ii | NewScientist | 3 December 2011
Particularly salient information gets transferred to the brains long-term storage facility, where it can remain for years or even decades. Your date of birth, phone number, car registration number and your mothers maiden name are all held here.
Unlike short-term memory, with its acoustic representations, we seem to store long-term memories by their meaning. If you try to recall information after a delay, for instance, you probably wont be able to reproduce the exact wording but its meaning or gist should come back fairly easily. This can lead to errors, however.
Long-term memories can take many different forms. Semantic memories, for example, concern your knowledge of facts, such as Paris being the capital of France, though you may not remember the exact circumstances in which you acquired this information. Episodic memories concern particular events from your life, such as the day you passed your driving test.
You can also categorise long-term memories by the way they influence your behaviour. Consciously recalled events or pieces of information are known as explicit memories, whereas implicit memory refers to experiences that influence your behaviour, feelings or thoughts without you actively recollecting the events or facts. For instance, if you pass an Italian restaurant on the way to work in the morning, you might later that day think about going out for an Italian meal, without being aware that you had been influenced by your morning journey.
From early influential work by Canadian psychologist Donald Hebb in the 1940s through to the more recent Nobel-prizewinning work of American neuropsychiatrist Eric Kandel, we now know that long-term memories are maintained by stable and permanent changes in neural connections. And with techniques such as fMRI we now have the capacity to study these processes non-invasively in humans. The diencephalon and the hippocampal regions seem to be essential for consolidating information from short-term into long-term memories (see diagram, above). Their exact roles in retrieving older memories from across our lifespan is still contentious; the hippcampus might be part of the library system that organises our long-term memories, or instead just the printing press which makes them.
PREFRONTAL CORTEXResponsible for short-term and working memory
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The exact mechanism for consolidating and retrieving memories is still unclear
When you hold a restaurants phone number in your mind as you dial the number, you rely on your short-term memory. This store is capable of holding roughly seven items of information for approximately 15 to 20 seconds, though actively rehearsing the information by repeating it several times can help you to retain it for longer.
Seven items of information may not seem much, but it is possible to get around this limit by chunking larger pieces of information into meaningful units. To recall a 10-digit telephone number, for instance, a person could chunk the digits into three groups: the area code (such as 021), then a three-digit chunk (639) and a four-digit chunk (4345).
Your short-term memory seems to store verbal and visuospatial information in different places. The verbal store has received most attention Its existence has been inferred from studies asking volunteers to remember lists of words: people tend to be much better at recalling the last few items in a list, but this effect disappears if the test is delayed by a few seconds, especially if the delay involves a verbal activity that interferes with the storage process, such as counting backwards.
Verbal short-term memories seem to be stored in
acoustic or phonological form. When you try to remember sequences of letters, for instance, lists of letters that are similar in sound, like P, D, B, V, C and T, are harder to recall correctly than sequences of dissimilar-sounding letters like W, K, L, Y, R and Z, even when the information is initially presented visually.
Short-term memory is closely linked to working memory, and the two terms are often used interchangeably. There is a difference, however: short-term memory refers to the passive storage and recall of information from the immediate past, whereas working memory refers to the active processes involved in manipulating this information. Your short-term memory might help you to remember what someone has just said to you, for example, but your working memory would allow you to recite it to them backwards or pick out the first letter of each word.
short-term and working memory
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Twirling a sparkler allows us to write letters and make circles in the air thanks to our sensory memory
During every moment of an organisms life, its eyes, ears and other sensory organs are taking in information and relaying it to the nervous system for processing. Our sensory memory store retains this information for a few moments. So twirling a sparkler, for example, allows us to write letters and make circles in the air thanks to the fleeting impression of its path.
Johann Segner, an 18th-century German scientist, was one of the first to explore this phenomenon. He reportedly attached a glowing coal to a cartwheel, which he rotated at increasing speeds until an unbroken circle of light could be perceived. His observations were followed by the systematic investigations of American psychologist George Sperling 100 years later. By studying peoples ability to recall an array of letters flashed briefly on a screen, he found that our fleeting visual impressions dubbed iconic memory last for just a few hundred milliseconds. Studies of echoic sound memories came soon afterwards, showing that we retain an impression of what we hear for several seconds. Of note, echoic memories may be impaired in children who are late talkers.
Sensory memories are thought to be stored as transient patterns of electrical activity in the sensory and perceptual regions of the brain. When this activity dissipates, the memory usually fades too. While they last, though, they provide a detailed representation of the entire sensory experience, from which relevant pieces of information can be extracted into short-term memory and processed further via working memory.
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WHAt iS memOry?Remembering the past is an integral part of human existence. Without a good memory, you would not be able to drive to work, hold a meaningful conversation with your children, read a book, or prepare a meal.
This capacity has fascinated humans since ancient times; Plato famously compared our memory to a wax tablet that is blank at birth and slowly takes on the impression of the events from our life. Only in the past hundred years, though, have psychologists developed objective techniques to study our recollections of the past with scientific accuracy and reproducibility. These range from straightforward laboratory tests of our ability to remember long lists of words to more recent brain-imaging approaches.
It has become clear through these studies that, unlike Platos wax tablet, human memory has many different components. If you consider how long a memory lasts, for example, there appear to be at least three subtypes of storage: sensory, short term and long term. Memories can also be distinguished by the type of information that is stored and the way it is recalled.
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Wha t is memory?Long-term memoryShort-term and working memorySensory Memory