The short-term memory
All the information which the ultra-short-term memory consideres as important(meaning worthwhile to be memorised)is transferred to the short- term memory. Lots of information is filtered in between. The short-term memory is also known as working memory or active memory. It is used for specific tasks and also contains very specialised neurons that are responsible for addition. Short-term memory has a limited store which is active during 10-20 seconds. It is vital, because it is responsible for all mental activities. Reading this sentence is also part of the activities.
Our short-term memory can store seven pieces of information. This is mentioned in the study "The Magical Number Seven, Plus or Minus Two" published by the American psychologist George Armitage Miller in 1956. He proofed that our short-term memory can keep about 7 pieces of information, chunks or clumps at the same time. Our receptiveness is thus limited. If you look at, for example, scattered coins on the table you can remember the position of seven coins. He assumed that we can memorise an infinite number of information if we split the information in seven groups, chunks or clumps. Our brains seems to divide information automatically into seven blocks.
We have not learned the alphabet by memorising 26 letters at the same time or by memorising 1 = a, 2 = b, 3 = c but we have learned it with rhymes. It sounded like:
abcd efg hijk lmnop qrs tuvw xyz
We splitted it in seven small blocks. Whether you test it with numbers, coins, words or colors does not make a difference. There are always seven ± two. Intelligence tests often include memo tests trying to test the capacity of short-term memory. Only persons who have an intelligence quotient higher than 150 are able to memorise 9 or even 10 pieces of information!
Any new piece of information replaces the existing piece. This also explains why it is so difficult to find the information again after having been distracted. Our short-term memory can be easily distracted by external influences. If the information is often repeated or if particular emotions are attached to it, it will be transferred to the long term memory.