Memory (from Middle High German Memories , "keepsake memory") or Mnestik refers to the ability of the nervous systems of living organisms , recorded information to convert, store, and retrieve it. Both terms are derived from mnḗstis , 'memory' or 'commemoration' (this from ancient Greek μνήμη mnḗmē , German 'memory, remembrance' ; compare also amnesia and amnesty ).
Information stored in memory is the result of conscious or unconscious learning processes . The memory formation is made possible by the neural plasticity . In a figurative sense, the word “memory” is also used generally for the storage of information in other biological and technical systems.
Primitive nervous systems (e.g. those of cnidarians ) are also capable of simple learning processes. The complexity and scope of possible memory functions have increased in the course of evolution.
The different types of memory can be classified according to two aspects at the psychological level: the duration of the storage or the type of memory content. With regard to the storage duration of the information, a distinction is made between sensory memory (e.g. iconic or echoic memory ), short-term memory and long-term memory .
Within long-term memory, there is also a difference in content between declarative and procedural memory. The declarative memory consciously stores accessible information. This includes facts and events that either belong to one's own biography ( episodic memory ) or constitute a person's so-called world knowledge ( semantic memory , e.g. professional knowledge, facts from history, politics, cooking recipes, etc.). The procedural memory, on the other hand, comprises skills that are usually used automatically and without thinking. Above all, this includes motor processes (cycling, swimming, dancing, skiing). Procedural memory content is mainly acquired through implicit learning , while declarative content is acquired through explicit learning .
Division into different types of memory
The memory can be divided into different subsystems according to the duration of the information storage. A distinction is usually made between three systems:
- Sensory memory (also sensory register ): Holds information for milliseconds to seconds
- Working memory (also short-term memory ): Stores information for about 20–45 seconds
- Long-term memory: Stores information for years
Another model is the levels of processing approach .
Sensory memory (ultra-short-term memory)
New information reaches the brain via the sensory organs and is temporarily stored in sensory memory (also known as sensory register , formerly also called immediate memory , ultra-short-term memory or ultra-short-term memory ). Sensory memory is specific to each sensory modality , it is also referred to as iconic memory for visual perception and echoic memory for auditory perception . The ability to repeat something that was said in a conversation while not listening is an example of auditory sensory memory.
Much more information is recorded in sensory memory than in working memory. However, these disintegrate after a few tenths of a second. One way to investigate the collapse of the information in this memory system, the so-called partial report method (Engl. Partial-report ), by George Sperling was developed (1960). In this, test subjects are presented with several rows of letters (set) in different lines, of which only individual lines are to be displayed when later retrieved. This is to prevent the others from being forgotten when individual parts from the set are being reproduced. If, in an experiment, the time between the presentation of the set and the indication of which line should be reproduced is varied and the memory performance is compared depending on the time in between, an estimate for the duration of the storage is obtained. With this method it could be shown that the visual sensory memory can store information for about 15 milliseconds, whereas the auditory sensory memory can store information for about 2 seconds.
With this type of memory, centrally controlled processes, such as awareness or attention , usually do not play a significant role. However, these can have a major impact on the transfer of information into working memory.
Short term memory and working memory
The basis of conscious information processing is short-term memory (in some models also working memory ). Short-term memory is a memory that holds a very limited amount of information in an immediately available state.
According to a hypothesis that is now considered to be historically obsolete, it had an approximate capacity of about 7 ± 2 information units, provided that it was a matter of items that could be listed in numbers. These were also called chunks (see there for more recent findings).
Short term memory
One aspect that received special attention in the context of research into short-term memory is “quick forgetting”. This was examined for the first time by Peterson & Peterson (1959). By showing their subjects individual words, word triads and consonant triads followed by a distracting task (counting backwards), they found a significant drop in memory performance depending on the length of the distracting task. It also made a difference whether the words were presented individually or in groups. Individual words showed a significantly lower forgetting rate than a group of three consonants or three words. The latter two were no different from each other. Murdock (1961) confirmed the results of Peterson & Peterson and could additionally show that the presentation of several things of the same semantic category caused a forward inhibition . The subjects found it more difficult to differentiate between things the more they had seen (list length effect). This was shown in a clear drop in memory performance.
Delos Wickens (1970) was able to show that the forward-looking inhibition can be lifted if test subjects are presented with words from different semantic categories. After a category change, the memory performance increased again significantly. Gunter et al. a. (1981) carried out three experiments in which they could demonstrate forward inhibition and its cancellation. They had their test subjects auditioned individual television news from various topics, for example domestic and foreign policy. One group was presented with four similar topics, the other three similar and a news item from a different topic. In the first group, the forward-looking inhibition showed in the sense of a falling memory performance and in the second group, the lifting of the inhibition through the change of topic was shown. Both effects could also be found with a reduced number of things and with the additional task of describing them precisely. In addition, the authors were able to demonstrate a learning effect if certain things had already been shown in a previous test. The subjects could then remember them better in a second test. Research on the period of the effect of the forward inhibition most likely indicated the retrieval phase.
- The spatial-visual notepad for short-term storage of visual impressions.
- The articulatory or phonological loop is used to store verbal information, which can remain available for a relatively long time through internal repetition.
- The central executive administers the two subsystems and links information from them with long-term memory.
Finally, the model was expanded to include an episodic buffer .
The long-term memory is the permanent storage system of the brain. It is not a single entity, but several storage services for different types of information. It can be stored in long-term memory from minutes to years (secondary memory) or even for a lifetime (tertiary memory). Nothing is known about limitations in the capacity of long-term memory. However, studies in so-called savants (French) or those with gifted islands suggest a significantly higher memory capacity than the one normally used. Forgetting does not seem to be a capacity problem, but rather a protection against too much knowledge. Forgetting apparently takes place less through the loss of information as in the other, short-term memory forms, but rather through the deleting or falsifying influence of other, previously or subsequently formed content.
A distinction must be made between various processes of long-term memory:
- Learning / encoding : new storage of information
- Remembering / recalling: becoming aware of memory contents
- Consolidate / Retain: Consolidation of information through repeated retrieval
- Linking new and old information
- Forgetting: The disintegration of memory content or modification by competing information
For the transfer of new memory contents into the long-term memory and the preservation of information, practice is often beneficial, for example through the conscious retrieval and rethinking of information in working memory. The anchoring in the memory increases with the meaning, the emotional weight and the number of associations (connection with other content).
Components of long-term memory
A basic distinction is made between two types of long-term memory that store different types of information: declarative (explicit) and procedural implicit memory . The different forms of information are independent of one another and are stored in different areas of the brain, so that, for example, patients with amnesia (memory disorder) of the declarative memory can have undisturbed procedural memory performance.
The "declarative memory", also knowledge memory, stores facts and events that can be consciously reproduced. Declarative memory is divided into two areas:
- The "semantic memory" contains the world knowledge, independent of the person, general facts ("Paris is the capital of France", "You have a mother and a father").
- The “episodic memory” contains episodes, events and facts from one's own life (memories of experiences during a visit to Paris, the face and name of one's own father).
The procedural memory , also behavioral memory, stores automated courses of action or skills. Examples are walking, cycling, dancing, driving a car, playing the piano. These are complex movements, the sequence of which has been learned and practiced and which are then called up and performed without thinking .
The capacity of human memory is difficult to determine and depends on the type of information we store. It was estimated that on average each person can recognize and thus remember around 5000 other people's faces.
Anatomy and Physiology of Memory
In contrast to other areas such as language , motor skills , vision or hearing, there is no definable, comprehensive “memory area” in the brain. Rather, memory is based primarily on additional services provided by other specialized parts of the brain. Nevertheless, different anatomical structures can be distinguished that are necessary for memory. Before doing this, it must be clarified what is the correlate (correspondence) of learning and memory at the lowest level, on the individual neuron .
Neural learning processes
The content of the memory is stored in the connections of the nerve cells , the synapses , more precisely in the synaptic efficiency of neural networks . After the hypothesis was put forward until the 1970s that chemical molecules could take on this role - Scotophobin has become especially famous - this hypothesis turned out to be no longer tenable.
It is estimated that there are between 100 and 500 billion synapses between the approximately 100 billion nerve cells. The decisive factor here is synaptic plasticity : many synapses are anatomically adaptable. This allows them to change the efficiency of the transmission between neurons. In addition, transmission properties are adapted through the formation and breakdown of synapses.
In 1949, Donald O. Hebb was the first to propose that synapses - depending on the extent to which they are activated by neuronal activity - change the strength of their signaling ability "permanently" through anatomical remodeling. The hypothesis he set up in the so-called Hebb's learning rule was confirmed experimentally. A synapse that becomes stronger through simultaneous activity in the pre- and post-synaptic neuron is called a "Hebb synapse". Such a permanent change in a synapse is referred to in neurophysiology as "homosynaptic" long-term potentiation (long-term strengthening ).
There are many other forms of synaptic plasticity. They differ primarily in their direction (potentiation or depression, i.e. amplification or weakening), in their duration (short-term or long-term change), in their synaptic specificity (homo- or heterosynaptic) and the molecular mechanisms of their formation and maintenance.
Various signal cascades have been described which start with the excitation of a nerve cell through a certain synapse and a subsequently triggered action potential and lead to short-term or long-term changes in synaptic efficiency. Such mechanisms include short term, the phosphorylation of receptor molecules, the release of a retrograde (backward acting) messengers for the presynaptic axon ( nerve fiber ), and for the long-term action, in particular the activation of transcription factors , the protein biosynthesis regulate and to increased synthesis of receptor molecules, enzymes for transmitter build-up and breakdown and structural proteins.
Gross anatomical structures
Today, certain brain regions are assigned to the various types of memory. The assignments could be made by comparing memory disorders with localized damage to the brain (e.g. from a stroke ).
Working memory is assigned to the prefrontal cortex . Long-term memory, on the other hand, is based on the interaction of the cortex and numerous subcortical areas. A distinction is made between the various information qualities.
The entire neocortex is involved in declarative memory, in episodic memory especially the right frontal and temporal cortex, in semantic memory especially the temporal lobe.
However, subcortical regions such as the limbic system , especially the medial temporal lobe system, the hippocampus and adjacent areas are also involved, especially in the process of storage . These are summarized in the so-called Papez neuron circle . The case of the patient HM , whose hippocampi were removed for the treatment of severe epilepsy , is often cited . Although the epilepsy was cured, the patient showed severe anterograde amnesia after the operation : he could no longer remember anything new. However, access to memory content acquired before the operation was not impaired.
In humans, in addition to cortical areas, such as the motor and prefrontal areas, the cerebellum and the basal ganglia in particular are involved in the learning of skills . The amygdala plays an important role in the storage of emotionally important memory contents, as well as fear reactions .
For forms of learning in the manner of classical conditioning , which are also present in more primitive animals, evolutionarily older areas of the brain are also involved. The place of learning here is often where the two stimuli to be linked anatomically converge. The cerebellum in particular plays a role here.
Fields of application of memory research
In court proceedings, testimony is very important, especially if it is the most important - or even the only - basis for decision-making. Therefore, it is important to know how reliable witness memories are. Situations in which people witness a crime are situations that are not expected, often only of a very short duration and usually very emotional. Due to the characteristics of these situations, it is particularly easy to falsify memories of them with additional information, for example during interviews. Loftus et al. a. (1978) showed test subjects a sequence of images in which a car hits a pedestrian after it has passed either a stop sign or a give way sign. In a subsequent questionnaire, either a stop sign or a give way sign was mentioned. Through this retrospective conceptual control, the researchers were able to ensure that the group of test persons who received a contradicting question, in a recognition test, opted for the sign that was mentioned "after the original image sequence" and "before the memory test" in the questionnaire submitted in between was.
Although it might be possible that the memory of faces should be more reliable, especially when these were at the center of the action, Loftus and Greene (1980) were able to show that this is also easily falsified. To do this, they showed test subjects faces of people in several experiments and exposed them to false information in the form of subsequent questions or reports. In one of these experiments, they showed them a man without a beard and later gave some of the test subjects the wrong information that the target person had a beard. The group of subjects with the wrong information was much more likely to choose someone with a beard in a recognition test than the group with the correct report (p <0.01). Overall, Loftus u. a. show that in witnesses the memory of faces can also be falsified unnoticed and permanently.
Together, these results show that memories are unreliable and easily falsified. It is therefore important that extreme caution is used in police investigations such as interviews and confrontations, as well as in court proceedings. See also false memories .
Emotion and memory
The process in which the human brain uses learning processes to influence the way in which certain stimuli evoke an emotion is known as "emotional memory". In order to understand which brain areas and neural mechanisms are involved in the processing and storage of such emotional memory contents, the classic fear conditioning was applied in connection with lesion studies. Fear conditioning (which is mostly done on rats) pairs a neutral stimulus (e.g. a sound) with an aversive stimulus (e.g. an electric shock), which causes the rats to subsequently react with fear to the neutral one Show stimulus. This can be the case after a single pairing of the stimuli. By means of selective lesions in rats, it was also possible to determine which brain areas are necessary for the development of such fear reactions (see below).
Neural basis of fear conditioning
It is believed that shock affects the way neurons in specific regions of the brain respond to the previously neutral stimulus. From the results of various lesion studies on rats, Joseph LeDoux et al. a. deduce that sensory signals do not have to be processed by the cortex for conditioning to be possible. Rather, it was found that the relevant area here is the amygdala , which has direct connections to the thalamus (sensory pathways) as well as to the brain stem (vital basic programs).
One region within the amygdala is the central nucleus, which is connected to both the brain stem and the hippocampus. The hippocampus is an important structure for consolidating memory and processing complex stimuli. The assumption is that through this connection memory content and the context of a stimulus get emotional assignments.
There are also connections between the cortex and the amygdala. It is assumed that emotional learning can take place on the one hand on the subcortical path (from the thalamus directly to the amygdala) and on the other hand on the cortical path (from the thalamus via the cortex to the amygdala). The subcortical path goes “faster”, but does not involve any further processing of the stimulus (something is moving - I'm afraid). The cortical path processes the stimulus more extensively (what is moving is a snake - it can bite me - I better get away), but requires "longer reaction time", which in some situations could be too long, which is why the faster subcortical path is evolutionary - right down to humans.
Memory training and sports
Memory training is possible in many ways. There are numerous memory trainers and numerous books. Most of these are based on mnemonics. The most famous is the loci method . Nowadays there are also memory athletes, memory sports championships and a world ranking list. The world record for memorizing , i.e. learning by heart, as many digits as possible in 5 minutes is 520, for example.
- Retrograde amnesia
- Anterograde amnesia
- Transient global amnesia
- Alzheimer's disease
- Korsakoff syndrome
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