Tower of Hanoi
a puzzle often used in studies of problem solving and tests of executive functions. In the most basic version, three disks of successively decreasing diameter are stacked on one of three vertical pegs. The solver’s task is to move the stack to another peg one disk at a time, never stacking a larger disk on a smaller one, in the fewest possible moves. The puzzle was invented in 1883 by French mathematician Edouard Lucas (1842–1891). A similar test, the Tower of London, was developed in the 1980s by British neuropsychologist Timothy Shallice (1940– ) to investigate planning ability in individuals with frontal lobe dysfunction; in this case, the solver’s task is to move three colored beads (or rings) from their initial position on vertical pegs to predetermined positions on other pegs, using the fewest number of moves possible.