Merzenich was an expert in a technique called "micromapping", a precise but time-consuming way of finding which parts of the brain responded to external stimuli, using extremely small electrodes that measure the electrical activity of a single neuron. Merzenich used macaque monkeys, measuring neuronal activity as he tapped different parts of the monkey's hand, in order to see which areas of it stimulated electrical activity in the monitored part of the brain. Mapping a whole hand using this method could take between 20 and 40 hours.
Source: wiktionary