How did the neuroscientist Michael Merzenich prove the brain was more changeable than was previously thought with his experiments in the 1960s?

The neuroscientist Michael Merzenich proved the brain was more changeable than was previously thought with his experiments on monkeys in the 1960s in which he created a micro-map of monkeys' brains in relation to what their hands felt. Then Merzenich injured the monkeys' hands and recorded how their brains changed and adapted to the injuries.