Take the following test: ask a friend to make a list of ten words or numbers. Read it once. Then try to remember the items and say them out loud. Most people repeat a maximum of seven items.
It all has to do withwork memory, a type of short-term memory that functions as aboardwhere we quickly write and erase information before it becomes part of our longest-lasting memory or quickly disappears. This mechanism is essential for having a conversation, touring a new city, or dialing a simple phone number. But its space is limited. Specifically, it islimited to “magic number seven”.
But why that figure? Mikhail Rabinovich, a neuroscientist at the University of California, San Diego, and Christian Bick of the German Max Planck Institute, seem to have the answer. In an article published in November in the magazinePhysical Review Letters, the researchers present a mathematical model that explains how neurons fire whenrecord a sequence of numbers or words. According to his scheme, for each item a group of neurons is activated that inhibits the rest momentarily while information is retained, and also while it is remembered. The longer a phrase or string of figures is, the more difficult it is for excited cells to keep other nerve cells inactive. Specific,remembering seven items requires 15 times more “neural suppression” than remembering three. And to remember ten we would need 50 times more inhibition capacity. Repeating more than ten figures or words using our temporary memory is physiologically impossible for almost everyone except theautistic, whose brain seems capable of creating much stronger circuits than that of a normal subject. “The brain is a complex biochemical machine,” says Rabonovich.
