of abstract knowledge about the world, such as "Paris is the capital of France". Episodic memory, on the other hand, is used for more personal memories, such as the sensations, emotions, and personal associations of a particular place or time. Autobiographical memory - memory for particular events within one's own life - is generally viewed as either equivalent to, or a subset of, episodic memory. Visual memory is part of memory preserving some characteristics of our senses pertaining to visual
experience. We are able to place in memory information that resembles objects, places, animals or people in sort of a mental image. Visual memory can result In priming and it is assumed some kind of perceptual representation system or PRS underlies this phenomenon. In contrast, procedural memory (or implicit memory) is not based on the conscious recall of information, but on implicit learning. Procedural memory is primarily employed in learning motor skills and should be considered a subset of implicit memory. It is revealed when we do better in a given task due only to repetition - no new explicit memories have been formed, but we are unconsciously accessing aspects of those previous experiences. Procedural memory involved In motor learning depends on the cerebellum and basal ganglia.. So far, nobody has successfully been able to isolate the time dependence of these suggested memory structures.

Classification by temporal direction

A further major way to distinguish different memory functions is whether the content to be remembered is in the past, retrospective memory, or whether the content is to be remembered in the future, prospective memory. Thus, retrospective memory as a category includes semantic memory and episodic/autobiographical memory. In contrast, prospective memory is memory for future intentions, or remembering to remember. Prospective memory can be further broken down into event- and time-based prospective remembering. Time-based prospective memories are triggered by a time-cue, such as going to the doctor (action) at 4pm (cue). Event-based prospective memories are intentions triggered by cues, such as remembering to post a letter (action) after seeing a mailbox (cue). Cues do not need to be related to the action (as the mailbox example is), and lists, sticky-notes, or string around the finger are all examples of cues that are produced by people as a strategy to enhance prospective memory.

Most important brain structures responsible for memory


The amygdale also are involved in the modulation of memory consolidation. Following any learning event, the long term memory for the event is not instantaneously formed. Rather, information regarding the event is slowly assimilated into long-term storage over time, a process referred to as memory consolidation, until it reaches a relatively permanent state. During the consolidation period, the memory can be modulated. In particular, it appears that emotional arousal following the learning event influences the strength of the subsequent memory for that event. Greater emotional arousal following a learning event enhances a person's retention of that event. Experiments have shown that administration of stress hormones to individuals immediately after they learn something enhances their retention when they are tested two weeks later.

The amygdale, especially the basolateral nuclei, are involved in mediating the effects of emotional arousal on the strength of the memory for the event. There were experiments conducted by James McGaugh on animals in a special laboratories. These laboratories have trained animals on a variety of learning tasks and found that drugs injected into the amygdala after training affect the animals' subsequent retention of the task. These tasks include basic Pavlovian tasks such as inhibitory avoidance, where a rat learns to associate a mild footshock with a particular compartment of an apparatus, and more complex tasks such as spatial or cued water maze, where a rat learns to swim to a platform to escape the water. If a drug that activates the amygdale is injected into the amygdale, the animals had better memory for the training in the task. If a drug that inactivates the amygdale is injected, the animals had impaired memory for the task.

Despite the importance of the amygdale in modulating memory consolidation, however, learning can occur without it, though such learning appears to be impaired, as in fear conditioning impairments following amygdale damage.

Evidence from work with humans indicates that the amygdale plays a similar role. Amygdale activity at the time of encoding information correlates with retention for that information. However, this correlation depends on the relative "emotionalness" of the information. More emotionally-arousing information increases amygdalar activity, and that activity correlates with retention.


Psychologists and neuroscientists dispute the precise role of the hippocampus, but, in general, agree that it has an essential role in the formation of new memories about experienced events (episodic or autobiographical memory). Some researchers prefer to consider the hippocampus as part of a larger medial temporal lobe memory system responsible for general declarative memory (memories that can be explicitly verbalized — these would include, for example, memory for facts in addition to episodic memory).

Some evidence supports the idea that, although these forms of memory often last a lifetime, the hippocampus ceases to play a crucial role in the retention of the memory after a period of consolidation. Damage to the hippocampus usually results in profound difficulties in forming new memories (anterograde amnesia), and normally also affects access to memories prior to the damage (retrograde amnesia). Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories remain - this sparing of older memories leads to the idea that consolidation over time involves the transfer of memories out of the hippocampus to other parts of the brain. However, experimentation has difficulties in testing the sparing of older memories; and, in some cases of retrograde amnesia, the sparing appears to affect memories formed decades before the damage to the hippocampus occurred, so its role in maintaining these older memories remains controversial.

Damage to the hippocampus does not affect some aspects of memory, such as the ability to learn new skills (playing a musical instrument, for example), suggesting that such abilities depend on a different type of memory (procedural memory) and different brain regions. And there is some evidence to suggest that patient HM (who had his medial temporal lobes removed bilaterally as a treatment for epilepsy) can form new semantic memories.

Types of Memory

In the following section we will discuss the three different types of memory and their respective characteristics: Sensory Memory, Short Term (STM) or Working Memory and Long Term Memory (LTM).

Sensory Memory

This type of memory has the shortest duration time, only 0.5 to 2.0 seconds. Roughly, Sensory Memory can be subdivided into two kinds: iconic and echoic memory. The first is concerned with visual input, the latter with auditory input. (It should be noted, though, that according to the Atkinson and Shiffrin model of memory, only iconic memory is equal to sensory memory. The addition of echoic memory to the level of sensory memory is due to research done by Darwin and others (1972).) Let us consider the following intuitive example for iconic memory: probably we all know the phenomenon
that it seems possible to draw lines, figures or names with lighted sparklers by moving the sparkler fast enough in a dark environment. Physically, however, there are no such things as lines of light. So how come we can nevertheless see such figures? This is due to iconic memory. Roughly speaking, we can think of this subtype of memory as a kind of photographic memory, but one which only lasts for a very short time. The image of the light of a sparkler remains in our memory (persistence of vision) and thus makes it seem to us like the light would leave lines in the dark. The same effect occurs, e.g., if we watch a sprinkler and we think that we can see a ring of water drops. As for echoic memory, as the name already suggests, it is meant to apply to auditory input. Here the persistence time is a little longer as with iconic memory (up to 4 seconds). At the level of sensory memory no manipulation of the incoming information occurs, it is simply transferred to, e.g., short term memory (at least the information that is somehow important at the time of perception is transferred).

Short Term Memory

The term "short term memory" stems from the modal model approach to memory by Atkinson and Shiffrin. In more modern approaches the idea of short term memory has been further investigated and there seems to be evidence that also short term memory consists of several separated, but of course closely related, subparts. Baddeley (2000) introduced the nowadays most often used term "working memory". We will first look at the modal model approach and then go on to the concept of working memory.

Short Term Memory

As the name suggests, information is stored in short term memory for a rather short period of time (15-20 seconds). If we look up a phone number in the phone book and memorize it long enough until we dialed that number, it is stored in short term memory. (Unless we want to remember that phone number for a longer period of time, it will most probably not be stored in long term memory.) Now we know how long information can be stored in short term memory, but what about the question about how much can be stored? George Miller in his seminal paper (1956) proposed "the magical number seven, plus minus two". He showed that between 5 and 9 items can usually be stored in short term memory at a time. The term "item" might strike one as a little vague, all of the following are considered items: single digits or letters, whole words or even sentences, and the like. It has been shown by experiments also done by Miller that chunking is a useful method to memorize more than just single items. Gobet et al. defined a chunk as "a collection of elements that are strongly associated with one another but are weakly associated with other chunks" (p. 157 in Goldstein). A famous experiment was conducted by Chase and Simon (1973) with amateur and experienced chess players. When asked to remember certain arrangements of chess pieces on the board, the experts performed significantly better that the amateurs. However, if the pieces were arranged arbitrarily, i.e. not corresponding to possible game situations, both the experts and the amateurs performed equally bad. This shows that chunking (as done by experienced chess players) enhances the performance in such memory tasks.

Problems with the Modal Model Approach: According to the memory-model proposed by Atkinson and Shiffrin, all information has to pass the STM in order to be stored in LTM. However, cases have been reported where patients can form long term memories even though their STM-abilities are severely reduced. This clearly poses a problem to the modal model approach. It was suggested by Shallice and Warrington (1970) that there must be another possible way for information to enter LTM than via STM. Baddeley and Hitch (1974) drew attention to another problem. Under certain conditions
it seems to be possible to do two different tasks simultaneously, even though STM, as suggested by Atkinson and Shiffrin, should be regarded as a single, undivided unit. An example for the performance of two tasks simultaneously would be the following: a person is asked to memorize 4 numbers and then read a text (unrelated to the first task). Most people are able to recall the 4 numbers correctly after the reading task, so apparently both memorizing numbers and reading a text carefully can be done at the same time. According to Baddeley and Hitch the result of this experiment indicates that the number-task and the reading-task are handled by two different components of short term memory. So they coined the term "working memory" instead of "short term memory".

Working Memory

Working memory is defined by Baddeley (2000) as follows: "Working memory is a limited capacity system for temporary storage and manipulation of information for complex tasks such as comprehension, learning and reasoning" (p. 162 in Goldstein). What is interesting here is that (a) the system is limited in its capacity (the same limitations hold as for short term memory) and (b) that the task of working memory is not only storage, but also manipulation of incoming information. Working memory consists of three parts: the phonological loop, the visuospatial sketch pad and the central executive. We will consider each subpart in turn. Let us begin with the phonological loop.

The phonological loop is responsible for auditory and verbal information, such as phone numbers, person's names or general understanding of what other people are talking about. We could roughly say that it is a system specialized for language. This system can again be subdivided into an active and a passive part. The storage of information belongs to the passive part and fades after 2 second if the information is not rehearsed explicitly. Rehearsal, on the other hand, is regarded as the active part of the phonological loop. The repetition of information deepens the memory. There are three well-known phenomena that support the idea that the phonological loop is specialized for language: the phonological similarity effect, the word-length effect and articulatory suppression. When words that sound similar are confused, we speak of the phonological similarity effect. The word-length effect refers to the fact that it is more difficult to memorize a list of long words and better results can be achieved if a list of short words should be memorized. Let us consider the phenomenon of articulatory suppression in a little more detail. Consider the following experiment: participants are asked to memorize words while saying "the, the, the ..." out loud. What we find is that, with respect to the word-length effect, the difference in performance between lists of long and short words is levelled out. Both lists can be memorized equally well. The explanation given by Baddeley et al. (1984), who conducted this experiment, is that the constant repetition of the word "the" prevents the rehearsal of the words in the lists, independent of whether the list contains long or short words. The findings become even more drastic if we compare the memory-performance in the following experiment (also conducted by Baddeley and his co-workers in 1984): participants were again asked to say out loud "the, the, the ...". But instead of memorizing words from a list of short or long words, their task was to remember words that were either spoken to them or shown to them written on paper. The results indicated that the participant's performance was significantly better if the words were presented to them and not read out aloud. Baddeley concluded from this fact that the performance in a memory task is improved if the two stimuli can be dealt with in distinct components of the working memory. In other words, because the reading of words is handled in the visuospatial sketch pad, whereas the saying of "the" belongs to the phonological loop, the two tasks do not "block" each other. The rather bad performance of hearing words while speaking could be explained by the fact that both hearing and speaking are dealt with in the phonological loop and thus the two tasks conflict with each other, decreasing the performance of memorization.

In the visuospatial sketch pad visual and spatial information is stored. As we have seen above, performance decreases if two tasks that are dealt with in the same component are to be done simultaneously. Let us consider a further example that illustrates this effect. Brandimonte and co-workers (1992) conducted an experiment where participants were asked to say out loud "la, la, la ...". At the same time they were given the task of subtracting a partial image from a given whole image. The subtraction had to be done mentally because the two images were presented only for a short time. The interesting result was that not only did the performance not decrease while saying "la, la, la ..." when compared to doing the subtraction-task alone, but the performance even increased. According to Brandimonte this was due to the fact that the subtraction task was easier if handled in the visuospatial sketch pad as opposed to the phonological loop (both the given and the resulting pictures were such that they could also be named, i.e. verbalized, a task that belongs to the phonological loop). In principle, the participants could freely choose whether they did the subtraction-task verbally or visually. But because the phonological loop was already occupied by saying "la, la, la ..." and would therefore have been overloaded if the subtraction-task had been done verbally as well, the participants were forced to do the task visually. As mentioned above, because of the fact that the subtraction of a partial image from a whole given image is easier if done visually, the performance increased if participants were forced to visually perform that task, i.e. if they were forced to use the component that is suited best for the given task. We have seen that the phonological loop and the visuospatial sketch pad deal with rather different kinds of information which nonetheless have to somehow interact in order to do certain tasks. The component that connects those two systems is the central executive.

The central executive co-ordinates the activity of both the phonological loop and the visuospatial sketch pad. Imagine the following situation: you are driving a car and your friend in the passenger seat has the map and gives you directions. The directions are given verbally, i.e. they are handled by the phonological loop, while the perception of the traffic, street lights, etc. is obviously visual, i.e. dealt with in the visuospatial sketch pad. If you now try to follow the directions given to you by your friend it is necessary to somehow combine both kinds of information, the verbal and the visual information. This important connection of the two components is done by the central executive. (It also links the working memory to long term memory, we will discuss long term memory below.) Currently, research is being done in order to find out how the central executive solves the complex task of co-ordinating and controlling the other components. Unfortunately, we do not know much about the operation of the central executive yet. Let us hope that the research will be as fruitful as it has been so far with respect to other parts of memory.

Long Term Memory

As the name already suggest, long term memory is the system where memories are stored for a long time. "Long" in this sense means something between a few minutes and several years or even decades. Similar to working memory, long term memory can again be subdivided into different types. Two major distinctions are being made between declarative (conscious) and implicit (unconscious) memory. Those two subtypes are again split into two components each: episodic and semantic memory with respect to declarative memory and priming effects and procedural memory with respect to implicit memory. In contrast to short term or working memory, the capacity of long term memory is theoretically infinite. The magic number seven obviously does not apply here, because, as mentioned above, information can be stored for a very long time and is not restricted to a few items. The opinions as to whether information remains in long term memory for ever, or whether information can get deleted differ. The main argument for the latter opinion is that apparently not all information that ever got stored in LTM can be recalled. However, theories that regard long term memories as not being
subject to deletion emphasize that there might be a useful distinction between the existence of information and the ability to retrieve or recall that information at a given moment. An example for the inability to retrieve a particular memory would be a situation in which someone tries to remember a name, but cannot come up with it. The saying of something like "I have it on the tip of my tongue..." indicates that the speaker is sure that the information is still existent in his memory, but that the retrieval is somehow blocked. It such situations the circumstances (in a rather broad sense) might enhance the retrieval of information. An example would be that someone helps the aforementioned speaker by giving the first letter of the name or something similar. Or another enhancement might be that in order to better recall memories about the childhood, it could be helpful to visit the places or people that are connected to childhood, like the kindergarten or an elementary school teacher.

Declarative Memory

Let us now consider the two types of declarative memory. As noted above, those two types are episodic and semantic memory. Episodic memory refers to memories for particular events that have happened to someone. Typically, those memories are connected to specific times and places. Semantic memory, on the other hand, refers to knowledge about the world that is not connected to personal events. Vocabulary, concepts, numbers or facts would be stored in semantic memory. The two types are usually closely related to one another, i.e. memory of facts might be enhanced by interaction with memory about personal events and vice versa. For example, the answer to the factual question of whether people put vinegar on their chips might be answered positively by remembering the last time you saw someone eating fish and chips. The other way around, good semantic memory about certain things such as football can contribute to more detailed episodic memory of a particular personal event, like watching a football match. A person that barely knows the rules of that game will most probably have a less specific memory for the personal event of watching the game than a football-expert will.

Implicit Memory

We now turn to the different types of implicit memory. As the name suggests, both types are usually active when unconscious memories are concerned. This becomes most evident for procedural memory, though it must be said that the distinction between both types is not as clearly cut as in the case of declarative memory and that often both categories are collapsed into the single category of procedural memory. But if we want to draw the distinction between priming effects and procedural memory, the latter category is responsible for highly skilled activities that can be performed without much conscious effort. Examples would be the tying of shoelaces or the driving of a car, if those activities have been practiced sufficiently. As regards the priming effect, consider the following experiment conducted by T.J. Perfect and C. Askew (1994). Participants were asked to read a magazine without paying attention to the advertisements. After that, different advertisements were presented to them, some had occurred in the magazine, others had not. The participants were told to rate the presented advertisement with respect to different criteria such as how appealing, how memorable or eye-catching they were. The result was that in general those advertisements that had been in the magazine received higher rankings than those that had not been in the magazine. Additionally, when asked which advertisements the participants had actually seen in the magazine, the recognition was very poor (only 2.8 of the 25 advertisements were recognized). This experiment shows that the participants performed implicit learning (as can be seen from the high rankings of advertisements they had seen before) without being conscious of it (as can be seen from the poor recognition rate). This is an example of the priming effect.

Errors in Memory

Finally we arrive at the errors or disorders. A definition of Memory: "Memory is the ability of an organism to store, retain, and subsequently recall information." So we are on our way to discover what blocked processes or defective mechanisms of the brain are leading to what kind of disfunction. We like to divide the errors in biochemical and hardware. (like a brain injury or operation).


A biochemical error is for example alzheimer where patients show a depletion of acetylcholine and glutamate. "Alzheimer disease is a neurodegenerative disease characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms or behavioral changes." Alzheimer - Wikipedia

But there are many more Neurodegenerative Diseases. Commonly known are Parkinson, Multiple sclerosis and Creutzfeld-Jakob. A Neurodegenerative Disease is a "disease caused by the irreversible deterioration of essential cell and tissue components of the nervous system.". So the basis gets lost and the brain loses its cognitive functions.

Hardware Errors

The most studied patient with a so called hardware error is Henry M. H.M.'s History A man who had as child a bicycle accident and suffered from epilepsy. In an experimental surgery Dr. William Scoville removed H.M.'s hippocampus and other parts of the brain. Good for Henry was that the frequency of the epilepsy was reduced, bad was that he was not able to store any new memories. By the way, his

Short Term Memory was normal. Corkin showed in Diagram of basic features of 1968 that he could learn new simple tasks. So he concluded that his procedural memory was working. H.M. is suffering from Anterograde amnesia. Amnesia - Wikipedia

a neuron.


• E. Bruce Goldstein: Cognitive Psychology: Connecting Mind, Research, and Everyday Experience. Thomson Wadsworth, USA 2005 ISBN 0-534-57726-1

• Lyle E. Bourne and Bruce R. Ekstrand: Einführung in die Psychologie. (4. Auflage). Verlag Dietmar Klotz GmbH, Eschborn 2005 ISBN 3-88074-500-5


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