PARTS OF THE BRAIN
The human brain is hugely interconnected but three major components can be identified: 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 well as 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 also involved in some cognitive functions such as attention, language, emotional functions (such as regulating fear and pleasure responses) and in the processing of procedural memories.
The cerebrum (or forebrain), which makes up 75% of the brain by volume and 85% 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 large bundle of nerve fibres called the corpus callosum, and also by other smaller connections called commissures.
Most of the important elements of the cerebrum, are split into symmetrical pairs in the left and right hemispheres. Thus, we often speak of the temporal lobes, hippocampi, etc (in the plural), although this website generally follows the convention 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 and perform different functions. The right hemisphere generally controls the left side of the body, and vice versa, although popular notions that logic, creativity, etc, are restricted to the left or right hemispheres are largely simplistic and unfounded.
Lobes of the cerebral cortex
Lobes of the cerebral cortex
Picture from Wikipedia (http://en.wikipedia.org/wiki/Human_brain)
The cerebrum is covered by a sheet of neural tissue known as the cerebral cortex (or neocortex), which envelops other brain organs such as the thalamus (which evolved to help relay information from the brain stem and spinal cord to the cerebral cortex) and the hypothalamus and pituitary gland (which control visceral functions, body temperature and behavioural responses such as feeding, drinking, sexual response, aggression and pleasure). The cerebral cortex itself is only 2 - 4 mm thick, and contains six distinct but interconnected layers. It is intricately grooved and folded into the familiar convoluted pattern of folds, or gyri, allowing a large surface area (typically almost 2m2) to fit within the confines of the skull. Consequently, more than two-thirds of the cerebral cortex is buried in the grooves, or sulci.
About 90% of all the brain’s neurons are located in the cerebral cortex, mainly in the "grey matter", which makes up the surface regions of the cerebral cortex, while the inner "white matter" consists mainly of myelinated axons, over 170,000 km of them. As many as five times that number of glial cells exist to support the active nerve cells.
The cerebral cortex plays a key role in memory, attention, perceptual awareness, thought, language and consciousness. It is divided into four main regions or lobes, which cover both hemispheres: the frontal lobe (involved in conscious thought and higher mental functions such as decision-making, particularly in that part of the frontal lobe known as the prefrontal cortex, and plays an important part in processing short-term memories and retaining longer term memories which are not task-based); the parietal lobe (involved in integrating sensory information from the various senses, and in the manipulation of objects in determining spatial sense and navigation); the temporal lobe (involved with the senses of smell and sound, the processing of semantics in both speech and vision, including the processing of complex stimuli like faces and scenes, and plays a key role in the formation of long-term memory); and the occipital lobe (mainly involved with the sense of sight).
The Limbic System and Basal Ganglia
The Limbic System and Basal Ganglia
Picture from How Stuff Works (http://people.howstuffworks.com/
swearing.htm/printable)
The medial temporal lobe (the inner part of the temporal lobe, near the divide between the left and right hemispheres) in particular is thought to be involved in declarative and episodic memory. Deep inside the medial temporal lobe is the region of the brain known as the limbic system, which includes the hippocampus, the amygdala, the cingulate gyrus, the thalamus, the hypothalamus, the epithalamus, the mammillary body and other organs, many of which are of particular relevance to the processing of memory.
The hippocampus, for example, is essential for memory function, particularly the transference from short- to long-term memory and control of spatial memory and behaviour. The hippocampus is one of the few areas of the brain capable actually growing new neurons, although this ability is impaired by stress-related glucocorticoids. The amygdala also performs a primary role in the processing and memory of emotional reactions and social and sexual behaviour, as well as regulating the sense of smell.
Another sub-cortical systems (inside the cerebral cortex) which is essential to memory function is the basal ganglia system, particularly the striatum (or neostriatum) which is important in the formation and retrieval of procedural memory.
Types of memories:
Sensory memory
1-SENSORY MEMORY
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Studies have shown that attention significantly affects memory during the encoding phase, but hardly at all during recall.
Thus, distractions or divided attention during initial learning may severely impair subsequent retrieval success, whereas distractions at the time of recall may slow down the process a little, but has little to no effect on its accuracy.
Sensory memory is the shortest-term element of memory. It is the ability to retain impressions of sensory information after the original stimuli have ended. It acts as a kind of buffer for stimuli received through the five senses of sight, hearing, smell, taste and touch, which are retained accurately, but very briefly. For example, the ability to look at something and remember what it looked 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 they disappear 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 totally outside of conscious control. The brain is designed to only process information that will be useful at a later date, and to allow the rest to pass by unnoted. As information is perceived, it is therefore stored in sensory memory automatically and unbidden. Unlike other types of memory, the sensory memory cannot be prolonged via rehearsal.
Sensory memory is an ultra-short-term memory and decays or degrades very quickly, typically in the region of 200 - 500 milliseconds (1/5 - 1/2 second) after the perception of an item, and certainly less than a second (although echoic memory is now thought to last a little longer, up to perhaps three or four seconds). Indeed, it lasts for such a short time that it is often considered part of the process of perception, but it nevertheless represents an essential step for storing information in short-term memory.
The sensory memory for visual stimuli is sometimes known as the iconic memory, the memory for aural stimuli is known as the echoic memory, and that for touch as the haptic memory. Smell may actally be even more closely linked to memory than the other senses, 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, smells may be more quickly and more strongly associated with memories and their associated emotions than the other senses, and memories 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 (50 milliseconds) 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 of selectively concentrating on one aspect of the environment while ignoring other things), which effectively filters the stimuli to only those which are of interest at any given time.
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2- short term memory :
SHORT-TERM (WORKING) MEMORY
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A 2010 University of Stirling study has suggested a possible link between poor short-term or working memory and depression.
The 10 to 15% with the poorest working memory in the study tended to mull things over and brood too much, leading to a risk of depression.
People with a good working memory, on the other hand, are more likely to be optimistic and self-assured, and more likely to lead a happy and successful life.
Short-term memory acts as a kind of “scratch-pad” for temporary recall of the information which is being processed at any point in time, and has been refered to as "the brain's Post-it note". It can be thought of as the ability to remember and process information at the same time. It holds a small amount of information (typically around 7 items or even less) in mind in an active, readily-available state for a short period of time (typically from 10 to 15 seconds, or sometimes up to a minute).
For example, in order to understand this sentence, the beginning of the sentence needs to be held in mind while the rest is read, a task which is carried out by the short-term memory. Other common examples of short-term memory in action are the holding on to a piece of information temporarily in order to complete a task (e.g. “carrying over” a number in a subtraction sum, or remembering a persuasive argument until another person finishes talking), and simultaneous translation (where the interpreter must store information in one language while orally translating it into another). What is actually held in short-term memory, though, is not complete concepts,but rather links or pointers (such as words, for example) which the brain can flesh out from it's other accumulated knowledge.
However, this information will quickly disappear forever unless we make a conscious effort to retain it, and short-term memory is a necessary step toward the next stage of retention, long-term memory. The transfer of information to long-term memory for more permanent storage can be facilitated or improved by mental repetition of the information or, even more effectively, by giving it a meaning and associating it with other previously acquired knowledge. Motivation is also a consideration, in that information relating to a subject of strong interest to a person, is more likely to be retained in long-term memory.
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A recent study at the University of Michigan suggests that attention and short-term memory processing are directly affected by a person's surroundings and environment.
Two groups of individuals were tested on their attention and working memory performance, one group after a relaxed walk in a quiet park and the other group after navigating busy city streets.
Those who had been walking the city streets scored far lower on the tests.
The term working memory is often used interchangeably with short-term memory, although technically working memory refers more to the whole theoretical framework of structures and processes used for the temporary storage and manipulation of information, of which short-term memory is just one component.
The central executive part of the prefrontal cortex at the front of the brain appears to play a fundamental role in short-term and working memory. It both serves as a temporary store for short-term memory, where information is kept available while it is needed for current reasoning processes, but it also "calls up" information from elsewhere in the brain. The central executive controls two neural loops, one for visual data (which activates areas near the visual cortex of the brain and acts as a visual scratch pad), and one for language (the "phonological loop", which uses Broca's area as a kind of "inner voice" that repeats word sounds to keep them in mind). These two scratch pads temporarily hold data until it is erased by the next job.
Although the prefrontal cortex is not the only part of the brain involved - it must also cooperate with other parts of the cortex from which it extracts information for brief periods - it is the most important, and Carlyle Jacobsen reported, as early as 1935, that damage to the prefrontal cortex in primates caused short-term memory deficits.
The short-term memory has a limited capacity, which can be readily illustrated by the simple expedient of trying to remember a list of random items (without allowing repetition or reinforcement) and seeing when errors begin to creep in. The often-cited experiments by George Miller in 1956 suggest that the number of objects an average human can hold in working memory (known as memory span) is between 5 and 9 (7 ± 2, which Miller described as the “magical number”, and which is sometimes referred to as Miller's Law). However, although this may be approximately true for a population of college students, for example, memory span varies widely with populations tested, and modern estimates are typically lower, of the order of just 4 or 5 items.
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Short-term working memory appears to operate phonologically.
For instance, whereas English speakers can typically hold seven digits in short-term memory, Chinese speakers can typically remember ten digits.
This is because Chinese number words are all single syllables, whereas English are not.
The type or characteristics of the information also affects the number of items which can be retained in short-term memory. For instance, more words can be recalled if they are shorter or more commonly used words, or if they are phonologically similar in sound, or if they are taken from a single semantic category (such as sports, for example) rather than from different categories, etc. There is also some evidence that short-term memory capacity and duration is increased if the words or digits are articulated aloud instead of being read sub-vocally (in the head).
The relatively small capacity of the short-term memory, compared to the huge capacity of long-term memory, has been attributed by some to the evolutionary survival advantage in paying attention to a relatively small number of important things (e.g. the approach of a dangerous predator, the proximity of a nearby safe haven, etc) and not to a plethora of other peripheral details which would only interfere with rapid decision-making.
"Chunking" of information can lead to an increase in the short-term memory capacity. Chunking is the organization of material into shorter meaningful groups to make them more manageable. For example, a hyphenated phone number, split into groups of 3 or 4 digits, tends to be easier to remember than a single long number. Experiments by Herbert Simon have shown that the ideal size for chunking of letters and numbers, whether meaningful or not, is three. However, meaningful groups may be longer (such as four numbers that make up a date within a longer list of numbers, for example). With chunking, each chunk represents just one of the 5 - 9 items that can be stored in short-term memory, thus extending the total number of items that can be held.
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The use of mnemonic devices can significantly increase memory, particularly the recall of long lists of names, numbers, etc.
One case, known as “S.F.”, was able to increase his digit span (the longest list of number that a person can repeat back in correct order) from 7 to 79 with the use of mnemonic strategies.
Akira Haraguchi and Lu Chao’s record-breaking recitations of the digits of the number Pi (100,000 and 67,890 digits respectively) also make use of mnemonic systems.
It is usually assumed that the short-term memory spontaneously decays over time, typically in the region of 10 - 15 seconds, but items may be retained for up to a minute, depending on the content. However, it can be extended by repetition or rehearsal (either by reading items out loud, or by mental simulation), so that the information re-enters the short-term store and is retained for a further period. When several elements (such as digits, words or pictures) are held in short-term memory simultaneously, they effectively compete with each other for recall. New content, therefore, gradually pushes out older content (known as displacement), unless the older content is actively protected against interference by rehearsal or by directing attention to it. Any outside interference tends to cause disturbances in short-term memory retention, and for this reason people often feel a distinct desire to complete the tasks held in short-term memory as soon as possible.
The forgetting of short-term memories involves a different process to the forgetting of long-term memories. When something in short-term memory is forgotten, it means that a nerve impulse has merely ceased being transmitted through a particular neural network. In general, unless an impulse is reactivated, it stops flowing through a network after just a few seconds.
Typically, information is transferred from the short-term or working memory to the long-term memory within just a few seconds, although the exact mechanisms by which this transfer takes place, and whether all or only some memories are retained permanently, remain controversial topics among experts. Richard Schiffrin, in particular, is well known for his work in the 1960s suggesting that ALL memories automatically pass from a short-term to a long-term store after a short time (known as the modal or multi-store or Atkinson-Schiffrin model).
However, this is disputed, and it now seems increasingly likely that some kind of vetting or editing procedure takes place. Some researchers (e.g. Eugen Tarnow) have proposed that there is no real distinction between short-term and long-term memory at all, and certainly it is difficult to demarcate a clear boundary between them. However, the evidence of patients with some kinds of anterograde amnesia, and experiments on the way distraction affect the short-term recall of lists, suggest that there are in fact two more or less separate systems.
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3-
LONG-TERM MEMORY
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While older people have more difficulty than the young with rote memorization, such as remembering lists of words or numbers, they actually tend to perform better than young people in the recognition and recall of facts and tasks.
This is partly because older people, having accumulated more real-life experience and information, have a denser network of linkages and associations in their long-term memory, and partly because they have had time to more efficiently organize their facts and experiences in a more easily accessible hierarchical form.
Long-term memory is, obviously enough, intended for storage of information over a long period of time. Despite our everyday impressions of forgetting, it seems likely that long-term memory actually decays very little over time, and can store a seemingly unlimited amount of information almost indefinitely. Indeed, there is some debate as to whether we actually ever “forget” anything at all, or whether it just becomes increasingly difficult to access or retrieve certain items from memory.
Short-term memories can become long-term memory through the process of consolidation, involving rehearsal and meaningful association. Unlike short-term memory (which relies mostly on an acoustic, and to a lesser extent a visual, code for storing information), long-term memory encodes 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 is usually based on the sound of a word, not its meaning.
Physiologically, the establishment of long-term memory involves a process of physical changes in the 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, are created, altered or strengthened. These neural circuits are composed of a number of neurons that communicate with one another through special junctions called synapses. Through a process involving the creation of new proteins within the body of neurons, and the electrochemical transfer of neurotransmitters across synapse gaps to receptors, the communicative strength of certain circuits of neurons in the brain is reinforced. With repeated use, the efficiency of these synapse connections increases, facilitating the passage of nerve impulses along particular neural circuits, which may involve many connections to the visual cortex, the auditory cortex, the associative regions of the cortex, etc.
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Several studies have shown that both episodic and semantic long-term memories can be better recalled when the same language is used for both encoding and retrieval.
For example, bilingual Russian immigrants to the United States can recall more autobiographical details of their early life when the questions and cues are presented in Russian than when they are questioned in English.
This process differs both structurally and functionally from the creation of working or short-term memory. Although the short-term memory is supported by transient patterns of neuronal communication in the regions of the frontal, prefrontal and parietal lobes of the brain, long-term memories are maintained by more stable and permanent changes in neural connections widely spread throughout the brain. The hippocampus area of the brain essentially acts as a kind of temporary transit point for long-term memories, and is not itself used to store information. However, it is essential to the consolidation of 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 more after the initial learning.
Unlike with short-term memory, forgetting occurs in long-term memory when the formerly strengthened synaptic connections among the neurons in a neural network become weakened, or when the activation of a new network is superimposed over an older one, thus causing interference in the older memory.
Over the years, several different types of long-term memory have been distinguished, including explicit and implicit memory, declarative and procedural memory (with a further sub-division of declarative memory into episodic and semantic memory) and retrospective and prospective memory.
