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Brain Uses WW2 Code-Cracking Stats To Make Decisions

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Human brains use statistics in a manner similar to that used by code breakers in World War Two, new research reveals.

The Enigma machine, designed by German intelligence agencies during the Second World War, encrypted Allied military messages. The key of the code changed regularly, confounding efforts to decipher the code.

Alan Turing was a pioneering computer programmer who was vital to Allied efforts to break codes of the Axis powers.

The statistical technique developed by Turing (and independently by Abraham Wald of Columbia) assigns values to aligned pairs of letters. If the two characters match, they are labeled with a positive value, and a negative number is given to those that are not similar. Today, that system is known as Wald's sequential probability ratio test. The technique makes it possible to identify pairs of messages that used the same key, providing code breakers with a tool to help translate the messages.

Some letters are more commonly used than others, so by measuring how frequently characters appear in matching pairs of messages, Allied code breakers were often able to determine which letters in the code matched to characters in the alphabet. By taking a sum of all the negative and positive values in pairs of messages, they could frequently tell which messages were coded using identical keys.

Rhesus monkeys were studied as they were shown sequences of symbols flashed upon a screen for just one-quarter second each. The animals then had to make a decision as to which symbol to press in order to be awarded a treat. Some of the characters were reliable indicators as to the location of the food, while others could not be counted on to lead the way to the treat.

Brain scans of the primate brains revealed that primate brains also assign negative values when symbols indicated the treat was to the right, and positive numbers when they shown the food was to the left of the subject. The primates weighted reliable symbols to a greater degree than those that were less likely to reveal the presence of the food.

Humans may also used a similar technique to make decisions every day, researchers revealed.

"They're decisions like, 'I'm going to pick up a book,' or 'I'm going to walk toward the left of the coffee table, not the right.' We make lots of these decisions every day, and it turns out, we're making them by using the laws of probability in a way that statisticians think is optimal," Michael Shadlen of Columbia University said.

Discovery of the brain-processing system similar to that used by Alan Turing was detailed in the journal Neuron.

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